Co-reporter:Graham M. McNally, Ángel M. Arévalo-López, Padraig Kearins, Fabio Orlandi, Pascal Manuel, and J. Paul Attfield
Chemistry of Materials October 24, 2017 Volume 29(Issue 20) pp:8870-8870
Publication Date(Web):September 25, 2017
DOI:10.1021/acs.chemmater.7b03556
Cation ordering in ABO3 perovskites can lead to interesting and useful phenomena such as ferrimagnetism and high magnetoresistance by spin polarized conduction in Sr2FeMoO6. We used high pressures and temperatures to synthesize the cation ordered AA′BB′O6 perovskites CaMnFeReO6, CaMnMnReO6, and Ca(Mn0.5Cu0.5)FeReO6. These have columnar A/A′ and rocksalt B/B′ cation orders, as found in the recently discovered double double perovskite MnNdMnSbO6, and partial Mn/Cu order over tetrahedral and square planar A′ sites in Ca(Mn0.5Cu0.5)FeReO6 demonstrates that triple double cation order is possible. Neutron diffraction reveals complex ferrimagnetic orders in all three materials; CaMnFeReO6 and Ca(Mn0.5Cu0.5)FeReO6 have large room temperature magnetizations with low temperature switching of magnetoresistance in the latter material, and CaMnMnReO6 displays a high coercivity of 1.3 T at low temperatures.
Co-reporter:Chun Che Lin, Yi-Ting Tsai, Hannah E. Johnston, Mu-Huai Fang, Fengjiao Yu, Wuzong Zhou, Pamela Whitfield, Ye Li, Jing Wang, Ru-Shi Liu, and J. Paul Attfield
Journal of the American Chemical Society August 30, 2017 Volume 139(Issue 34) pp:11766-11766
Publication Date(Web):August 2, 2017
DOI:10.1021/jacs.7b04338
Optimizing properties of phosphors for use in white-light-emitting diodes (WLEDs) is an important materials challenge. Most phosphors have a low level of lattice disorder due to mismatch between the host and activator cations. Here we show that deliberate introduction of high levels of cation disorder leads to significant improvements in quantum efficiency, stability to thermal quenching, and emission lifetime in Sr1.98–x(Ca0.55Ba0.45)xSi5N8:Eu0.02 (x = 0–1.5) phosphors. Replacing Sr by a (Ca0.55Ba0.45) mixture with the same average radius increases cation size variance, resulting in photoluminescence emission increases of 20–26% for the x = 1.5 sample relative to the x = 0 parent across the 25–200 °C range that spans WLED working temperatures. Cation disorder suppresses nonradiative processes through disruption of lattice vibrations and creates deep traps that release electrons to compensate for thermal quenching. Introduction of high levels of cation disorder may thus be a very useful general approach for improving the efficiency of luminescent materials.
Co-reporter:A. M. Arevalo-Lopez;A. Srinath;J. P. Attfield
Materials Chemistry Frontiers 2017 vol. 1(Issue 1) pp:172-175
Publication Date(Web):2016/11/30
DOI:10.1039/C6QM00136J
Sr1−xCaxCrO3−δ (x = 0.25, 0.50, 0.75) solid solutions have been synthesised using the ‘hard–soft’ technique and structurally characterised. Sr1−xCaxCrO3 phases were prepared under ‘hard’ high pressure and high temperature synthesis and then reduced at ‘soft’ low temperature conditions. Analysis by powder X-ray diffraction and Rietveld refinements reveals that the reduction of each composition gives two crystalline products, with 6H and 15R structure types previously reported in SrCrO3−δ. analogues. These results demonstrate that the 6H and 15R structures are stable for very high calcium contents up to x = 0.75 in Sr1−xCaxCrO3−δ, although different structural motifs are observed for CaCrO3−δ. The Sr1−xCaxCrO3−δ phase diagram shows that the lower temperature for superstructure formation has a maximum near x = 0.5, evidencing an influence of Ca/Sr cation disorder on oxygen vacancy order.
Co-reporter:A. M. Arévalo-López, F. Stegemann and J. P. Attfield
Chemical Communications 2016 vol. 52(Issue 32) pp:5558-5560
Publication Date(Web):22 Mar 2016
DOI:10.1039/C6CC01290F
The new double perovskite Mn2MnReO6 has been synthesised at high pressure. Mn2+ and Re6+ spins order antiferromagnetically through two successive transitions that are coupled by magnetoelastic effects, as order of the Mn spins at 109 K leads to lattice distortions that reduce frustration prompting Re order at 99 K.
Co-reporter:G. Perversi, J. Cumby, E. Pachoud, J. P. Wright and J. P. Attfield
Chemical Communications 2016 vol. 52(Issue 27) pp:4864-4867
Publication Date(Web):19 Feb 2016
DOI:10.1039/C5CC10495E
A remarkably complex electronic order of Fe2+/Fe3+ charges, Fe2+ orbital states, and weakly metal–metal bonded Fe3 units known as trimerons, was recently discovered in stoichiometric magnetite (Fe3O4) below the 125 K Verwey transition. Here, the low temperature crystal structure of a natural magnetite from a mineral sample has been determined using the same microcrystal synchrotron X-ray diffraction method. Structure refinement demonstrates that the natural sample has the same complex electronic order as pure synthetic magnetite, with only minor reductions of orbital and trimeron distortions. Chemical analysis shows that the natural sample contains dopants such as Al, Si, Mg and Mn at comparable concentrations to extraterrestrial magnetites, for example, as reported in the Tagish Lake meteorite. Much extraterrestrial magnetite exists at temperatures below the Verwey transition and hence our study demonstrates that the low temperature phase of magnetite represents the most complex long-range electronic order known to occur naturally.
