Co-reporter:Weiwei Xie;Robert J. Cava
Journal of Materials Chemistry C 2017 vol. 5(Issue 29) pp:7215-7221
Publication Date(Web):2017/07/27
DOI:10.1039/C7TC01967J
The new complex intermetallic alloy, Cr22Zn72Sn24, with a giant cubic unit cell (space group Fmc, Z = 8, Pearson symbol cF944 with a = 25.083(3) Å) is reported. The structure can be described as a CsCl-type packing of two “Russian Doll” type intermetallic clusters: a 99-atom cluster, Cr@(Cr8Zn6)(Sn)24(Zn)60, composed of a central Cr atom encapsulated by three shells: a distorted rhombic dodecahedron, a snub-cube, and a rhombi-icosidodecahedron; and a 55-atom cluster, Cr@(Zn)12(Zn)30(Zn)12, also composed of a central Cr atom, but now surrounded by three different shells: an icosahedron, an icosidodecahedron, and a larger icosahedron. The two distinct clusters can be considered as “superatoms” that form the resulting nanometer-scale CsCl-type structure. Evaluating the electronic structure of this complex alloy structure yielded an alternative description, involving a 3-d network of vertex-connected, Cr-centered icosahedra and fragments of bcc metals. This description closely parallels the effective chemical bonding interactions, allowing for an understanding of the stability of this CMA. Magnetic and Seebeck coefficient measurements on self-flux grown crystals of Cr22Zn72Sn24 show it to be paramagnetic and metallic, in agreement with the results of electronic structure calculations.
Co-reporter:Miles A. White, Gordon J. Miller, and Javier Vela
Journal of the American Chemical Society 2016 Volume 138(Issue 44) pp:14574-14577
Publication Date(Web):October 21, 2016
DOI:10.1021/jacs.6b10054
The first example of polytypism in the I–II–V semiconductors has been demonstrated with the synthesis of cubic LiZnSb by a low-temperature solution-phase method. This phase exhibits a unique coloring pattern that is novel for this class of compounds. The choice of site configuration has a considerable impact on the band structure of these materials, which in turn affects the transport properties. While the hexagonal polytype has been suggested as a promising n-type and extremely poor p-type thermoelectric material, the cubic analogue is calculated to have high efficiencies for both the n- and p-type derivatives (1.64 and 1.43, respectively, at 600 K). Furthermore, the cubic phase is found to be the energetically favored polytype. This surprising result provides a rationale for the lack of success in synthesizing the hexagonal polytype in either stoichiometric or n-type compositions.
Co-reporter:Joyce Pham, Andreas Kreyssig, Alan I. Goldman, and Gordon J. Miller
Inorganic Chemistry 2016 Volume 55(Issue 20) pp:10425-10437
Publication Date(Web):September 28, 2016
DOI:10.1021/acs.inorgchem.6b01636
A new icosahedral quasicrystalline phase, CaAu4.5–xAl1.5+x [0.11 ≤ x ≤ 0.40(6); CaAu4.4Al1.6, aQC = 5.383(4) Å, and Pm3̅ 5̅], and its lowest-order 1/0 cubic crystalline approximant phase, CaAu3+xAl1–x [0 ≤ x ≤ 0.31(1); a = 9.0766(5)–9.1261(8) Å, Pa3̅ (No. 205), and Pearson symbol cP40], have been discovered in the Ca-poor region of the Ca–Au–Al system. In the crystalline approximant, eight [Au3–xAl1+x] tetrahedra fill the unit cell, and each tetrahedron is surrounded by four Ca atoms, thus forming a three-dimensional network of {Ca4/4[Au3–xAl1+x]} tetrahedral stars. A computational study of Au and Al site preferences concurs with the experimental results, which indicate a preference for near-neighbor Au–Al interactions over Au–Au and Al–Al interactions. Analysis of the electronic density of states and the associated crystal orbital Hamilton population curves was used to rationalize the descriptions of CaAu4.5–xAl1.5+x [0.11 ≤ x ≤ 0.46(6)] and CaAu3+xAl1–x [0 ≤ x ≤ 0.31(1)] as polar intermetallic species, whereby Ca atoms engage in polar covalent bonding with the electronegative, electron-deficient [Au3–xAl1+x] tetrahedral clusters and the observed phase width of the crystalline approximant.
Co-reporter:Siméon Ponou, Sven Lidin, Daniel Grüner, and Gordon J. Miller
Crystal Growth & Design 2016 Volume 16(Issue 10) pp:5946
Publication Date(Web):August 30, 2016
DOI:10.1021/acs.cgd.6b01002
Combined experimental and theoretical efforts to conceptually understand the structure directing forces in intergrowth structures have led to the discovery of the new ternary phase Ca4Ag2+xGe4–x (x = 0.5), obtained from high-temperature reaction of the elements. It crystallizes in a new structure type according to single-crystal diffraction methods: monoclinic space group C2/m–i10 with a = 10.7516(2) Å, b = 4.5475(1) Å, c = 18.7773(4) Å, β = 93.69(2)°, V = 916.17(3) Å3, Z = 4. The compound corresponds to the n = 2 member of the homologous series Ca2+nAg2+xGe2+n–x, that are built up by linear intergrowths of slabs cut from the CaGe (CrB-type) and the CaAg1+xGe1–x (KHg2 or TiNiSi-type) structures, and may be partitioned in Ag-rich and Ag-free domains. Instead of the predicted Zr2CoSi2-type (C2/m–i5), a simultaneous doubling of the size of the two building blocks is observed with the dimerization of the (Ge2) pairs into Ag-substituted tetramers (AgxGe4–x) due to valence electron shortage. However, the Ag/Ge mixing at one atomic site with roughly one-to-one atomic ratio is therefore unexplained. The electronic band structure calculations and analysis of the chemical bonding provided evidence that the Ag/Ge mixing is rather the result of a direct conflict between the Zintl-Klemm concept and empirically established “structure-directing rules”. The implications of these findings for the poorly understood ordered staging structural interfaces, typically observed in secondary Li-ion batteries during charge/discharge process, are briefly discussed.
Co-reporter:Isa Doverbratt, Siméon Ponou, Yuemei Zhang, Sven Lidin, and Gordon J. Miller
Chemistry of Materials 2015 Volume 27(Issue 1) pp:304
Publication Date(Web):December 12, 2014
DOI:10.1021/cm503985h
A series of four new analogue phases Ca2M2X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Δχ) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two different but closely related structure types. The first compound, Ca2Pd2Ge (I), is an unprecedented ternary ordered variant of the Zr2Al3-type (orthorhombic, Fdd2). The three other phases, Ca2Pt2Ge (II), Ca2Pd2Al (III) and Ca2Pt2Al (IV), adopt the Gd2Ge2Al-type structure (monoclinic, C2/c). All title structures feature linear chains of the noble metals (Pd or Pt). The Pd linear chains in I are undistorted with equidistant Pd···Pd atoms, whereas the metal chains in II–IV are pairwise distorted, resulting in short connected {Pd2} or {Pt2} dumbbells that are separated by longer M···M contacts. The occurrence and magnitude of the pairing distortion in these chains are controlled by the vec and the Δχ between the constituent elements, a result which is supported by analysis of the calculated Bader effective charges. The metal chains act as charge modulation units, critical for the stability and the electronic flexibility of the structures by an adequate adjustment of the metal–metal bond order to both the vec and the degree of charge transfer. Thus, Ca2Pd2Ge (28 ve/f.u) is a Zintl-like, charge optimized phase with formally zerovalent Pd atoms forming the undistorted metal chains; semimetallic properties are predicted by TB-LMTO calculations. In contrast, the isoelectronic Ca2Pt2Ge is predicted to be a good metal with the Fermi level located at a local maximum of the DOS, a fingerprint of potential electronic instability. This is due to greater charge transfer to the more electronegative Pt atoms forming the metal chains and probably to packing frustration in the well packed structure that may prevent a larger distortion of the Pt chains. However, the instability is suppressed in the aliovalent but isostructural phases Ca2M2Al (27 ve/f.u) with an enhancement of the pairing distortion within the metal chains but lower M–M bond order. Further reduction of the vec as in Ca2M2Cd (26 ve/f.u) may induce a transition toward the more geometrically flexible W2CoB2-type with a low dimensional structure, to create more room for a larger distortion of the metal chain as dictated by the shortage of valence electrons.
