•High purity sample of Sr2MnO2Ag1.5Se2 obtained.•Magnetic structure determined.•Compared with related mixed-valent manganite oxide chalcogenides.The synthesis of a high-purity sample of the layered oxide selenide Sr2MnO2Ag1.5Se2 is reported. At ambient temperature it crystallises in the space group I4/mmm with two formula units in the unit cell and lattice parameters a=4.08771(1) Å, c=19.13087(8) Å. The compound displays mixed-valent manganese in a formal oxidation state close to +2.5 and powder neutron diffraction measurements reveal that below the Néel temperature of 63(1) K this results in an antiferromagnetic structure which may be described as A-type, modelled in the magnetic space group PI4/mnc (128.410 in the Belov, Neronova and Smirnova (BNS) scheme) in which localised Mn moments of 3.99(2) μB are arranged in ferromagnetic layers which are coupled antiferromagnetically. In contrast to the isostructural compound Sr2MnO2Cu1.5S2, Sr2MnO2Ag1.5Se2 does not display long range ordering of coinage metal ions and vacancies, nor may significant amounts of the coinage metal readily be deintercalated using soft chemical methods.Sr2MnO2Ag1.5Se2 containing mixed valent Mn ions undergoes magnetic ordering with ferromagnetic coupling within MnO2 sheets and antiferromagnetic coupling between MnO2 sheets.Figure optionsDownload full-size imageDownload as PowerPoint slide

CaCoSO, synthesized from CaO, Co, and S at 900 °C, is isostructural with CaZnSO and CaFeSO. The structure is non-centrosymmetric by virtue of the arrangement of the vertex-sharing CoS3O tetrahedra which are linked by their sulfide vertices to form layers. The crystal structure adopts space group P63mc (No. 186), and the lattice parameters are a = 3.7524(9) Å and c = 11.138(3) Å at room temperature with two formula units in the unit cell. The compound is highly insulating, and powder neutron diffraction measurements reveal long-range antiferromagnetic order with a propagation vector k = (1/3, 1/3, 1/2). The magnetic scattering from a powder sample can be modeled starting from a 120° arrangement of Co2+ spin vectors in the triangular planes and then applying a canting out of the planes which can be modeled in the magnetic space group Ccc (space group 9.40 in the Belov, Neronova, and Smirnova (BNS) scheme) with Co2+ moments of 2.72(5) μB. The antiferromagnetic structure of the recently reported compound BaCoSO, which has a very different crystal structure from CaCoSO, is also described, and this magnetic structure and the magnitude of the ordered moment (2.75(2) μB) are found by experiment to be similar to those predicted computationally.

Lithiation of hydrothermally synthesized Li1–xFex(OH)Fe1–ySe turns on high-temperature superconductivity when iron ions are displaced from the hydroxide layers by reductive lithiation to fill the vacancies in the iron selenide layers. Further lithiation results in reductive iron extrusion from the hydroxide layers, which turns off superconductivity again as the stoichiometric composition Li(OH)FeSe is approached. The results demonstrate the twin requirements of stoichiometric FeSe layers and reduction of Fe below the +2 oxidation state as found in several iron selenide superconductors.

The structural complexity of the antiferromagnetic oxide selenide CaFeSeO is described. The compound contains puckered FeSeO layers composed of FeSe2O2 tetrahedra sharing all their vertexes. Two polymorphs coexist that can be derived from an archetype BaZnSO structure by cooperative tilting of the FeSe2O2 tetrahedra. The polymorphs differ in the relative arrangement of the puckered layers of vertex-linked FeSe2O2 tetrahedra. In a noncentrosymmetric Cmc21 polymorph (a = 3.89684(2) Å, b = 13.22054(8) Å, c = 5.93625(2) Å) the layers are related by the C-centering translation, while in a centrosymmetric Pmcn polymorph, with a similar cell metric (a = 3.89557(6) Å, b = 13.2237(6) Å, c = 5.9363(3) Å), the layers are related by inversion. The compound shows long-range antiferromagnetic order below a Neél temperature of 159(1) K with both polymorphs showing antiferromagnetic coupling via Fe–O–Fe linkages and ferromagnetic coupling via Fe–Se–Fe linkages within the FeSeO layers. The magnetic susceptibility also shows evidence for weak ferromagnetism which is modeled in the refinements of the magnetic structure as arising from an uncompensated spin canting in the noncentrosymmetric polymorph. There is also a spin glass component to the magnetism which likely arises from the disordered regions of the structure evident in the transmission electron microscopy.