Co-reporter:Elena Solana-Madruga;Dr. Ángel M. Arévalo-López;Dr. Antonio J. DosSantos-García;Dr. Esteban Urones-Garrote;Dr. David Ávila-Bre; Regino Sáez-Puche; J. Paul Attfield
Angewandte Chemie International Edition 2016 Volume 55( Issue 32) pp:9340-9344
Publication Date(Web):
DOI:10.1002/anie.201603526
Abstract
Cation ordering in ABO3 perovskites adds to their chemical variety and can lead to properties such as ferrimagnetism and magnetoresistance in Sr2FeMoO6. Through high-pressure and high-temperature synthesis, a new type of “double double perovskite” structure has been discovered in the family MnRMnSbO6 (R=La, Pr, Nd, Sm). This tetragonal structure has a 1:1 order of cations on both A and B sites, with A-site Mn2+ and R3+ cations ordered in columns and Mn2+ and Sb5+ having rock salt order on the B sites. The MnRMnSbO6 double double perovskites are ferrimagnetic at low temperatures with additional spin-reorientation transitions. The ordering direction of ferrimagnetic Mn spins in MnNdMnSbO6 changes from parallel to [001] below TC=76 K to perpendicular below the reorientation transition at 42 K at which Nd moments also order. Smaller rare earths lead to conventional monoclinic double perovskites (MnR)MnSbO6 for Eu and Gd.
Co-reporter:Elena Solana-Madruga;Dr. Ángel M. Arévalo-López;Dr. Antonio J. DosSantos-García;Dr. Esteban Urones-Garrote;Dr. David Ávila-Bre; Regino Sáez-Puche; J. Paul Attfield
Angewandte Chemie 2016 Volume 128( Issue 32) pp:9486-9490
Publication Date(Web):
DOI:10.1002/ange.201603526
Abstract
Cation ordering in ABO3 perovskites adds to their chemical variety and can lead to properties such as ferrimagnetism and magnetoresistance in Sr2FeMoO6. Through high-pressure and high-temperature synthesis, a new type of “double double perovskite” structure has been discovered in the family MnRMnSbO6 (R=La, Pr, Nd, Sm). This tetragonal structure has a 1:1 order of cations on both A and B sites, with A-site Mn2+ and R3+ cations ordered in columns and Mn2+ and Sb5+ having rock salt order on the B sites. The MnRMnSbO6 double double perovskites are ferrimagnetic at low temperatures with additional spin-reorientation transitions. The ordering direction of ferrimagnetic Mn spins in MnNdMnSbO6 changes from parallel to [001] below TC=76 K to perpendicular below the reorientation transition at 42 K at which Nd moments also order. Smaller rare earths lead to conventional monoclinic double perovskites (MnR)MnSbO6 for Eu and Gd.
Co-reporter:A. Kitada, A. M. Arevalo-Lopez and J. P. Attfield
Chemical Communications 2015 vol. 51(Issue 57) pp:11359-11361
Publication Date(Web):28 May 2015
DOI:10.1039/C5CC02800K
Gentle oxidation of lithium titanate spinel (LiTi2O4) with water at room temperature gives Li-deficient Li0.33Ti2O4. Combined X-ray and neutron Rietveld analysis shows that 28% of the Ti cations are displaced to alternative octahedral sites, in keeping with a proposed model based on Ti-migration limited by Li-vacancy concentration.
Co-reporter:Angel M. Arevalo-Lopez and J. Paul Attfield
Dalton Transactions 2015 vol. 44(Issue 23) pp:10661-10664
Publication Date(Web):19 Jan 2015
DOI:10.1039/C4DT03780D
Powder neutron diffraction and magnetic susceptibility measurements at 10–300 K have been used to determine the crystal and magnetic structures of brownmillerite type Ca2Cr2O5, which was obtained by reduction of the high pressure phase CaCrO3 through hard–soft chemistry. The ambient temperature crystal structure of Ca2Cr2O5 is refined in space group I2mb and the unusual tetrahedral coordination of Cr3+ results in local structural distortions. Cr3+ spins order antiferromagnetically below 220 K and a substantial observed canting of moments shows that Heisenberg exchange is weak or frustrated and competes with antisymmetric Dzialoshinskii–Moriya interactions.
Co-reporter:Dr. Angel M. Arévalo-López;Graham M. McNally ; J. Paul Attfield
Angewandte Chemie 2015 Volume 127( Issue 41) pp:12242-12245
Publication Date(Web):
DOI:10.1002/ange.201506540
Abstract
Ferrimagnetic A2BB′O6 double perovskites, such as Sr2FeMoO6, are important spin-polarized conductors. Introducing transition metals at the A-sites offers new possibilities to increase magnetization and tune magnetoresistance. Herein we report a ferrimagnetic double perovskite, Mn2FeReO6, synthesized at high pressure which has a high Curie temperature of 520 K and magnetizations of up to 5.0 μB which greatly exceed those for other double perovskite ferrimagnets. A novel switching transition is discovered at 75 K where magnetoresistance changes from conventional negative tunneling behavior to large positive values, up to 265 % at 7 T and 20 K. Neutron diffraction shows that the switch is driven by magnetic frustration from antiferromagnetic Mn2+ spin ordering which cants Fe3+ and Re5+ spins and reduces spin-polarization. Ferrimagnetic double perovskites based on A-site Mn2+ thus offer new opportunities to enhance magnetization and control magnetoresistance in spintronic materials.
Co-reporter:Dr. Angel M. Arévalo-López;Graham M. McNally ; J. Paul Attfield
Angewandte Chemie International Edition 2015 Volume 54( Issue 41) pp:12074-12077
Publication Date(Web):
DOI:10.1002/anie.201506540
Abstract
Ferrimagnetic A2BB′O6 double perovskites, such as Sr2FeMoO6, are important spin-polarized conductors. Introducing transition metals at the A-sites offers new possibilities to increase magnetization and tune magnetoresistance. Herein we report a ferrimagnetic double perovskite, Mn2FeReO6, synthesized at high pressure which has a high Curie temperature of 520 K and magnetizations of up to 5.0 μB which greatly exceed those for other double perovskite ferrimagnets. A novel switching transition is discovered at 75 K where magnetoresistance changes from conventional negative tunneling behavior to large positive values, up to 265 % at 7 T and 20 K. Neutron diffraction shows that the switch is driven by magnetic frustration from antiferromagnetic Mn2+ spin ordering which cants Fe3+ and Re5+ spins and reduces spin-polarization. Ferrimagnetic double perovskites based on A-site Mn2+ thus offer new opportunities to enhance magnetization and control magnetoresistance in spintronic materials.