Co-reporter:Volodymyr Smetana; Simon Steinberg; Nathan Card; Anja-Verena Mudring
Inorganic Chemistry 2015 Volume 54(Issue 3) pp:1010-1018
Publication Date(Web):December 10, 2014
DOI:10.1021/ic502402y
Five new polar intermetallic compounds in the Ae–Ga–Au system (Ae = Ba, Eu), BaAu5Ga2 (I), BaAu4.3Ga2.7 (II), Ba1.0Au4.5Ga2.4 (III), EuAu4.8Ga2.2 (IV), and Eu1.1Au4.4Ga2.2 (V), have been synthesized and their crystal structures determined by single-crystal X-ray diffraction. I crystallizes in the orthorhombic crystal system with a large unit cell [Pearson symbol oP64; Pnma, Z = 8, a = 8.8350(5) Å, b = 7.1888(3)Å, c = 20.3880(7) Å], whereas all other compounds are hexagonal [hP24; P6̅2m, Z = 3, a = 8.54–8.77(1) Å, c = 7.19–7.24(1) Å]. Both structures contain mutually orthogonal layers of Au6 hexagons in chair and boat conformations, resulting in a hexagonal diamond-like network. Ae atoms and additional (Au/Ga)3 groups are formally encapsulated by (Au6)2 distorted hexagonal prisms formed of three edge-sharing hexagons in the boat conformation or, alternatively, lie between two Au6 hexagons in the chair conformation. The (Au/Ga)3 groups can be substituted by Ae atoms in some of the hexagonal structures with no change to the structural symmetry. Tight-binding electronic structure calculations using linear-muffin-tin-orbital methods on idealized models “BaAu5Ga2” and “BaAu4Ga3” show both compounds to be metallic with evident pseudogaps near the corresponding Fermi levels. The integrated crystal orbital Hamilton populations are dominated by Au–Au and Au–Ga orbital interactions, although Ba–Au and Ba–Ga contributions are significant. Furthermore, Au–Au interactions vary considerably along different directions in the unit cells, with the largest values for the hexagons in the boat conformation and the lowest values for those in the chair conformation. II revealed that partial substitution of Au atoms in the hexagonal diamond net by a post-transition element (Ga) may occur in this family, whereas the sizes of the (Au/Ga)3 groups and strong Ba–Au covalent interactions allow for their mutual replacement in the voids.
Co-reporter:Yuemei Zhang
The Journal of Physical Chemistry C 2015 Volume 119(Issue 1) pp:580-589
Publication Date(Web):December 9, 2014
DOI:10.1021/jp5090185
The electronic and magnetic structures of the tetragonal and hexagonal MnFeAs were examined using density functional theory to understand the reported magnetic orderings and structural change induced by high-pressure synthesis. The reported magnetic ground states were confirmed using VASP total energy calculations. Effective exchange parameters for metal–metal contacts obtained from SPRKKR calculations indicate indirect exchange couplings are dominant in tetragonal MnFeAs. Weak direct exchange couplings for adjacent Fe–Fe and Fe–Mn contacts cause the coexistence of several low-energy magnetic structures in tetragonal MnFeAs and result in a near zero magnetic moment on the Fe atoms. On the other hand, the nearest-neighbor Fe–Fe and Fe–Mn interactions in hexagonal MnFeAs are a combination of direct and indirect exchange couplings. In addition, indirect exchange couplings in tetragonal MnFeAs are rationalized by both RKKY and superexchange mechanisms. Finally, to probe the high-pressure-induced phase transition, total energy changes with the change of volume was studied on both tetragonal and hexagonal MnFeAs.
Co-reporter:Andriy Palasyuk ; Yuri Grin
Journal of the American Chemical Society 2014 Volume 136(Issue 8) pp:3108-3117
Publication Date(Web):January 31, 2014
DOI:10.1021/ja411150e
A new homologous series of intermetallic compounds containing three-dimensional (3-d) tetrahedral frameworks of gold atoms, akin to hexagonal diamond, have been discovered in four related Sr–Au–Al systems: (I) hexagonal SrAl3–xAu4+x (0.06(1) ≤ x ≤ 0.46(1), P6̅2m, Z = 3, a = 8.633(1)–8.664(1) Å, c = 7.083(2)–7.107(1) Å); (II) orthorhombic SrAl2–yAu5+y (y ≤ 0.05(1); Pnma, Z = 4, a = 8.942(1) Å, b = 7.2320(4) Å, c = 9.918(1) Å); (III) Sr2Al2–zAu7+z (z = 0.32(2); C2/c, Z = 4, a = 14.956(4) Å, b = 8.564(2) Å, c = 8.682(1) Å, β = 123.86(1)°); and (IV) rhombohedral Sr2Al3–wAu6+w (w ≈ 0.18(1); R3̅c, Z = 6, a = 8.448(1) Å, c = 21.735(4) Å). These remarkable compounds were obtained by fusion of the pure elements and were characterized by X-ray diffraction and electronic structure calculations. Phase I shows a narrow phase width and adopts the Ba3Ag14.6Al6.4-type structure; phase IV is isostructural with Ba2Au6Zn3, whereas phases II and III represent new structure types. This novel series can be formulated as Srx[M3]1–xAu2, in which [M3] (= [Al3] or [Al2Au]) triangles replace some Sr atoms in the hexagonal prismatic-like cavities of the Au network. The [M3] triangles are either isolated or interconnected into zigzag chains or nets. According to tight-binding electronic structure calculations, the greatest overlap populations belong to the Al–Au bonds, whereas Au–Au interactions have a substantial nonbonding region surrounding the calculated Fermi levels. QTAIM analysis of the electron density reveals charge transfer from Sr to the Al–Au framework in all four systems. A study of chemical bonding by means of the electron-localizability indicator indicates two- and three-center interactions within the anionic Al–Au framework.
Co-reporter:Saroj L. Samal, Abhishek Pandey, David C. Johnston, John D. Corbett, and Gordon J. Miller
Chemistry of Materials 2014 Volume 26(Issue 10) pp:3209
Publication Date(Web):April 23, 2014
DOI:10.1021/cm500871c
Ternary R4Mn3–xAu10+x (R = Gd or Y; 0.2 ≤ x ≤ 1) compounds have been synthesized and characterized using single-crystal X-ray diffraction. The structure is a ternary variant of orthorhombic Zr7Ni10 (oC68, space group Cmca) and is isostructural with Ca4In3Au10. The structure contains layers of Mn-centered rectangular prisms of gold (Mn@Au8), interbonded via Au atoms in the b-c plane, and stacked in a hexagonal close packed arrangement along the a direction. These layers are bonded via additional Mn atoms along the a direction. The rare-earth metals formally act as cations and fill the rest of the space. The structure could also be described as sinusoidal layers of gold atoms, which are interconnected through Au–Au bonds. The magnetic characteristics of both compounds reveal the presence of nearly localized Mn magnetic moments. Magnetization M measurements of Y4Mn2.8Au10.2 versus temperature T and applied magnetic field H demonstrate the dominance of antiferromagnetic (AFM) interactions in this compound and indicate the occurrence of noncollinear AFM ordering at TN1 = 70 K and a spin reorientation transition at TN2 = 48 K. For the Gd analogue Gd4Mn2.8Au10.2, the M(H,T) data instead indicate the dominance of ferromagnetic interactions and suggest a ferrimagnetic transition at TC ≈ 70 K for which two potential ferrimagnetic structures are suggested. Linear muffin-tin orbital calculations on the stoichiometric composition “Y4Mn3Au10” using the local spin density approximation indicate a ∼1 eV splitting of the Mn 3d states with nearly filled majority spin states and partially filled minority spin states at the Fermi level resulting in approximately four unpaired electrons per Mn atom in the metallic ground state. The crystal orbital Hamilton population analyses demonstrate that ∼94% of the total Hamilton populations originate from Au–Au and polar Mn–Au and Y–Au bonding.
Co-reporter:Siméon Ponou, Sven Lidin, Yuemei Zhang, and Gordon J. Miller
Inorganic Chemistry 2014 Volume 53(Issue 9) pp:4724-4732
Publication Date(Web):April 18, 2014
DOI:10.1021/ic500449d
The quaternary phase Ca5Mg0.95Ag1.05(1)Ge5 (3) was synthesized by high-temperature solid-state techniques, and its crystal structure was determined by single-crystal diffraction methods in the orthorhombic space group Pnma – Wyckoff sequence c12 with a = 23.1481(4) Å, b = 4.4736(1) Å, c = 11.0128(2) Å, V = 1140.43(4) Å3, Z = 4. The crystal structure can be described as linear intergrowths of slabs cut from the CaGe (CrB-type) and the CaMGe (TiNiSi-type; M = Mg, Ag) structures. Hence, 3 is a hettotype of the hitherto missing n = 3 member of the structure series with the general formula R2+nT2X2+n, previously described with n = 1, 2, and 4. The member with n = 3 was predicted in the space group Cmcm – Wyckoff sequence f5c2. The experimental space group Pnma (in the nonstandard setting Pmcn) corresponds to a klassengleiche symmetry reduction of index two of the predicted space group Cmcm. This transition originates from the switching of one Ge and one Ag position in the TiNiSi-related slab, a process that triggers an uncoupling of each of the five 8f sites in Cmcm into two 4c sites in Pnma. The Mg/Ag site preference was investigated using VASP calculations and revealed a remarkable example of an intermetallic compound for which the electrostatic valency principle is a critical structure-directing force. The compound is deficient by one valence electron according to the Zintl concept, but LMTO electronic structure calculations indicate electronic stabilization and overall bonding optimization in the polyanionic network. Other stability factors beyond the Zintl concept that may account for the electronic stabilization are discussed.
Co-reporter:Yuemei Zhang, Jakoah Brgoch, and Gordon J. Miller
Inorganic Chemistry 2013 Volume 52(Issue 6) pp:3013-3021
Publication Date(Web):February 22, 2013
DOI:10.1021/ic3024716
The electronic and magnetic structures of the tetragonal Cu2Sb-type 3d metal arsenides (M2As, M = Cr, Mn, Fe) were examined using density functional theory to identify chemical influences on their respective patterns of magnetic order. Each compound adopts a different antiferromagnetic (AFM) ordering of local moments associated with the 3d metal sites, but every one involves a doubled crystallographic c-axis. These AFM ordering patterns are rationalized by the results of VASP calculations on several magnetically ordered models using a × a × 2c supercell. Effective exchange parameters obtained from SPRKKR calculations indicate that both direct and indirect exchange couplings play essential roles in understanding the different magnetic orderings observed. The nature of nearest-neighbor direct exchange couplings, that is, either ferromagnetic (FM) or AFM, were predicted by analysis of the corresponding crystal orbital Hamilton population (COHP) curves obtained by TB-LMTO calculations. Interestingly, the magnetic structures of Fe2As and Mn2As show tetragonal symmetry, but a magnetostrictive tetragonal-to-orthorhombic distortion could occur in Cr2As through AFM Cr1–Cr2 coupling between symmetry inequivalent Cr atoms along the a-axis, but FM coupling along the b-axis. A LSDA+U approach is required to achieve magnetic moment values for Mn2As in better agreement with experimental values, although computations always predict the moment at the M1 site to be lower than that at the M2 site. Finally, a rigid-band model applied to the calculated DOS curve of Mn2As correctly assesses the magnetic ordering patterns in Cr2As and Fe2As.
Co-reporter:Weiwei Xie, Srinivasa Thimmaiah, Jagat Lamsal, Jing Liu, Thomas W. Heitmann, Dante Quirinale, Alan I. Goldman, Vitalij Pecharsky, and Gordon J. Miller
Inorganic Chemistry 2013 Volume 52(Issue 16) pp:9399-9408
Publication Date(Web):August 2, 2013
DOI:10.1021/ic4009653
The results of crystallographic analysis, magnetic characterization, and theoretical assessment of β-Mn-type Co–Zn intermetallics prepared using high-temperature methods are presented. These β-Mn Co–Zn phases crystallize in the space group P4132 [Pearson symbol cP20; a = 6.3555(7)–6.3220(7)], and their stoichiometry may be expressed as Co8+xZn12–x [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co8+xZn12–x reveals that the β-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed β-Mn structure type, a result that offers greater insight into the β-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).
Co-reporter:Volodymyr Smetana ; Gordon J. Miller ;John D. Corbett
Inorganic Chemistry 2013 Volume 52(Issue 21) pp:12502-12510
Publication Date(Web):October 18, 2013
DOI:10.1021/ic401580y
A systematic exploration of Na- and Au-poor parts of the Na–Au–Ga system (less than 15 at. % Na or Au) uncovered several compounds with novel structural features that are unusual for the rest of the system. Four ternary compounds Na1.00(3)Au0.18Ga1.82(1) (I), NaAu2Ga4 (II), Na5Au10Ga16 (III), and NaAu4Ga2 (IV) have been synthesized and structurally characterized by single crystal X-ray diffraction: Na1.00(3)Au0.18Ga1.82(1)(I, P6/mmm, a = 15.181(2), c =9.129(2)Å, Z = 30); NaAu2Ga4 (II, Pnma, a = 16.733(3), b = 4.3330(9), c =7.358(3) Å, Z = 4); Na5Au10Ga16 (III, P63/m, a = 10.754(2), c =11.457(2) Å, Z = 2); and NaAu4Ga2 (IV, P21/c, a = 8.292(2), b = 7.361(1), c =9.220(2)Å, β = 116.15(3), Z = 4). Compound I lies between the large family of Bergman-related compounds and Na-poor Zintl-type compounds and exhibits a clathrate-like structure containing icosahedral clusters similar to those in cubic 1/1 approximants, as well as tunnels with highly disordered cation positions and fused Na-centered clusters. Structures II, III, and IV are built of polyanionic networks and clusters that generate novel tunnels in each that contain isolated, ordered Na atoms. Tight-binding electronic structure calculations using linear muffin-tin-orbital (LMTO) methods on II, III, IV and an idealized model of I show that all are metallic with evident pseudogaps at the Fermi levels. The integrated crystal orbital Hamilton populations for II–IV are typically dominated by Au–Ga, Ga–Ga, and Au–Au bonding, although Na–Au and Na–Ga contributions are also significant. Sodium’s involvement into such covalency is consistent with that recently reported in Na–Au–M (M = Ga, Ge, Sn, Zn, and Cd) phases.
Co-reporter:Jakoah Brgoch, Sheng Ran, Srinivasa Thimmaiah, Paul C. Canfield, Gordon J. Miller
Journal of Alloys and Compounds 2013 Volume 546() pp:300-306
Publication Date(Web):5 January 2013
DOI:10.1016/j.jallcom.2012.08.046
A new structure type has been discovered in the system Sc–Fe–Ge–Sn by employing Sn as a flux medium. According to single crystal X-ray diffraction, the new structure has a composition of Sc4Fe5Ge6.10(3)Sn1.47(2) and crystallizes in the space group Immm (No. 71, oI144) with lattice parameters of a = 5.230(1) Å, b = 13.467(3) Å, and c = 30.003(6) Å. The structure is composed of square anti-prismatic clusters that are condensed into zig-zag chains along the [0 1 0] direction. These chains are further condensed through a split Sn/Ge position, forming a three-dimensional network. Magnetization measurements indicate an antiferromagnetic phase transition near 240 K. Electronic structure calculations identified the most favorable bonding network in this new system. Using crystal orbital Hamilton population (COHP) curves and their integrated values (ICOHP), a polar intermetallic bonding network involving Sc–Ge as well as Fe–Sn and Fe–Ge contacts can be assigned to this new structure type.Graphical abstractHighlights► A new structure type with the composition Sc4Fe5Ge6.10(3)Sn1.47(2). ► Crystallizes in the space group Immm (No. 71, oI144). ► Sample obtained using a reactive Sn flux. ► Electronic structure calculations indicate polar intermetallic bonding network.
Co-reporter:Simon Steinberg, Jakoah Brgoch, Gordon J. Miller, and Gerd Meyer
Inorganic Chemistry 2012 Volume 51(Issue 21) pp:11356-11364
Publication Date(Web):October 5, 2012
DOI:10.1021/ic300838a
The structures of two new cubic {TnLa3}Br3 (Tn = Ru, Ir; I4132, Z = 8; Tn = Ru: a = 12.1247(16) Å, V = 1782.4(4) Å3; Tn = Ir: a = 12.1738(19) Å, V = 1804.2(5) Å3) compounds belonging to a family of reduced rare-earth metal halides were determined by single-crystal X-ray diffraction. Interestingly, the isoelectronic compound {RuLa3}I3 crystallizes in the monoclinic modification of the {TnR3}X3 family, while {IrLa3}I3 was found to be isomorphous with cubic {PtPr3}I3. Using electronic structure calculations, a pseudogap was identified at the Fermi level of {IrLa3}Br3 in the new cubic structure. Additionally, the structure attempts to optimize (chemical) bonding as determined through the crystal orbital Hamilton populations (COHP) curves. The Fermi level of the isostructural {RuLa3}Br3 falls below the pseudogap, yet the cubic structure is still formed. In this context, a close inspection of the distinct bond frequencies reveals the subtleness of the structure determining factors.
Co-reporter:Qisheng Lin, Volodymyr Smetana, Gordon J. Miller, and John D. Corbett
Inorganic Chemistry 2012 Volume 51(Issue 16) pp:8882-8889
Publication Date(Web):August 3, 2012
DOI:10.1021/ic300866q
Bergman-type phases in the Na–Au–T (T = Ga, Ge, and Sn) systems were synthesized by solid-state means and structurally characterized by single-crystal X-ray diffraction studies. Two structurally related (1/1) Bergman phases were found in the Na–Au–Ga system: (a) a conventional Bergman-type (CB) structure, Na26AuxGa54–x, which features empty innermost icosahedra, as refined with x = 18.1 (3), Im3̅, a = 14.512(2) Å, and Z = 2; (b) a stuffed Bergman-type (SB) structure, Na26AuyGa55–y, which contains Ga-centered innermost icosahedra, as refined with y = 36.0 (1), Im3̅, a = 14.597(2) Å, and Z = 2. Although these two subtypes have considerable phase widths along with respective tie lines at Na ≈ 32.5 and 32.1 atom %, they do not merge into a continuous solid solution. Rather, a quasicrystalline phase close to the Au-poor CB phase and an orthorhombic derivative near the Au-rich SB phase lie between them. In contrast, only Au-rich SB phases exist in the Ge and Sn systems, in which the innermost icosahedra are centered by Au rather than Ge or Sn. These were refined for Na26Au40.93(5)Ge14.07(5) (Im3̅, a = 14.581(2) Å, and Z = 2) and Na26Au39.83(6)Sn15.17(6) (Im3̅, a = 15.009(2) Å, and Z = 2), respectively. Occupations of the centers of Bergman clusters are rare. Such centering and coloring correlate with the sizes of the neighboring icosahedra, the size ratios between electropositive and electronegative components, and the values of the average valence electron count per atom (e/a). Theoretical calculations revealed that all of these phases are Hume–Rothery phases, with evident pseudogaps in the density of states curves that arise from the interactions between Fermi surface and Brillouin zone boundaries corresponding to a strong diffraction intensity.