Co-reporter:Angel M. Arevalo-Lopez, Bing Liang, Mark S. Senn, Claire Murray, Chiu Tang and J. Paul Attfield
Journal of Materials Chemistry A 2014 vol. 2(Issue 44) pp:9364-9367
Publication Date(Web):09 Oct 2014
DOI:10.1039/C4TC01808G
New CaCrO3−δ superstructure phases (δ = 0.33, 0.4, and 0.5) have been discovered through ‘soft’ low temperature reduction of the perovskite CaCrO3 made at ‘hard’ high pressure–temperature conditions. Their structures form a related series based on stacking of octahedral and tetrahedral chromium oxide layers.
Co-reporter:Dr. Alexra Sinclair;Dr. Jennifer A. Rodgers;Craig V. Topping;Dr. Martin Mí&x161;ek;Ross D. Stewart;Dr. Winfried Kockelmann;Dr. Jan-Willem G. Bos; J. Paul Attfield
Angewandte Chemie 2014 Volume 126( Issue 32) pp:8483-8487
Publication Date(Web):
DOI:10.1002/ange.201403223
Abstract
An extensive series of new LnRuO3 perovskites has been synthesized at high pressure. These ruthenium(III)-based oxides are ruthenium deficient, and high-pressure samples have compositions close to LnRu0.9O3. These phases stabilize ruthenium(III) which is very unusual in oxides. X-ray and neutron powder diffraction studies show that the materials adopt orthorhombic perovskite superstructures in which the RuO6 octahedra are tetragonally compressed. These distortions, and the Mott insulator properties of the materials, are driven by strong spin-orbit coupling.
Co-reporter:Dr. Alexra Sinclair;Dr. Jennifer A. Rodgers;Craig V. Topping;Dr. Martin Mí&x161;ek;Ross D. Stewart;Dr. Winfried Kockelmann;Dr. Jan-Willem G. Bos; J. Paul Attfield
Angewandte Chemie International Edition 2014 Volume 53( Issue 32) pp:8343-8347
Publication Date(Web):
DOI:10.1002/anie.201403223
Abstract
An extensive series of new LnRuO3 perovskites has been synthesized at high pressure. These ruthenium(III)-based oxides are ruthenium deficient, and high-pressure samples have compositions close to LnRu0.9O3. These phases stabilize ruthenium(III) which is very unusual in oxides. X-ray and neutron powder diffraction studies show that the materials adopt orthorhombic perovskite superstructures in which the RuO6 octahedra are tetragonally compressed. These distortions, and the Mott insulator properties of the materials, are driven by strong spin-orbit coupling.
Co-reporter:Lucy Clark, Judith Oró-Solé, Kevin S. Knight, Amparo Fuertes, and J. Paul Attfield
Chemistry of Materials 2013 Volume 25(Issue 24) pp:5004
Publication Date(Web):December 9, 2013
DOI:10.1021/cm4037132
The presence and thermal stability of anion order in the oxynitride perovskites SrTaO2N and LaTaON2 have been determined using high resolution powder neutron and electron diffraction data. Partial order of oxide and nitride anions consistent with the formation of planes of disordered cis-anion chains is observed in both materials, with a chemical symmetry between distributions in SrTaO2N and LaTaON2. No loss of anion order is observed up to 1100 °C and extrapolations based on lattice strains show the order to be stable to remarkably high temperatures >2000 °C, demonstrating that anions are segregated when the materials are synthesized. SrTaO2N has an apparent tetragonal I4/mcm superstructure at room temperature due to ordered octahedral tilts, but anion order lowers symmetry to an orthorhombic Fmmm supercell (with lattice parameters a = 8.0657(8), b = 8.0614(7), and c = 8.0775(4) Å). Anion order also lowers the symmetry of LaTaON2 from apparent orthorhombic Imma to monoclinic I2/m (a = 5.7140(6), b = 8.0595(6), c = 5.7506(5) Å, and β = 90.239(4)° at 20 °C) and this superstructure persists up to 1100 °C with an extrapolated loss of tilting at 1540 °C. Anion order appears to direct octahedral tilting such that the more rigid Ta–N–Ta bridges retain bond angles closer to 180° than the Ta–O–Ta connections in these superstructures.Keywords: anion order; oxynitrides; perovskites; powder neutron diffraction;
Co-reporter:Angel M. Arévalo-López, Falak Sher, James Farnham, Adam J. Watson, and J. Paul Attfield
Chemistry of Materials 2013 Volume 25(Issue 11) pp:2346
Publication Date(Web):May 22, 2013
DOI:10.1021/cm401062u
15-layer rhombohedral (15R) SrCrO2.8-type superstructures have been discovered in Sr(Cr1-xFex)O3-y perovskites (0.4 ≤ x ≤ 0.6; space group R3̅m; a ≈ 5.56 and c ≈ 34.6 Å). Cr/Fe cations are segregated between layers of tetrahedrally and octahedrally coordinated sites. The 15R-Sr(Cr1-xFex)O3-y materials are semiconducting and order ferrimagnetically below 225–342 K. The magnetic structure of an x = 0.5 sample shows spin canting consistent with a simple spin disorder model. Samples with x ≥ 0.7 have a disordered cubic perovskite structure, and we propose that locally reconstructed (111) planes like those in the 15R materials facilitate oxide ion migration in Cr-based perovskite mixed conductors used in solid oxide fuel cells.Keywords: magnetic structure; perovskite oxides; solid oxide fuel cells;
Co-reporter:Judith Oró-Solé, Lucy Clark, William Bonin, J. Paul Attfield and Amparo Fuertes
Chemical Communications 2013 vol. 49(Issue 24) pp:2430-2432
Publication Date(Web):06 Feb 2013
DOI:10.1039/C3CC38736D
The correlated anion order in the oxynitride perovskite NdVO2N, where disordered zig-zag VN chains segregate into planes within a pseudo-cubic lattice, is similar to that in materials such as SrTaO2N containing d0 transition metal cations. However, NdVO2N has 3d1 V4+ cations and the 3d-electrons are itinerant, showing that the anion chain order in oxynitride perovskites is robust to electron-doping.
Co-reporter:Sandra Carlsson, Leokadiya Zorina, David R. Allan, J. Paul Attfield, Enric Canadell, and Patrick Batail
Inorganic Chemistry 2013 Volume 52(Issue 6) pp:3326-3333
Publication Date(Web):February 22, 2013
DOI:10.1021/ic302790m
(EDT-TTF-CONH2)6[Re6Se8(CN)6] is a molecular solid with R3̅ space group symmetry and has the remarkable feature of exhibiting hybrid donor layers with a kagome topology which sustain metallic conductivity. We report a detailed study of the structural evolution of the system as a function of temperature and pressure. This rhombohedral phase is maintained on cooling down to 220 K or up to 0.7 GPa pressure, beyond which a symmetry-breaking transition to a triclinic P1̅ phase drives a metal to insulator transition. Band structures calculated from the structural data lead to a clear description of the effects of temperature and pressure on the structural and electronic properties of this system. Linear energy dispersion is calculated at the zero-gap Fermi level where valence and conduction bands touch for the rhombohedral phase. (EDT-TTF-CONH2)6[Re6Se8(CN)6] thus exhibits a regular (right circular) Dirac-cone like that of graphene at the Fermi level, which has not been reported previously in a molecular solid. The Dirac-cone is robust over the stability region of the rhombohedral phase, and may result in exotic electronic transport and optical properties.
Co-reporter:J. Paul Attfield
Crystal Growth & Design 2013 Volume 13(Issue 10) pp:4623-4629
Publication Date(Web):September 5, 2013
DOI:10.1021/cg4011168
Metal oxynitrides are emerging materials that may combine the advantages of oxides and nitrides. Anion order is important for controlling and tuning properties, and neutron diffraction provides good O/N contrast for experimental determinations of local or long-range O/N order in solids. Differences between oxide and nitride in charge, size, and covalent bonding are the important factors that drive anion order. An important example is the robust partial anion order in SrMO2N (M = Nb, Ta) and related oxynitride perovskites driven by covalency that results in disordered zigzag MN chains which segregate into planes within the perovskite lattice. This leads to unusual subextensive scaling of entropy, described as “open order”. Local anion order is important to optical materials. Size mismatch between host and dopant cations leads to local O/N clustering that tunes photoluminescence shifts systematically in M1.95Eu0.05Si5–xAlxN8–xOx phosphors, leading to a red shift when the M = Ba and Sr host cations are larger than the Eu2+ dopant but a blue shift when the M = Ca host is smaller.
Co-reporter:Philip J. Camp ; Amparo Fuertes
Journal of the American Chemical Society 2012 Volume 134(Issue 15) pp:6762-6766
Publication Date(Web):March 23, 2012
DOI:10.1021/ja300847m
Unusual subextensive configurational entropies that vary with particle size and tend to zero per atom in macroscopic samples are predicted for AMO3–zNz oxynitrides with perovskite type crystal structures. These materials are crystallographically disordered on the atomic scale, but local anion order produces chains of M–N–M bonds that undergo a 90° turn at each M cation, giving rise to subextensive entropies in materials such as SrTaO2N, LaNbON2, and EuWO1.5N1.5. A general Pauling ice-rules formula is used to calculate the extensive molar entropies for other cases such as SrMoO2.5N0.5 and BaTaO2N. The subextensive oxynitrides are usefully classified as showing an “open order”, related to the correlated order of displacements in ferroelectric perovskites such as BaTiO3. This raises the possibility that further open-ordered oxynitride or molecular structures may be accessible, and other states such as spins and charges may also show novel open orders.
Co-reporter:Wei-Ting Chen ; Hwo-Shuenn Sheu ; Ru-Shi Liu
Journal of the American Chemical Society 2012 Volume 134(Issue 19) pp:8022-8025
Publication Date(Web):April 26, 2012
DOI:10.1021/ja301593z
Red or yellow phosphors excited by a blue light-emitting diode are an efficient source of white light for everyday applications. Many solid oxides and nitrides, particularly silicon nitride-based materials such as M2Si5N8 and MSi2O2N2 (M = Ca, Sr, Ba), CaAlSiN3, and SiAlON, are useful phosphor hosts with good thermal stabilities. Both oxide/nitride and various cation substitutions are commonly used to shift the emission spectrum and optimize luminescent properties, but the underlying mechanisms are not always clear. Here we show that size-mismatch between host and dopant cations tunes photoluminescence shifts systematically in M1.95Eu0.05Si5–xAlxN8–xOx lattices, leading to a red shift when the M = Ba and Sr host cations are larger than the Eu2+ dopant, but a blue shift when the M = Ca host is smaller. Size-mismatch tuning of thermal quenching is also observed. A local anion clustering mechanism in which Eu2+ gains excess nitride coordination in the M = Ba and Sr structures, but excess oxide in the Ca analogues, is proposed for these mismatch effects. This mechanism is predicted to be general to oxynitride materials and will be useful in tuning optical and other properties that are sensitive to local coordination environments.
Co-reporter:Mikael Markkula, Angel M. Arevalo-Lopez, J. Paul Attfield
Journal of Solid State Chemistry 2012 Volume 192() pp:390-393
Publication Date(Web):August 2012
DOI:10.1016/j.jssc.2012.04.029
A variable-temperature powder neutron diffraction study of the monoclinic brannerite-type CoV2O6 (space group C2/m, a=9.2531(2), b=3.5040(1), c=6.6201(1) Å and β=111.617(1)° at 300 K) is reported. No structural transition is observed down to 4 K, but a magnetostriction accompanying antiferromagnetic order at TN=15 K is discovered. Antiferromagnetic order observed below TN has an a×b×2c supercell in which Co2+ moments of magnitude 4.77(4) μB at 4 K lie in the ac plane and are ferromagnetically coupled within chains of edge-sharing CoO6 octahedra parallel to b. Ferromagnetic chains are coupled antiferromagnetically to neighbouring chains in the a and c directions, and a model for the interchain order in the reported 1/3 magnetization plateau region is proposed.Graphical abstractAntiferromagnetic order of Co2+ moments in monoclinic brannerite type CoV2O6 results in a magnetostriction at the 15 K Neel transition. An alternative coupling between ferromagnetic chains is proposed to account for a 1/3 magnetization plateau in this material.Highlights► Variable-temperature powder neutron diffraction study of the monoclinic brannerite-type CoV2O6 is reported. ► Magnetostriction accompanying antiferromagnetic order at TN=15 K in monoclinic CoV2O6 is discovered. ► Antiferromagnetic order in a×b×2c supercell of CoV2O6 is determined. ► Model for spin order in the reported 1/3 magnetization plateau of CoV2O6 is proposed.