Co-reporter:Volodymyr Smetana, John D. Corbett, and Gordon J. Miller
Inorganic Chemistry 2012 Volume 51(Issue 3) pp:1695-1702
Publication Date(Web):January 19, 2012
DOI:10.1021/ic201999u
The K–Au–Ga system has been investigated at 350 °C for <50 at. % K. The potassium gold gallides K0.55Au2Ga2, KAu3Ga2, KAu2Ga4 and the solid solution KAuxGa3–x (x = 0–0.33) were synthesized directly from the elements via typical high-temperature reactions, and their crystal structures were determined by single crystal X-ray diffraction: K0.55Au2Ga2 (I, I4/mcm, a = 8.860(3) Å, c = 4.834(2) Å, Z = 4), KAu3Ga2 (II, Cmcm, a = 11.078(2) Å, b = 8.486(2) Å, c = 5.569(1) Å, Z = 4), KAu2Ga4 (III, Immm, a = 4.4070(9) Å, b = 7.339(1) Å, c = 8.664(2) Å, Z = 2), KAu0.33Ga2.67 (IV, I-4m2, a = 6.0900(9) Å, c = 15.450(3) Å, Z = 6). The first two compounds contain different kinds of tunnels built of puckered six- (II) or eight-membered (I) ordered Au/Ga rings with completely different cation placements: uniaxial in I and III but in novel 2D-zigzag chains in II. III contains only infinite chains of a potassium-centered 20-vertex polyhedron (K@Au8Ga12) built of ordered 6–8–6 planar Au/Ga rings. The main structural feature of IV is dodecahedral (Au/Ga)8 clusters. Tight-binding electronic structure calculations by linear muffin-tin-orbital methods were performed for idealized models of I, II, and III to gain insights into their structure–bonding relationships. Density of states curves reveal metallic character for all compounds, and the overall crystal orbital Hamilton populations are dominated by polar covalent Au–Ga bonds. The relativistic effects of gold lead to formation of bonds of greater population with most post-transition elements or to itself, and these appear to be responsible for a variety of compounds, as in the K–Au–Ga system.
Co-reporter:Volodymyr Smetana, Gordon J. Miller, and John D. Corbett
Inorganic Chemistry 2012 Volume 51(Issue 14) pp:7711-7721
Publication Date(Web):June 27, 2012
DOI:10.1021/ic300740u
Six new intermetallic compounds have been characterized in the alkali metal (A = Na, Rb, Cs)–gold–gallium systems. Three isostructural compounds with the general composition A0.55Au2Ga2, two others of AAu3Ga2 (A = Rb, Cs), and the related Na13Au41.2Ga30.3 were synthesized via typical high-temperature reactions and their crystal structures determined by single-crystal X-ray diffraction analysis: Na0.56(9)Au2Ga2 (I, I4/mcm, a = 8.718(1) Å, c = 4.857(1) Å, Z = 4), Rb0.56(1)Au2Ga2 (II, I4/mcm, a = 8.950(1) Å, c = 4.829(1) Å, Z = 4), Cs0.54(2)Au2Ga2 (III, I4/mcm, a = 9.077(1) Å, c = 4.815(1) Å, Z = 4), RbAu3Ga2 (IV, Pnma, a = 13.384(3) Å, b = 5.577(1) Å, c = 7.017(1) Å, Z = 4), CsAu3Ga2 (V, Pnma, a = 13.511(3) Å, b = 5.614(2) Å, c = 7.146(1) Å, Z = 4), Na13Au41.2(1)Ga30.3(1) (VI, P6 mmm, a = 19.550(3) Å, c = 8.990(2) Å, Z = 2). The first three compounds (I–III) are isostructural with tetragonal K0.55Au2Ga2 and likewise contain planar eight-member Au/Ga rings that stack along c to generate tunnels and that contain varying degrees of disordered Na–Cs cations. The cation dispositions are much more clearly and reasonably defined by electron density mapping than through least-squares refinements with conventional anisotropic ellipsoids. Orthorhombic AAu3Ga2 (IV, V) are ordered ternary Rb and Cs derivatives of the SrZn5 type structure, demonstrating structural variability within the AAu3Ga2 family. All attempts to prepare an isotypic “NaAu3Ga2” were not successful, but yielded only a similar composition Na13Au41.2Ga30.3 (NaAu3.17Ga2.33) (VI) in a very different structure with two types of cation sites. Crystal orbital Hamilton population (COHP) analysis obtained from tight-binding electronic structure calculations for idealized I–IV via linear muffin-tin-orbital (LMTO) methods emphasized the major contributions of heteroatomic Au–Ga bonding to the structural stability of these compounds. The relative minima (pseudogaps) in the DOS curves for IV correspond well with the valence electron counts of known representatives of this structure type and, thereby, reveal some magic numbers to guide the search for new isotypic compounds. Theoretical calculation of total energies vs volumes obtained by VASP (Vienna Ab initio Simulation Package) calculations for KAu3Ga2 and RbAu3Ga2 suggest a possible transformation from SrZn5- to BaZn5-types at high pressure.
Co-reporter:Jakoah Brgoch, Yassir A. Mahmoud, Gordon J. Miller
Journal of Solid State Chemistry 2012 Volume 196() pp:168-174
Publication Date(Web):December 2012
DOI:10.1016/j.jssc.2012.06.010
The site preference for a class of intermetallic borides following the general formula M2Fe(Ru0.8T0.2)5B2 (M=Sc, Ti, Zr; T=Ru, Rh, Ir), has been explored using ab initio and semi-empirical electronic structure calculations. This intermetallic boride series contains two potential sites, the Wyckoff 2c and 8j sites, for Rh or Ir to replace Ru atoms. Since the 8j site is a nearest neighbor to the magnetically active Fe atom, whereas the 2c site is a next nearest neighbor, the substitution pattern should play an important role in the magnetic structure of these compounds. The substitution preference is analyzed based on the site energy and bond energy terms, both of which arise from a tight-binding evaluation of the electronic band energy, and are known to influence the locations of atoms in extended solids. According to these calculations, the valence electron-rich Rh and Ir atoms prefer to occupy the 8j site, a result also corroborated by experimental evidence. Additionally, substitution of Rh or Ir at the 8j site results in a modification of the magnetic structure that ultimately results in larger local magnetic moment on the Fe atoms.Graphical abstractThe site preference for electron rich atoms to occupy the 8j (gray) site is identified in these intermetallic borides, while the magnetic structure is modified as a function of the substituted atoms band center.Highlights► We identify the energetics dictating the site preference in a series of intermetallic borides. ► Establish substitution rules for use in future directed synthetic preparations. ► Identified changes in magnetic structure that accompany the site preference.
Co-reporter:Dr. Volodymyr Smetana;Dr. Qisheng Lin;Dr. Daniel K. Pratt;Dr. Andreas Kreyssig;Dr. Mehmet Ramazanoglu; John D. Corbett; Alan I. Goldman; Gordon J. Miller
Angewandte Chemie 2012 Volume 124( Issue 51) pp:
Publication Date(Web):
DOI:10.1002/ange.201209047
Co-reporter:Jakoah Brgoch and Gordon J. Miller
The Journal of Physical Chemistry A 2012 Volume 116(Issue 9) pp:2234-2243
Publication Date(Web):January 19, 2012
DOI:10.1021/jp206992z
A theoretical investigation of the relationship between chemical composition and electronic structure was performed on the nonstoichiometric iron sulfide, mackinawite (Fe1+xS), which is isostructural and isoelectronic with the superconducting Fe1+xSe and Fe1+x(Te1–ySey) phases. Even though Fe1+xS has not been measured for superconductivity, the effects of stoichiometry on transport properties and electronic structure in all of these iron-excess chalcogenide compounds has been largely overlooked. In mackinawite, the amount of Fe that has been reported ranges from a large excess, Fe1.15S, to nearly stoichiometric, Fe1.00(7)S. Here, we analyze, for the first time, the electronic structure of Fe1+xS to justify these nonstoichiometric phases. First principles electronic structure calculations using supercells of Fe1+xS yield a wide range of energetically favorable compositions (0 < x < 0.30). The incorporation of interstitial Fe atoms originates from a delicate balance between the Madelung energy and the occupation of Fe–S and Fe–Fe antibonding orbitals. A theoretical assessment of various magnetic structures for “FeS” and Fe1.06S indicate that striped magnetic ordering along [110] is the lowest energy structure and the interstitial Fe affects the values of moments in the square planes as a function of distance. Moreover, the formation of the magnetic moment is dependent on the unit cell volume, thus relating it to composition. Finally, changes in the composition cause a modification of the Fermi surface and ultimately the loss of a nested vector.
Co-reporter:Dr. Volodymyr Smetana;Dr. Qisheng Lin;Dr. Daniel K. Pratt;Dr. Andreas Kreyssig;Dr. Mehmet Ramazanoglu; John D. Corbett; Alan I. Goldman; Gordon J. Miller
Angewandte Chemie International Edition 2012 Volume 51( Issue 51) pp:12699-12702
Publication Date(Web):
DOI:10.1002/anie.201207076
Co-reporter:Dr. Volodymyr Smetana;Dr. Qisheng Lin;Dr. Daniel K. Pratt;Dr. Andreas Kreyssig;Dr. Mehmet Ramazanoglu; John D. Corbett; Alan I. Goldman; Gordon J. Miller
Angewandte Chemie International Edition 2012 Volume 51( Issue 51) pp:
Publication Date(Web):
DOI:10.1002/anie.201209047
Co-reporter:Dr. Volodymyr Smetana;Dr. Qisheng Lin;Dr. Daniel K. Pratt;Dr. Andreas Kreyssig;Dr. Mehmet Ramazanoglu; John D. Corbett; Alan I. Goldman; Gordon J. Miller
Angewandte Chemie 2012 Volume 124( Issue 51) pp:12871-12874
Publication Date(Web):
DOI:10.1002/ange.201207076
Co-reporter:Jakoah Brgoch ; Christian Goerens ; Boniface P. T. Fokwa
Journal of the American Chemical Society 2011 Volume 133(Issue 17) pp:6832-6840
Publication Date(Web):April 7, 2011
DOI:10.1021/ja200909r
The electronic structures of “Ti9-nFe2+nRu18B8” (n = 0, 0.5, 1, 2, 3), in connection to the recently synthesized Ti9-nFe2+nRu18B8 (n = 1, 2), have been investigated and analyzed using LSDA tight-binding calculations to elucidate the distribution of Fe and Ti, to determine the maximum Fe content, and to explore possible magnetic structures to interpret experimental magnetization results. Through a combination of calculations on specific models and using the rigid band approximation, which is validated by the DOS curves for “Ti9-nFe2+nRu18B8” (n = 0, 0.5, 1, 2, 3), mixing of Fe and Ti is anticipated at both the 2b- and 4h-chain sites. The model “Ti8.5Fe2.5Ru18B8” (n = 0.5) revealed that both Brewer-type Ti−Ru interactions as well as ligand field splitting of the Fe 3d orbitals regulated the observed valence electron counts between 220 and 228 electrons/formula unit. Finally, models of magnetic structures were created using “Ti6Fe5Ru18B8” (n = 3). A rigid band analysis of the LSDA DOS curves concluded preferred ferromagnetic ordering at low Fe content (n ≤ 0.75) and ferrimagnetic ordering at higher Fe content (n > 0.75). Ferrimagnetism arises from antiferromagnetic exchange coupling in the scaffold of Fe1-ladder and 4h-chain sites.
Co-reporter:Fei Wang and Gordon J. Miller
Inorganic Chemistry 2011 Volume 50(Issue 16) pp:7625-7636
Publication Date(Web):July 20, 2011
DOI:10.1021/ic200643f
To enhance understanding of the Zintl–Klemm concept, which is useful for characterizing chemical bonding in semimetallic and semiconducting valence compounds, and to more effectively rationalize the structures of Zintl phases, we present a partitioning scheme of the total energy calculated on numerous possible structures of the alkali metal trielides, LiAl, LiTl, NaTl, and KTl, using first-principles quantum mechanical calculations. This assessment of the total energy considers the relative effects of covalent, ionic, and metallic interactions, all of which are important to understand the complete structural behavior of Zintl phases. In particular, valence electron transfer and anisotropic covalent interactions, explicitly employed by the Zintl–Klemm concept, are often in competition with isotropic, volume-dependent metallic and ionic interaction terms. Furthermore, factors including relativistic effects, electronegativity differences, and atomic size ratios between the alkali metal and triel atoms can affect the competition by enhancing or weakening one of the three energetic contributors and thus cause structural variations. This partitioning of the total energy, coupled with analysis of the electronic density of states curves, correctly predicts and rationalizes the structures of LiAl, LiTl, NaTl, and KTl, as well as identifies a pressure-induced phase transition in KTl from its structure, based on [Tl6]6– distorted octahedra, to the double diamond NaTl-type.
Co-reporter:Hui Wang, Fei Wang, Karah Jones, and Gordon J. Miller
Inorganic Chemistry 2011 Volume 50(Issue 24) pp:12714-12723
Publication Date(Web):November 22, 2011
DOI:10.1021/ic201840q
A crystallographic study and theoretical analysis of the structural and La/Y site preferences in the La5–xYxSi4 (0 ≤ x ≤ 5) series prepared by high-temperature methods is presented. At room temperature, La-rich La5–xYxSi4 phases with x ≤ 3.0 exhibit the tetragonal Zr5Si4-type structure (space group P41212, Z = 4, Pearson symbol tP36), which contains only Si–Si dimers. On the other hand, Y-rich phases with x = 4.0 and 4.5 adopt the orthorhombic Gd5Si4-type structure (space group Pnma, Z = 4, Pearson symbol oP36), also with Si–Si dimers, whereas Y5Si4 forms the monoclinic Gd5Si2Ge2 structure (space group P21/c, Z = 4, Pearson symbol mP36), which exhibits 50% “broken” Si–Si dimers. Local and long-range structural relationships among the tetragonal, orthorhombic, and monoclinic structures are discussed. Refinements from single crystal X-ray diffraction studies of the three independent sites for La or Y atoms in the asymmetric unit reveal partial mixing of these elements, with clearly different preferences for these two elements. First-principles electronic structure calculations, used to investigate the La/Y site preferences and structural trends in the La5–xYxSi4 series, indicate that long- and short-range structural features are controlled largely by atomic sizes. La 5d and Y 4d orbitals, however, generate distinct, yet subtle effects on the electronic density of states curves, and influence characteristics of Si–Si bonding in these phases.
Co-reporter:Fei Wang
European Journal of Inorganic Chemistry 2011 Volume 2011( Issue 26) pp:3989-3998
Publication Date(Web):
DOI:10.1002/ejic.201100312
Abstract
Alkali metal gold bismuthides, A2AuBi, are isoelectronic with alkali metal thallides, ATl = A2TlTl, and yet Na2AuBi adopts an orthorhombic structure with a 1-D zigzag “ribbon” structural motif rather than the cubic double diamond structure type of NaTl as well as Li2AuBi. Using first principles quantum mechanical calculations applied to A2AuBi, hypothetical “A2HgPb,” and A2TlTl, and comprehensively decomposing the total energies into metallicity, ionicity, and covalency components to establish parallels with the qualitative Zintl–Klemm formalism, the factors determining the relative stability between the zigzag “ribbon” and the diamond network are examined. An interplay between volume-dependent energy terms, i.e., metallicity or ionicity, and covalency among the electronegative components determines which structural motif is favored. In Na2AuBi, there are two factors stabilizing the zigzag “ribbon.” Au 5d states significantly interact with Bi 6p states, especially Au 5d with Bi 6pz to promote stronger Au–Bi covalent interactions than in the diamond network. This factor does not exist in Na2TlTl and “A2HgPb,” where Hg, Tl, and Pb 5d states are well localized. Secondly, the zigzag ribbons provide effective covalent interactions at larger volumes, as in Na2AuBi, while effective covalent interactions occur in the diamond network only at smaller volume, as in Li2AuBi.
Co-reporter:Arthur Mar;Julia Y. Chan;Raymond E. Schaak;Myung-Hwan Whangbo;Mercouri G. Kanatzidis;Michael Shatruk
European Journal of Inorganic Chemistry 2011 Volume 2011( Issue 26) pp:
Publication Date(Web):
DOI:10.1002/ejic.201190075
Abstract
The front cover picture shows the clock tower, the “Campanile”, of Iowa State University where John Corbett did the ground-breaking research in polar intermetallics that forms the basis of his Viewpoint in this cluster issue. Superimposed on this background are structures and data to visualize the broad scope of topic. The complexity of structure is displayed by the ternary rare-earth cobalt gallides that contain interstitial atoms (top left, A. Mar et al.), a calcium-poor intermetallic phase of the Ca/Ni/Ge system (top right from the lab of T. Fässler), and a single crystal of a polymorph of thallium nickel gallide (bottom right, J. Chan et al.). The potentially general synthesis of colloidal nanoparticles – Au3Li from the lab of R. E. Schaak – is outlined mid left. The groups of M. H. Whangbo and G. Miller devote their contributions to the theoretical aspects of bonding (depicted top centre, the plots showing Au–Au bonding and antibonding interactions in Dy2Au2In and mid right, the effects of ionic interactions on the structural properties of isoelectronic intermetallic compounds, respectively). Representative of the range of properties discussed is the magnetic susceptibility of Yb5Ni4Ge10 (bottom left, M. G. Kanatzidis et al.). We thank the authors for the use of the graphics from their papers on the cover.
Co-reporter:Fei Wang, Karen N. Pearson, Warren E. Straszheim and Gordon J. Miller
Chemistry of Materials 2010 Volume 22(Issue 5) pp:1798
Publication Date(Web):January 7, 2010
DOI:10.1021/cm903300y
EuAgxAl11−x phases adopting the BaHg11-type structure (space group Pm3̅m, Z = 3) were synthesized with high yield by arc melting a mixture loaded as “EuAg3.5Al7.5” and annealing at 500 °C for 40 days. This phase has a very narrow phase width around EuAg4.0Al7.0; and it is unstable at 600 and 700 °C, at which it transforms into other phases. Magnetometry indicates that Eu is divalent, which gives the valence electron concentration per Ag/Al atom as 2.45 e−/atom, higher than in the BaCd11-type phases in the Eu−Ag−Al system (2.10−2.30 e−/atom). First principles electronic structure calculations, using a computational model structure built by simulating the crystallographic results as well as maximizing the number of heteroatomic (Ag−Al) contacts, can explain why the cubic BaHg11-type structure is favored at higher valence electron concentration than the tetragonal BaCd11-type structure.