Co-reporter:Dr. Angel M. Arévalo-López;Dr. Jennifer A. Rodgers;Mark S. Senn;Dr. Falak Sher;James Farnham;William Gibbs; J. Paul Attfield
Angewandte Chemie International Edition 2012 Volume 51( Issue 43) pp:10791-10794
Publication Date(Web):
DOI:10.1002/anie.201206203
Co-reporter:Dr. Angel M. Arévalo-López;Dr. Jennifer A. Rodgers;Mark S. Senn;Dr. Falak Sher;James Farnham;William Gibbs; J. Paul Attfield
Angewandte Chemie 2012 Volume 124( Issue 43) pp:10949-10952
Publication Date(Web):
DOI:10.1002/ange.201206203
Co-reporter:J. Paul Attfield
Journal of Materials Chemistry A 2011 vol. 21(Issue 13) pp:4756-4764
Publication Date(Web):12 Jan 2011
DOI:10.1039/C0JM03274C
Seven distinct families of superconductors with critical temperatures at ambient pressure that equal or surpass the historic 23 K limit for Nb3Ge have been discovered in the last 25 years. Each family is reviewed briefly and their common chemical features are discussed. High temperature superconductors are distinguished by having a high (≥50%) content of nonmetallic elements and fall into two broad classes. ‘Metal–nonmetal’ superconductors require a specific combination of elements such as Cu–O and Fe–As which give rise to the highest known Tc's, probably through a magnetic pairing mechanism. ‘Nonmetal-bonded’ materials contain covalently bonded nonmetal anion networks and are BCS-like superconductors. Fitting an extreme value function to the distribution of Tc values for the known high-Tc families suggests that the probability of a newly discovered superconductor family having maximum Tc > 100 K is ∼0.1 to 1%, decreasing to ∼0.02 to 0.2% for room temperature superconductivity.
Co-reporter:Minghui Yang ; Judith Oró-Solé ; Anna Kusmartseva ; Amparo Fuertes
Journal of the American Chemical Society 2010 Volume 132(Issue 13) pp:4822-4829
Publication Date(Web):March 10, 2010
DOI:10.1021/ja910745b
A remarkable electronic flexibility and colossal magnetoresistance effects have been discovered in the perovskite oxynitrides EuWO1+xN2−x. Ammonolysis of Eu2W2O9 yields scheelite-type intermediates EuWO4−yNy with a very small degree of nitride substitution (y = 0.04) and then EuWO1+xN2−x perovskites that show a wide range of compositions −0.16 ≤ x ≤ 0.46. The cubic lattice parameter varies linearly with x, but electron microscopy reveals a tetragonal superstructure. The previously unobserved x < 0 regime corresponds to oxidation of Eu (hole doping of the Eu:4f band), whereas x > 0 materials have chemical reduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and the hole/electron doping are tuned by varying the O/N ratio. EuWO1+xN2−x phases order ferromagnetically at 12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x = −0.04) sample. Distinct mechanisms for the hole and electron magnetotransport regimes are identified.
Co-reporter:Minghui Yang, Judith Oró-Solé, Amparo Fuertes and J. Paul Attfield
Chemistry of Materials 2010 Volume 22(Issue 14) pp:4132
Publication Date(Web):June 25, 2010
DOI:10.1021/cm101726b
Co-reporter:Congling Yin, Guobao Li, Winfried A. Kockelmann, Fuhui Liao, J. Paul Attfield and Jianhua Lin
Chemistry of Materials 2010 Volume 22(Issue 10) pp:3269
Publication Date(Web):April 21, 2010
DOI:10.1021/cm100578u
A 10-layer hexagonal perovskite Ba5Sb1−xMn4+xO15−δ solid solution (0.24 ≤ x ≤ 0.36; space group P63/mmc, a = 5.7095(1) Å, and c = 23.4866(3) Å for x = 0.36) has been synthesized by solid state reaction and studied using powder X-ray and neutron diffraction and magnetization measurements. This 10H polytype structure contains one corner-sharing (Sb,Mn)O6 octahedron and a tetramer of four face-sharing MnO6 octahedra per formula unit. Two magnetic transitions are observed at 129−167 K (TM) and 13−22 K (TF). The frustration index |θ|/TM, where θ is the Weiss temperature, is very close to unity, but considerable magnetic frustration is evidenced through the long-range order of a small ∼0.5 μB antiferromagnetic moment below TM, and the formation of ferro- or ferrimagnetic clusters. These freeze at TF with the |θ|/TF ≈ 7−10 index representing high magnetic frustration. TF increases directly with the excess of Mn, x, in Ba5Sb1−xMn4+xO15−δ rather than with the overall Mn content, showing that the occupation of the corner-sharing Sb/Mn sites is key to the extended magnetism.
Co-reporter:Anna F. Kusmartseva, Minghui Yang, Angel M. Arevalo-Lopez, Konstantin V. Kamenev and J. Paul Attfield
Chemical Communications 2010 vol. 46(Issue 36) pp:6681-6683
Publication Date(Web):23 Aug 2010
DOI:10.1039/C0CC01708F
Resistivity and powder X-ray diffraction measurements on Cs2Au2I6 up to 320 kbar pressure show that the suppression of Au+/Au3+ charge order above 55 kbar results in a non-metallic phase containing localised Au2+ states that is irreversibly amorphised above 120 kbar, and a gradual metallisation observed above 175 kbar may result from decomposition within the amorphous material.
Co-reporter:Luis Ortega-San-Martin, Anthony J. Williams, Alistair Storer and J. Paul Attfield
Chemistry of Materials 2009 Volume 21(Issue 12) pp:2436
Publication Date(Web):May 4, 2009
DOI:10.1021/cm900148a
A new perovskite, (Bi0.5Sr0.5)CrO3, has been prepared at 1000 °C and ambient pressure, although BiCrO3 and SrCrO3 are both high pressure phases. The crystal structure is rhombohedral (R3̅c, a = 5.4652(5) and c = 13.3502(2) Å at 300 K). Well-defined local Bi3+ displacements are evident despite an absence of long-range Bi/Sr cation order, but the degree of ferroelectric order is unclear. Long range G-type antiferromagnetic order occurs below 30 K; however, the low value of the ordered magnetic moment (1.2 μB), a high frustration index, and low temperature divergence of zero field and field cooled susceptibilities show that a substantial glassy component is also present.