Co-reporter:Hui Wang ; Sumohan Misra ; Fei Wang
Inorganic Chemistry 2010 Volume 49(Issue 10) pp:4586-4593
Publication Date(Web):April 16, 2010
DOI:10.1021/ic100142u
A crystallographic study and theoretical analysis of the Si/Ga site preferences in the Gd5GaxSi4−x series is presented. Gd5GaxSi4−x adopt the orthorhombic Gd5Si4-type structure (space group Pnma, Z = 4) with a maximum Ga content near x = 1.00, as determined by single crystal and powder X-ray diffraction. Refinements from single crystal X-ray diffraction studies of the three independent sites for Si/Ga atoms in the asymmetric unit (interslab T1, intraslab T2 and T3) reveal partial mixing of these elements, with a clear preference for Ga substitution at the interslab T1 sites. To investigate site preferences of Si/Ga atoms, first-principles electronic structure calculations were carried out using the Vienna ab initio simulation package (VASP) and the Stuttgart tight-binding, linear-muffin-tin orbital program with the atomic sphere approximation (TB-LMTO-ASA). Analysis of various crystal orbital Hamilton population (COHP) curves provide some further insights into the structural tendencies and indicate the roles of both sizes and electronegativities of Ga and Si toward influencing the observed upper limit in Ga content in Gd5GaxSi4−x. The magnetic properties of two Gd5GaxSi4−x phases are also reported: both show ferromagnetic behavior with Curie temperatures lower than that for Gd5Si4.
Co-reporter:Hyunjin Ko, Olivier Gourdon, Delphine Gout, Eun-Deok Mun, Srinivasa Thimmaiah, and Gordon J. Miller
Inorganic Chemistry 2010 Volume 49(Issue 24) pp:11505-11515
Publication Date(Web):November 15, 2010
DOI:10.1021/ic101671k
A series of rhombohedrally distorted γ-brass structures containing a mixture of magnetically active 3d elements, Cr and Fe, Cr1−xFexGa, is investigated crystallographically. These structures consist of chains of trans-face-sharing Ga-centered transition metal icosahedra. Neutron powder diffraction specifically on Cr0.5Fe0.5Ga, which could be prepared as a single phase material, gave lattice constants (11 K) a = 12.5172(2) Å and c = 7.8325(2) Å and a refined composition of Cr0.502(6)Fe0.498Ga = Cr6.523Fe6.477Ga13 and revealed partial ordering of Cr and Fe atoms among three crystallographic sites. Magnetic susceptibility and magnetization studies of Cr0.5Fe0.5Ga showed the onset of magnetic ordering at ca. 25 K. Theoretical calculations suggested both site-energy and bond-energy factors influencing the Cr/Fe distribution. Heteroatomic interactions significantly affect exchange interactions and create low local magnetic moments. Models created to mimic Cr0.5Fe0.5Ga showed ferromagnetic Fe−Fe and antiferromagnetic Cr−Fe interactions, with an overall ferrimagnetic ordering.
Co-reporter:Jakoah Brgoch, Steven Yeninas, Ruslan Prozorov, Gordon J. Miller
Journal of Solid State Chemistry 2010 Volume 183(Issue 12) pp:2917-2924
Publication Date(Web):December 2010
DOI:10.1016/j.jssc.2010.09.025
Polycrystalline samples of two complex intermetallic borides Zr2Fe1−δRu5+δB2 and Zr2Fe1−δ(Ru1−xRhx)5+δB2 (δ=ca. 0.10; x=0.20) were synthesized by high-temperature methods and characterized by single-crystal X-ray diffraction, energy dispersive spectroscopy, and magnetization measurements. Both structures are variants of Sc2Fe(Ru1−xRhx)5B2 and crystallize in the space group P4/mbm (no. 127) with the Ti3Co5B2-type structure. These structures contain single-atom, Fe-rich Fe/Ru or Fe/Ru/Rh chains along the c-axis with an interatomic metal-metal distance of 3.078(1) Å, a feature which makes them viable for possible low-dimensional temperature-dependent magnetic behavior. Magnetization measurements indicated weak ferrimagnetic ordering with ordering temperatures ca. 230 K for both specimens. Tight-binding electronic structure calculations on a model “Zr2FeRu5B2” using LDA yielded a narrow peak at the Fermi level assigned to Fe–Fe antibonding interactions along the c-axis, a result that indicates an electronic instability toward ferromagnetic coupling along these chains. Spin-polarized calculations of various magnetic models were examined to identify possible magnetic ordering within and between the single-atom, Fe-rich chains.Zr2FeRu5−xRhxB2 (x=0, 1) crystallizes with magnetic atoms forming chains which have been shown to order magnetically depending on the total valence electron count. Magnetic measurements and tight-binding electronic structure calculations are employed to investigate the ordering.
Co-reporter:Srinivasa Thimmaiah Dr. ;GordonJ. Miller
Chemistry - A European Journal 2010 Volume 16( Issue 18) pp:5461-5471
Publication Date(Web):
DOI:10.1002/chem.200903300
Abstract
Novel ternary phases, (Pd1−xZnx)18(Zn1−yAly)86−δ (0≤x≤0.162, 0.056≤y≤0.088, 0≤δ≤4), which adopt a superstructure of the γ-brass type (called γ′-brass), have been synthesized from the elements at 1120 K. Single-crystal X-ray structural analysis reveals a phase width (F3m, a=18.0700(3)–18.1600(2) Å, Pearson symbols cF400–cF416), which is associated with structural disorder based on both vacancies as well as mixed site occupancies. These structures are constructed of four independent 26-atom γ-clusters per primitive unit cells and centered at the four special positions A (0, 0, 0), B (1/4, 1/4, 1/4), C (1/2, 1/2, 1/2) and D (3/4, 3/4, 3/4). Two of these, centered at B and C, are completely ordered Pd4Zn22 clusters, whereas the other two, centered at A and D, contain all structural disorder in the system. According to our single-crystal X-ray results, Al substitutions are restricted to the A- and D-centered clusters. Moreover, the outer tetrahedron (OT) site of the 26-atom cluster at D is completely vacant at the Al-rich boundary of these phases. Electronic structure calculations, using the tight-binding linear muffin-tin orbital atomic-spheres approximation (TB-LMTO-ASA) method, on models of these new, ternary γ′-brass phases indicate that the observed chemical compositions and atomic distributions lead to the presence of a pseudogap at the Fermi level in the electronic density of states curves, which is consistent with the Hume-Rothery interpretation of γ-brasses, in general.
Co-reporter:Fei Wang, Karen Nordell Pearson and Gordon J. Miller
Chemistry of Materials 2009 Volume 21(Issue 2) pp:230
Publication Date(Web):January 5, 2009
DOI:10.1021/cm803021u
The EuAgxAl11−x (loading composition, x ≈ 3−8) ternary system was experimentally and theoretically investigated. According to powder X-ray diffraction, phases adopting the BaCd11-type structure (space group I41/amd, Z = 4) were obtained for a narrow composition range, i.e., x ≈ 5−6. Single-crystal X-ray crystallography showed that Ag and Al atoms share 4b, 8d, and 32i sites and that 4b site distinctly prefers Ag to Al. Eu is divalent in these phases, which was supported by both magnetometry and unit-cell dimensional analysis. Comparison with other isostructural RE (rare earth)−Ag−Al compounds showed that the BaCd11-type structure is stable specifically at the valence electron concentrations (vec) of 2.1−2.3 e− per atom. A Mulliken population analysis was performed with Extended Hückel calculations, the result of which explained the observed site preferences of the Ag and Al atoms. TB-LMTO-ASA calculations were used to study the relative energies of various models established according to crystallography and the coloring problem was included by maximizing the number of Ag−Al contacts. The calculated density of states (DOS) and crystal orbital Hamiltonian population (COHP) curves explain the stability of the BaCd11-type structure at specifically vec ≈ 2.1−2.3 e− per atom in RE−Ag−Al ternary compounds.
Co-reporter:Tae-Soo You and Gordon J. Miller
Inorganic Chemistry 2009 Volume 48(Issue 14) pp:6391-6401
Publication Date(Web):June 16, 2009
DOI:10.1021/ic900629t
The electronic structures of EuZn2, Eu(Zn0.75Ge0.25)2, Eu(Zn0.5Ge0.5)2, Eu(Zn0.25Ge0.75)2, and EuGe2 have been investigated using tight-binding, linear muffin-tin orbital (TB-LMTO) and pseudopotential methods to understand the structural preferences influenced by valence electron counts and to explain the observed homogeneity range of the AlB2-type phases as reported in the companion article. A crystal orbital Hamilton population (COHP) analysis for Zn−Zn contacts in EuZn2 suggests a possible homogeneity width for the KHg2-type phase, which is indicated from analysis of X-ray powder diffraction patterns. Total electronic energy comparisons, as well as density of states (DOS) and COHP analysis for a hypothetical Zn-rich compound, Eu(Zn0.75Ge0.25)2, indicate that two distinct phases, KHg2-type EuZn2 and AlB2-type Eu(Zn1−xGex)2 (0.5 ≤ x ≤ 0.70), are more favorable than a single Zn-rich composition adopting the AlB2-type phase. Among 10 structural models of Eu(Zn0.5Ge0.5)2, the one with heteroatomic Zn−Ge interactions both within and perpendicular to the 63 nets is energetically the most favorable structure. The experimentally observed Zn−Ge bond distance is attributed to the contribution of both σ- and π-bond interactions. Zn−Ge, Eu−Zn, and Eu−Ge COHP curves of the minimum energy form of Eu(Zn0.5Ge0.5)2 show bonding character above the Fermi level and explain the observed wide homogeneity width of the AlB2-type phase. In the Ge-rich case, Eu(Zn0.25Ge0.75)2, the planar hexagonal nets are not energetically favorable due to the significant antibonding character of Ge−Ge bonding at the Fermi level. Structural relaxation using pseudopotentials also revealed that the hexagonal nets tend to pucker rather than being planar, in agreement with the observed incommensurately modulated superstructure. An electron localization function analysis for Eu(Zn0.5Ge0.5)2 reveals that there exists no two-center, two-electron bond or multicentered interactions between interlayer Zn···Ge contacts.
Co-reporter:Tae-Soo You, Sven Lidin, Olivier Gourdon, Yaqiao Wu and Gordon J. Miller
Inorganic Chemistry 2009 Volume 48(Issue 14) pp:6380-6390
Publication Date(Web):June 16, 2009
DOI:10.1021/ic900628k
The series of ternary polar intermetallics Eu(Zn1−xGex)2 (0 ≤ x ≤ 1) has been investigated and characterized by powder and single-crystal X-ray diffraction as well as physical property measurements. For 0.50(2) ≤ x < 0.75(2), this series shows a homogeneity width of hexagonal AlB2-type phases (space group P6/mmm, Pearson symbol hP3) with Zn and Ge atoms statistically distributed in the planar polyanionic 63 nets. As the Ge content increases in this range, a decreases from 4.3631(6) Å to 4.2358(6) Å, while c increases from 4.3014(9) Å to 4.5759(9) Å, resulting in an increasing c/a ratio. Furthermore, the Zn−Ge bond distance in the hexagonal net drops from 2.5190(3) Å to 2.4455(3) Å, while the anisotropy of the displacement ellipsoids significantly increases along the c direction. For x < 0.50 and x > 0.75, respectively, orthorhombic KHg2-type and trigonal EuGe2-type phases occur as a second phase in mixtures with an AlB2-type phase. Diffraction of the x = 0.75(2) sample shows incommensurate modulation along the c direction; a structural model in super space group P3m̅1(00γ)00s reveals puckered 63 nets. Temperature-dependent magnetic susceptibility measurements for two AlB2-type compounds show Curie−Weiss behavior above 40.0(2) K and 45.5(2) K with magnetic moments of 7.98(1) μB for Eu(Zn0.48Ge0.52(2))2 and 7.96(1) μB for Eu(Zn0.30Ge0.70(2))2, respectively, indicating a (4f)7 electronic configuration for Eu atoms (Eu2+). The Zintl−Klemm formalism accounts for the lower limit of Ge content in the AlB2-type phases but does not identify the observed upper limit. In a companion paper, the intrinsic relationships among chemical structures, compositions, and electronic structures are analyzed by electronic structure calculations.
Co-reporter:Sumohan Misra, Yurij Mozharivskyj, Alexandra O. Tsokol, Deborah L. Schlagel, Thomas A. Lograsso, Gordon J. Miller
Journal of Solid State Chemistry 2009 Volume 182(Issue 11) pp:3031-3040
Publication Date(Web):November 2009
DOI:10.1016/j.jssc.2009.08.016
Temperature-dependent, single crystal and powder X-ray diffraction studies as well as magnetization, and heat capacity measurements were carried out on two phases of the Gd5GaxGe4−x system: for x=0.7 and 1.0. Gd5Ga0.7Ge3.3 shows three structure types as a function of temperature: (i) from 165 K to room temperature, the orthorhombic Sm5Ge4-type structure exists; (ii) below 150 K, it transforms to a orthorhombic Gd5Si4-type structure; and (iii) a monoclinic Gd5Si2Ge2-type component is observed for the intermediate temperature range of 150 K≤T≤165 K. This is the first time that all these three structure types have been observed for the same composition. For Gd5Ga1.0Ge3.0, the room temperature phase belongs to the orthorhombic Pu5Rh4-type structure with interslab contacts between main group atoms of 2.837(4) Å. Upon heating above 523 K, it transforms to a Gd5Si4-type structure with this distance decreasing to 2.521(7) Å before decomposing above 573 K.Phase transformations in Gd5GaxGe4−x magnetocaloric materials as a function of temperature.
Co-reporter:Tae-Soo You, Jing-Tai Zhao, Rainer Pöttgen, Walter Schnelle, Ulrich Burkhardt, Yuri Grin, Gordon J. Miller
Journal of Solid State Chemistry 2009 Volume 182(Issue 9) pp:2430-2442
Publication Date(Web):September 2009
DOI:10.1016/j.jssc.2009.06.032
Two isoelectronic series, Eu(Ga1−xTtx)2 (Tt=Si, Ge, 0≤x≤1), have been synthesized and characterized by powder and single-crystal X-ray diffraction, physical property measurements, and electronic structure calculations. In Eu(Ga1−xSix)2, crystal structures vary from the KHg2-type to the AlB2-type, and, finally, the ThSi2-type structure as x increases. The hexagonal AlB2-type structure is identified for compositions 0.18(2)≤x<0.70(2) with Ga and Si atoms statistically distributed in the polyanionic 63 nets. As smaller Si atoms replace Ga atoms while the number of valence electrons increases, the lattice parameters, unit cell volumes, and Ga–Si distances in this phase region decrease significantly. Although aspects of X-ray diffraction results suggest puckering of the 63 nets for the Si-richest example of the AlB2-type Eu(Ga1−xSix)2, the complete experimental evidence remains inconclusive. On the other hand, in Eu(Ga1−xGex)2, six different structural types were observed as x varies. In addition to EuGa2 (KHg2-type; space group Imma) and EuGe2 (own structure type, space group P3¯m1), the ternary phases studied show four different structures: the AlB2-type for Ga-rich compositions; the YPtAs-type structure for EuGaGe; and two new structures, which are intergrowths of the YPtAs-type EuGaGe and EuGe2, for Ge-rich compositions. These two Ge-rich phases include: (1) Eu(Ga0.45(2)Ge0.55(2))2 containing two YPtAs-type motifs of EuGaGe plus one EuGe2 motif; and (2) Eu(Ga0.40(2)Ge0.60(2))2 containing one YPtAs-type motif alternating with a split site at x=23,y=13 and z=0.4798(2) with ca. 50% site occupancy by Ga and Ge along the c-axis. Magnetic susceptibilities of three Eu(Ga1−xGex)2 compounds display Curie–Weiss behavior above ca. 100 K, and show effective magnetic moments indicative of divalent Eu with a 4f7 electronic configuration, consistent with. X-ray absorption spectra (XAS). Density of states (DOS) and crystal orbital Hamilton population (COHP) analyses, based on first principles electronic structure calculations, rationalize the observed homogeneity ranges of the AlB2-type phases in both systems and the structural variations as a function of Tt content.A study of Eu(Ga1−xSix)2 and Eu(Ga1−xGex)2 shows different compositional ranges for puckering of 63 nets and, for the germanides, two new commensurately modulated superstructures.
Co-reporter:Tae-Soo You, Mi-Kyung Han, Gordon J. Miller
Inorganica Chimica Acta 2008 Volume 361(Issue 11) pp:3053-3062
Publication Date(Web):27 July 2008
DOI:10.1016/j.ica.2008.02.022
During exploration in the Y–Mg–Zn system for quasicrystal approximants, three new phases, YMg1−xZn1+x (0 ⩽ x ⩽ 0.17) adopting the hexagonal ZrNiAl structure type, have been discovered. In these structures, the elements are completely ordered to minimize both the site energies and the bond energies as calculated by tight-binding calculations. Evaluation of the electron density in YMgZn suggests that Mg–Zn and Y–Zn bonding coupled with maximizing the Zn⋯Zn separations is the main factor influencing the atomic arrangements. Analysis of the electronic density of states of YMgZn indicates an optimized bonding situation for eight valence electrons per formula unit, e.g., as in YMgGa. Subsequently, YMgAl, YMgGa, and YMgIn were successfully prepared and structurally characterized. Their structures show relationships to both densely-packed structures common for intermetallics as well as three-dimensional networks common for valence compounds.The structures of three new intermetallic phases, YMg1−xZn1+x (x ⩽ 0.17(3)), as well as YMgAl, YMgGa, and YMgIn, have been characterized by diffraction and electronic structure theory. These hexagonal structures display features bordering densely-packed metals and covalently-bonded networks.