Co-reporter:Wei-tin Chen, Anthony J. Williams, Luis Ortega-San-Martin, Ming Li, Derek C. Sinclair, Wuzong Zhou and J. Paul Attfield
Chemistry of Materials 2009 Volume 21(Issue 10) pp:2085
Publication Date(Web):April 27, 2009
DOI:10.1021/cm8031048
An investigation of Ca substitution in the multiferroic material BiFeO3 shows that a wide range of perovskites BixCa1−xFeO3 (0.4 ≤ x ≤ 1.0) can be prepared by sintering in air at 810−960 °C. 0.4 ≤ x < 0.8 samples are cubic Pm3̅m, whereas x = 0.8 and 0.9 show a coexistence of the cubic and the rhombohedral R3c BiFeO3-type structure. Considerable disorder arising from Bi3+ lone-pair distortions is evidenced by synchrotron X-ray and neutron studies of the average structures of the cubic phases, and electron microscopy reveals commensurate and incommensurate local superstructures. The BixCa1−xFeO3 (0.4 ≤ x ≤ 1.0) materials show a remarkably robust antiferromagnetic order with TN = 623−643 K and ordered moments of 3.6−4.1 μB. They are “leaky” dielectrics with relative permittivities of ∼30−100 and bulk resistivities ∼50−500 kΩ cm at room temperature. The activation energy for bulk conduction increases from 0.27 eV for x = 0.4 to 0.5 eV for x = 1, but with a discontinuity at the cubic-rhombohedral boundary. Further processing of the x = 0.8 and 0.9 compositions to reduce conductivity through control of oxygen content could lead to improved BiFeO3-based multiferroics.
Co-reporter:Minghui Yang, Jennifer A. Rodgers, Lawrence C. Middler, Judith Oró-Solé, A. Belén Jorge, Amparo Fuertes and J. Paul Attfield
Inorganic Chemistry 2009 Volume 48(Issue 24) pp:11498-11500
Publication Date(Web):November 13, 2009
DOI:10.1021/ic902020r
New oxynitrides of RZrO2N (R = Pr, Nd, and Sm) have been synthesized via a direct solid-state reaction of R2O3 with Zr2ON2 at 1200−1500 °C under 2−3 GPa pressure. Powder X-ray diffraction shows that all three phases adopt an orthorhombic Pnma perovskite superstructure [a = 5.8537(1) Å, b = 8.1707(1) Å, and c = 5.7093(1) Å for NdZrO2N] and the structural distortion increases with decreasing R3+ ionic radius. This method may enable new mixed-metal oxynitrides to be synthesized without the use of nitriding gas atmospheres.
Co-reporter:J. A. Rodgers, A. J. Williams, M. J. Martinez-Lope, J. A. Alonso and J. P. Attfield
Chemistry of Materials 2008 Volume 20(Issue 15) pp:4797
Publication Date(Web):July 11, 2008
DOI:10.1021/cm801263c
Co-reporter:Jan-Willem G. Bos, George B. S. Penny, Jennifer A. Rodgers, Dmitry A. Sokolov, Andrew D. Huxley and J. Paul Attfield
Chemical Communications 2008 (Issue 31) pp:3634-3635
Publication Date(Web):30 Jun 2008
DOI:10.1039/B808474B
New TbFeAs(O,F) and DyFeAs(O,F) superconductors with critical temperatures Tc = 46 and 45 K and very high critical fields, ≥100 T, have been prepared at 1100–1150 °C and 10–12 GPa, demonstrating that high pressure may be used to synthesise late rare earth derivatives of the recently reported RFeAs(O,F) (R = La–Nd, Sm, Gd) high temperature superconductors.
Co-reporter:Sandra J.E. Carlsson, Masaki Azuma, Yuichi Shimakawa, Mikio Takano, Alan Hewat, J. Paul Attfield
Journal of Solid State Chemistry 2008 Volume 181(Issue 3) pp:611-615
Publication Date(Web):March 2008
DOI:10.1016/j.jssc.2007.12.037
The crystal and magnetic structures of the charge ordered perovskite BiNiO3 have been studied at temperatures from 5 to 300 K using neutron diffraction. Rietveld analysis of the data shows that the structure remains triclinic (space group P1¯) throughout the whole temperature range. Bond-valence sum calculations based on the Bi–O and Ni–O bond distances confirm that the charge distribution is Bi3+0.5Bi5+0.5Ni2+O3 down to 5 K. The magnetic cell is identical to that of the triclinic superstructure and a G-type antiferromagnetic model gives a good fit to the magnetic intensities, with an ordered Ni2+ moment of 1.76(3) μB at 5 K. However, BiNiO3 is ferrimagnetic due to the inexact cancellation of opposing, inequivalent moments in the low symmetry cell.A neutron diffraction study shows that the perovskite BiNiO3 retains the unusual charge distribution Bi3+0.5Bi5+0.5Ni2+O3 down to 5 K. The Ni2+ moments are ordered in the G-type antiferromagnetic arrangement shown; however, BiNiO3 is ferrimagnetic due to the inexact cancellation of the four inequivalent moments in the triclinic unit cell.
Co-reporter:Falak Sher, J. Paul Attfield
Solid State Sciences 2006 Volume 8(3–4) pp:277-279
Publication Date(Web):March–April 2006
DOI:10.1016/j.solidstatesciences.2006.02.013
A new mixed metal oxide, Ba2CrMoO6, has been prepared at 1200 °C under reducing conditions. Ba2CrMoO6, adopts a 6-layered (6H) hexagonal perovskite structure (space group P63/mmc, a=5.7155(1)a=5.7155(1), c=13.9510(4) Åc=13.9510(4) Å). Rietveld analysis of powder X-ray diffraction data reveals a partial Cr/Mo ordering over the two transition metal sites. Ba2CrMoO6 is semiconducting with a spin glass transition at 37 K.