Co-reporter:Sumohan Misra, Gordon J. Miller
Journal of Solid State Chemistry 2006 Volume 179(Issue 8) pp:2290-2297
Publication Date(Web):August 2006
DOI:10.1016/j.jssc.2006.03.001
A crystallographic study of the Si/Ge site preferences in the Si-rich regime of Gd5(SixGe1−x)4 and a crystal chemical analysis of these site preferences for the entire range is presented. The room temperature crystal structure of Gd5Si4 as well as four pseudobinary phases, Gd5(SixGe1−x)4 for x⩾0.6x⩾0.6, is reported. All structures are orthorhombic (space group Pnma), Gd5Si4-type and show decreasing volume as the Si concentration increases. Refinements of the site occupancies for the three crystallographic sites for Si/Ge atoms in the asymmetric unit reveal a nonrandom, but still incompletely ordered arrangement of Si and Ge atoms. The distribution of Si and Ge atoms at each site impacts the fractions of possible homonuclear and heteronuclear Si–Si, Si–Ge and Ge–Ge dimers in the various structures. This distribution correlates with the observed room temperature crystal structures for the entire series of Gd5(SixGe1−x)4.The Ge occupation in each T site in Gd5(SixGe1−x)4 is studied as a function of Si concentration, x. The different crystal structures are related to the fractions of Ge–Ge (solid), Si–Ge (dashed) and Si–Si (solid) dimers at the T1–T1 sites.
Co-reporter:Olivier Gourdon, Gordon J. Miller
Journal of Solid State Chemistry 2003 Volume 173(Issue 1) pp:137-147
Publication Date(Web):June 2003
DOI:10.1016/S0022-4596(02)00031-2
The crystal structure of the binary gallide compound GaMn is reinvestigated using X-ray diffraction. The structure is quite different from that proposed previously. Although GaMn is reported to crystallize with the Al8Cr5 structure type, space group R3m, we found that the centrosymmetric space group , with a=12.605(2) Å and c=8.0424(11) Å, was more accurate. Moreover, the atomic positions and the atomic displacement parameters, which are missing in the previous study, are now refined. Thereafter, band structure calculations have been performed using the TB-LMTO-ASA method to understand the electronic and magnetic properties of this compound. Analyses from the band structure, the density of states and the magnetic moments obtained using spin-polarized calculations show the stability of two different magnetic models relative to the nonmagnetic one.
Co-reporter:Delphine Gout, Evan Benbow, Olivier Gourdon, Gordon J. Miller
Journal of Solid State Chemistry 2003 Volume 174(Issue 2) pp:471-481
Publication Date(Web):September 2003
DOI:10.1016/S0022-4596(02)00167-6
The Al-rich portion of the ternary Ce–Ni–Al has been investigated and a new ternary phase of composition Ce4Ni6Al23 has been found. This compound crystallizes in the monoclinic space group C2/m with the cell parameters a=16.042(8), b=4.140(4), c=18.380(8) Å and β=113.24(5)°. The structure has been determined by single crystal X-ray diffraction. The local environment of Ni and Ce is close to what is observed in the CeNi2Al5 and CeNiAl4 structures. Band structure calculations, using the tight-binding–linear muffin-tin orbital–atomic-spheres approximation (TB-LMTO-ASA) method, have been performed to understand the electronic structure of Ce4Ni6Al23 and the results are discussed in connection with those two other Ce–Ni–Al intermetallic compounds, which possess heavy-fermion behavior. Magnetic and heat capacity measurements have also been measured to analyze the low-temperature magnetic behavior of this new compound.
Co-reporter:Chi-Shen Lee, Gordon J. Miller
Journal of Solid State Chemistry 2003 Volume 170(Issue 1) pp:94-105
Publication Date(Web):January 2003
DOI:10.1016/S0022-4596(02)00032-4
A new ternary, intermetallic compound, Ba14Zn5−xAl22+x, was synthesized by heating the pure elements at 900°C. This compound crystallizes in the monoclinic space group I2/m, Z=2, with a=10.474(2) Å, b=6.0834(14) Å, c=34.697(8) Å and β=90.814(4)°. The crystal structure of Ba14Zn5−xAl22+x consists of [Zn5−xAl22+x] slabs that are built with a novel, two-dimensional (2D) network of Zn and Al atoms involving eight-membered rings sandwiched between two layers of trigonal bipyramids interconnected by three-center bonding. Tight-binding, linear muffin–tin orbital (TB-LMTO-ASA) calculations have been performed to understand the relationship between composition and orbital interactions in the electronegative element framework. This new structure is closely related to the high-pressure, cubic Laves-type structure of BaAl2 as well as the ambient pressure binary compound, Ba7Al13. The degree of valence electron charge transfer from the electropositive Ba atoms is related to the Al:Ba molar ratio in the Ba–Zn–Al system.
Co-reporter:Delphine Gout, Evan Benbow, Olivier Gourdon, Gordon J. Miller
Journal of Solid State Chemistry 2003 Volume 176(Issue 2) pp:538-548
Publication Date(Web):December 2003
DOI:10.1016/S0022-4596(03)00301-3
The electronic structures of Ce4Ni6Al23, CeNiAl4, CeNi2Al5, CeNiAl and CeNi4Al have been calculated using the TB-LMTO-ASA (tight-binding, linear muffin-tin orbital, atomic-spheres approximation) approach to probe relationships between chemical bonding and physical properties in this series of intermetallic compounds. Analysis from crystal orbital Hamilton populations (COHP) reveal that the Al-rich compounds may be considered as “polar intermetallic” because the Fermi level coincides to the separation of bonding and antibonding states of the Ni–Al framework. On the other hand, although the densities of states (DOS) of CeNiAl suggest “polar intermetallic” behavior, the bonding is more complex. Finally, the Ni-rich example, CeNi4Al, has significant Ni-3d character at the Fermi level. The results of these calculations are also discussed in connection with heavy fermion or possible valence fluctuation behavior observed for some of these intermetallic compounds: those showing exceptional properties also exhibit significant “lattice covalency” between Ce and the Ni–Al nets.
Co-reporter:Chi-Shen Lee
Angewandte Chemie International Edition 2001 Volume 40(Issue 24) pp:
Publication Date(Web):18 DEC 2001
DOI:10.1002/1521-3773(20011217)40:24<4740::AID-ANIE4740>3.0.CO;2-Z
A pseudo-pentagonal kernel is the central unit in the new quaternary intermetallic phase Li10Mg6Zn31Al3 (see structure). This unit can form the basis for decagonal and, possibly, icosahedral quasicrystals. The synthesis, crystal structure, and bonding of Li10Mg6Zn31Al3 are discussed in detail.
Co-reporter:Chi-Shen Lee
Angewandte Chemie 2001 Volume 113(Issue 24) pp:
Publication Date(Web):18 DEC 2001
DOI:10.1002/1521-3757(20011217)113:24<4876::AID-ANGE4876>3.0.CO;2-#
Ein pseudopentagonaler Kern bildet die zentrale Einheit der neuen quaternären intermetallischen Phase Li10Mg6Zn31Al3 (siehe Struktur). Mit dieser Einheit können dekagonale und möglicherweise ikosaedrische Quasikristalle aufgebaut werden; Synthese, Kristallstruktur und Bindungsverhältnisse der Verbindung werden im Detail diskutiert.
Co-reporter:Gordon J. Miller
European Journal of Inorganic Chemistry 1998 Volume 1998(Issue 5) pp:
Publication Date(Web):7 DEC 1998
DOI:10.1002/(SICI)1099-0682(199805)1998:5<523::AID-EJIC523>3.0.CO;2-L
The “coloring problem,” as applied to the field of solid state chemistry, addresses the issues of structural preference as well as the distribution of different elements within a given structure. Both the connectivity and arrangement of elements in a solid affect physical and chemical properties, so a clear understanding of the forces controlling how elements will arrange themselves in a solid state structure creates the ability to predict structure-composition-property relationships. There are two fundamental energetic contributions that influence how elements in a structure order: the site energy and the bond energy. This review examines how these two parts of the total electronic energy work together to influence the observed structures, compositions, and properties of intermetallics and transition metal cluster compounds.
Co-reporter:Karen J. Nordell; Dr. Gordon J. Miller
Angewandte Chemie 1997 Volume 109(Issue 18) pp:
Publication Date(Web):31 JAN 2006
DOI:10.1002/ange.19971091827
Co-reporter:Weiwei Xie, Robert J. Cava and Gordon J. Miller
Journal of Materials Chemistry A 2017 - vol. 5(Issue 29) pp:NaN7221-7221
Publication Date(Web):2017/07/03
DOI:10.1039/C7TC01967J
The new complex intermetallic alloy, Cr22Zn72Sn24, with a giant cubic unit cell (space group Fmc, Z = 8, Pearson symbol cF944 with a = 25.083(3) Å) is reported. The structure can be described as a CsCl-type packing of two “Russian Doll” type intermetallic clusters: a 99-atom cluster, Cr@(Cr8Zn6)(Sn)24(Zn)60, composed of a central Cr atom encapsulated by three shells: a distorted rhombic dodecahedron, a snub-cube, and a rhombi-icosidodecahedron; and a 55-atom cluster, Cr@(Zn)12(Zn)30(Zn)12, also composed of a central Cr atom, but now surrounded by three different shells: an icosahedron, an icosidodecahedron, and a larger icosahedron. The two distinct clusters can be considered as “superatoms” that form the resulting nanometer-scale CsCl-type structure. Evaluating the electronic structure of this complex alloy structure yielded an alternative description, involving a 3-d network of vertex-connected, Cr-centered icosahedra and fragments of bcc metals. This description closely parallels the effective chemical bonding interactions, allowing for an understanding of the stability of this CMA. Magnetic and Seebeck coefficient measurements on self-flux grown crystals of Cr22Zn72Sn24 show it to be paramagnetic and metallic, in agreement with the results of electronic structure calculations.
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