Co-reporter:R.J. Goff, J.P. Attfield
Journal of Solid State Chemistry 2006 Volume 179(Issue 5) pp:1369-1374
Publication Date(Web):May 2006
DOI:10.1016/j.jssc.2006.01.050
The room temperature structure of Bi0.75Sr0.25MnO3 has been fitted to high-resolution synchrotron X-ray and time-of-flight neutron powder diffraction data. Constrained structural models were refined using a (1200)Pn 11 supercell (a=11.0286(2)Å, b=7.7351(1)Å, c=5.53419(9)Å, and α=89.894(1)°α=89.894(1)°) of the underlying Pnma perovskite structure. The best-fit model evidences a 3:1 Mn3+/Mn4+charge ordering with only 30% of the ideal separation of bond valence sums. An ordered intergrowth of antiferro-orbitally ordered (LaMnO3 type) and charge and ferro-orbitally ordered (YBaMn2O6 type) blocks is observed. Off-centre Bi/Sr displacements are ferroelectrically ordered in this model.A model for charge and orbital order in Bi0.75Sr0.25MnO3 is proposed from a high-resolution powder X-ray and neutron diffraction study. An ordered intergrowth of antiferro-orbitally ordered (LaMnO3 type) and charge and ferro-orbitally ordered (YBaMn2O6 type) blocks is found, in agreement with previous electron microscopy images.
Co-reporter:Elizabeth Castillo-Martínez, Anthony J. Williams, J. Paul Attfield
Journal of Solid State Chemistry 2006 Volume 179(Issue 11) pp:3505-3510
Publication Date(Web):November 2006
DOI:10.1016/j.jssc.2006.07.019
The miscibility of TbBaMn2O5+x and TbBaMn2O5.5−y has been investigated at 100–600 °C using in situ powder neutron diffraction. No miscibility is observed, and the two phases remain oxygen stoichiometric (x,y=0x,y=0) at 600 °C. Structure refinement results show that neither material undergoes a phase transition in this temperature range. TbBaMn2O5 is Mn2+/Mn3+ charge ordered and any charge melting transition is >600 °C. This symmetry-broken charge ordering is remarkably robust in comparison to that in other oxides.In situ powder neutron diffraction shows that, surprisingly, there no miscibility between TbBaMn2O5+x and the oxygen intercalated product TbBaMn2O5.5−y up to 600 °C. TbBaMn2O5 is Mn2+/Mn3+ charge ordered and any charge melting transition occurs >600 °C. This charge ordering is remarkably robust in comparison to that in other oxides.
Co-reporter:Jan-Willem G. Bos and J. Paul Attfield
Journal of Materials Chemistry A 2005 vol. 15(Issue 6) pp:715-720
Publication Date(Web):25 Nov 2004
DOI:10.1039/B412918K
The crystal and magnetic structures of the double perovskite La2CoRuO6 have been investigated using neutron powder diffraction. La2CoRuO6 has a monoclinic P21/n superstructure (a
= 5.5724(3)
Å, b
= 5.6288(3)
Å, c
= 7.8843(4)
Å and β
= 89.99(2)°) with a rocksalt ordered arrangement of Co and Ru over the perovskite B sites (showing 10% Co/Ru inversion). Bond valence sums indicate the presence of Co2+ and Ru4+. Magnetization and transport measurements confirm La2CoRuO6 is an antiferromagnetic variable range hopping semiconductor with a Neel temperature of 25 K. The antiferromagnetic ordering is described by a (½ 0 ½) magnetic propagation vector but the spin directions cannot be determined unambiguously from neutron powder diffraction data because of the high pseudosymmetry.
Co-reporter:Falak Sher, A. Venimadhav, M.G. Blamire, B. Dabrowski, S. Kolesnik, J. Paul Attfield
Solid State Sciences 2005 Volume 7(Issue 8) pp:912-919
Publication Date(Web):August 2005
DOI:10.1016/j.solidstatesciences.2005.03.002
A series of Sr2Fe1−xMgxMoO6(0⩽x⩽1)(0⩽x⩽1) double perovskite oxides have been prepared and their crystal, magnetic and transport properties studied. Rietveld refinements, in tetragonal space group I4/m, show an almost complete B-cation ordering in the doped samples. A systematic decrease in Curie temperature, TC and saturation magnetization, MS with increase of x has been observed in this solid solution series. Resistivities show a semiconducting behavior for all samples and increase with x . The x=0x=0 sample becomes metallic above a metal-insulator transition temperature of 45 K in applied fields of 30 and 50 kOe. Negative colossal magnetoresistances (CMR) have been observed (up to 47% for the x=0.6x=0.6 sample at 5 K) in all the samples and these MR effects show a strong dependence on the microstructural nature (grain boundaries) of the material.
Co-reporter:A. C. Mclaughlin;F. Sher;J. P. Attfield
Nature 2005 436(7052) pp:829-832
Publication Date(Web):2005-08-11
DOI:10.1038/nature03828
The mechanism of high-transition-temperature (high-Tc) superconductivity in doped copper oxides is an enduring problem. Antiferromagnetism is established as the competing order1, 2, but the relationship between the two states in the intervening ‘pseudogap’ regime has become a central puzzle3. The role of the crystal lattice, which is important in conventional superconductors, also remains unclear. Here we report an anomalous increase of the distance between copper oxide planes on cooling, which results in negative thermal volume expansion, for layered ruthenium copper oxides4, 5 that have been doped to the boundary of antiferromagnetism and superconductivity. We propose that a crossover between these states is driven by spin ordering in the ruthenium oxide layers, revealing a novel mechanism for negative lattice expansion in solids. The differences in volume and lattice strain between the distinct superconducting and antiferromagnetic states can account for the phase segregation phenomena found extensively in low-doped copper oxides, and show that Cooper pair formation is coupled to the lattice. Unusually large variations of resistivity with magnetic field are found in these ruthenium copper oxides at low temperatures through coupling between the ordered Ru and Cu spins.
Co-reporter:A. Kitada, A. M. Arevalo-Lopez and J. P. Attfield
Chemical Communications 2015 - vol. 51(Issue 57) pp:NaN11361-11361
Publication Date(Web):2015/05/28
DOI:10.1039/C5CC02800K
Gentle oxidation of lithium titanate spinel (LiTi2O4) with water at room temperature gives Li-deficient Li0.33Ti2O4. Combined X-ray and neutron Rietveld analysis shows that 28% of the Ti cations are displaced to alternative octahedral sites, in keeping with a proposed model based on Ti-migration limited by Li-vacancy concentration.
Co-reporter:Jan-Willem G. Bos, George B. S. Penny, Jennifer A. Rodgers, Dmitry A. Sokolov, Andrew D. Huxley and J. Paul Attfield
Chemical Communications 2008(Issue 31) pp:NaN3635-3635
Publication Date(Web):2008/06/30
DOI:10.1039/B808474B
New TbFeAs(O,F) and DyFeAs(O,F) superconductors with critical temperatures Tc = 46 and 45 K and very high critical fields, ≥100 T, have been prepared at 1100–1150 °C and 10–12 GPa, demonstrating that high pressure may be used to synthesise late rare earth derivatives of the recently reported RFeAs(O,F) (R = La–Nd, Sm, Gd) high temperature superconductors.
Co-reporter:J. Paul Attfield
Journal of Materials Chemistry A 2011 - vol. 21(Issue 13) pp:NaN4764-4764
Publication Date(Web):2011/01/12
DOI:10.1039/C0JM03274C
Seven distinct families of superconductors with critical temperatures at ambient pressure that equal or surpass the historic 23 K limit for Nb3Ge have been discovered in the last 25 years. Each family is reviewed briefly and their common chemical features are discussed. High temperature superconductors are distinguished by having a high (≥50%) content of nonmetallic elements and fall into two broad classes. ‘Metal–nonmetal’ superconductors require a specific combination of elements such as Cu–O and Fe–As which give rise to the highest known Tc's, probably through a magnetic pairing mechanism. ‘Nonmetal-bonded’ materials contain covalently bonded nonmetal anion networks and are BCS-like superconductors. Fitting an extreme value function to the distribution of Tc values for the known high-Tc families suggests that the probability of a newly discovered superconductor family having maximum Tc > 100 K is ∼0.1 to 1%, decreasing to ∼0.02 to 0.2% for room temperature superconductivity.
Co-reporter:Judith Oró-Solé, Lucy Clark, William Bonin, J. Paul Attfield and Amparo Fuertes
Chemical Communications 2013 - vol. 49(Issue 24) pp:NaN2432-2432
Publication Date(Web):2013/02/06
DOI:10.1039/C3CC38736D
The correlated anion order in the oxynitride perovskite NdVO2N, where disordered zig-zag VN chains segregate into planes within a pseudo-cubic lattice, is similar to that in materials such as SrTaO2N containing d0 transition metal cations. However, NdVO2N has 3d1 V4+ cations and the 3d-electrons are itinerant, showing that the anion chain order in oxynitride perovskites is robust to electron-doping.
Co-reporter:G. Perversi, J. Cumby, E. Pachoud, J. P. Wright and J. P. Attfield
Chemical Communications 2016 - vol. 52(Issue 27) pp:NaN4867-4867
Publication Date(Web):2016/02/19
DOI:10.1039/C5CC10495E
A remarkably complex electronic order of Fe2+/Fe3+ charges, Fe2+ orbital states, and weakly metal–metal bonded Fe3 units known as trimerons, was recently discovered in stoichiometric magnetite (Fe3O4) below the 125 K Verwey transition. Here, the low temperature crystal structure of a natural magnetite from a mineral sample has been determined using the same microcrystal synchrotron X-ray diffraction method. Structure refinement demonstrates that the natural sample has the same complex electronic order as pure synthetic magnetite, with only minor reductions of orbital and trimeron distortions. Chemical analysis shows that the natural sample contains dopants such as Al, Si, Mg and Mn at comparable concentrations to extraterrestrial magnetites, for example, as reported in the Tagish Lake meteorite. Much extraterrestrial magnetite exists at temperatures below the Verwey transition and hence our study demonstrates that the low temperature phase of magnetite represents the most complex long-range electronic order known to occur naturally.
Co-reporter:A. M. Arévalo-López, F. Stegemann and J. P. Attfield
Chemical Communications 2016 - vol. 52(Issue 32) pp:NaN5560-5560
Publication Date(Web):2016/03/22
DOI:10.1039/C6CC01290F
The new double perovskite Mn2MnReO6 has been synthesised at high pressure. Mn2+ and Re6+ spins order antiferromagnetically through two successive transitions that are coupled by magnetoelastic effects, as order of the Mn spins at 109 K leads to lattice distortions that reduce frustration prompting Re order at 99 K.
Co-reporter:Anna F. Kusmartseva, Minghui Yang, Angel M. Arevalo-Lopez, Konstantin V. Kamenev and J. Paul Attfield
Chemical Communications 2010 - vol. 46(Issue 36) pp:NaN6683-6683
Publication Date(Web):2010/08/23
DOI:10.1039/C0CC01708F
Resistivity and powder X-ray diffraction measurements on Cs2Au2I6 up to 320 kbar pressure show that the suppression of Au+/Au3+ charge order above 55 kbar results in a non-metallic phase containing localised Au2+ states that is irreversibly amorphised above 120 kbar, and a gradual metallisation observed above 175 kbar may result from decomposition within the amorphous material.
Co-reporter:A. M. Arevalo-Lopez, A. Srinath and J. P. Attfield
Inorganic Chemistry Frontiers 2017 - vol. 1(Issue 1) pp:
Publication Date(Web):
DOI:10.1039/C6QM00136J
Co-reporter:Angel M. Arevalo-Lopez, Bing Liang, Mark S. Senn, Claire Murray, Chiu Tang and J. Paul Attfield
Journal of Materials Chemistry A 2014 - vol. 2(Issue 44) pp:NaN9367-9367
Publication Date(Web):2014/10/09
DOI:10.1039/C4TC01808G
New CaCrO3−δ superstructure phases (δ = 0.33, 0.4, and 0.5) have been discovered through ‘soft’ low temperature reduction of the perovskite CaCrO3 made at ‘hard’ high pressure–temperature conditions. Their structures form a related series based on stacking of octahedral and tetrahedral chromium oxide layers.
Co-reporter:Angel M. Arevalo-Lopez and J. Paul Attfield
Dalton Transactions 2015 - vol. 44(Issue 23) pp:NaN10664-10664
Publication Date(Web):2015/01/19
DOI:10.1039/C4DT03780D
Powder neutron diffraction and magnetic susceptibility measurements at 10–300 K have been used to determine the crystal and magnetic structures of brownmillerite type Ca2Cr2O5, which was obtained by reduction of the high pressure phase CaCrO3 through hard–soft chemistry. The ambient temperature crystal structure of Ca2Cr2O5 is refined in space group I2mb and the unusual tetrahedral coordination of Cr3+ results in local structural distortions. Cr3+ spins order antiferromagnetically below 220 K and a substantial observed canting of moments shows that Heisenberg exchange is weak or frustrated and competes with antisymmetric Dzialoshinskii–Moriya interactions.