High-resolution X-ray and neutron powder diffraction are used to reveal details of the spin-reorientation transition in the layered oxide pnictide CeMnAsO. Above 38 K, the localized moments on Mn2+ are antiferromagnetically ordered in a checkerboard fashion within the antifluorite-type MnAs planes and are oriented perpendicular to the planes. Below 38 K, reorientation of these moments into the planes commences. This is complete by 34 K and is coincident with long-range ordering of the Ce3+ moments. The Ce3+ and Mn2+ moments have an arrangement that is different in detail from that in the isostructural NdMnAsO and PrMnSbO. There is no evidence for structural distortion, as found for PrMnSbO and related Pr3+-containing compounds, although there is evidence for a very slight (0.025%) misfit between the magnetic and structural cells below the spin-reorientation transition. It is clarified that neutron powder diffraction methods are unable to distinguish between collinear and noncollinear arrangements of manganese and lanthanide moments when the moments have a component parallel to the MnAs planes. A proposal from computational analysis that NdMnAsO and CeMnAsO should adopt different magnetic structures on the basis of the different balances between biquadratic and antisymmetric exchange interactions should be tested using alternative methods.

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li1–xFex(OH)Fe1–ySe (x ∼ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li1–xFex(OH) reservoir layer to fill vacancies in the selenide layer.

The development of a technique for following in situ the reactions of solids with alkali metal/ammonia solutions, using time-resolved X-ray diffraction methods, reveals high-temperature superconducting ammonia-rich intercalates of iron selenide which reversibly absorb and desorb ammonia around ambient temperatures.

The series Sr2MnO2Cu1.5(S1–xSex)2 (0 ≤ x ≤ 1) contains mixed-valent Mn ions (Mn2+/Mn3+) in MnO2 sheets which are separated by copper-deficient antifluorite-type Cu2−δCh2 layers with δ ∼ 0.5. The compounds crystallize in the structure type first described for Sr2Mn3Sb2O2 and are described in the I4/mmm space group at ambient temperatures. Below about 250 K, ordering between Cu+ ions and tetrahedral vacancies occurs which is long-range and close to complete in the sulfide-containing end member of the series Sr2MnO2Cu1.5S2 but which occurs over shorter length scales as the selenide content increases. The superstructure is an orthorhombic 2√2a × √2a × c expansion in Ibam of the room temperature cell. For x > 0.3 there are no superstructure reflections evident in the X-ray or neutron diffraction patterns, and the I4/mmm description is valid for the average structure at all temperatures. However, in the pure selenide end member, Sr2MnO2Cu1.5Se2, diffuse scattering in electron diffractograms and modulation in high resolution lattice image profiles may arise from short-range Cu/vacancy order. All members of the series exhibit long-range magnetic order. In the sulfide-rich end member and in compounds with x < 0.1 in the formula Sr2MnO2Cu1.5(S1–xSex)2, which show well developed superstructures due to long-range Cu/vacancy order, the magnetic structure has a (1/41/4 0) propagation vector in which ferromagnetic zigzag chains of Mn moments in the MnO2 sheets are coupled antiferromagnetically in an arrangement described as the CE-type magnetic structure and found in many mixed-valent perovskite and Ruddlesden–Popper type oxide manganites. In these cases the magnetic cell is an a × 2b × c expansion of the low temperature Ibam structural cell. For x ≥ 0.2 in the formula Sr2MnO2Cu1.5(S1–xSex)2 the magnetic structure has a (0 0 0) propagation vector and is similar to the A-type structure, also commonly adopted by some perovskite-related manganites, in which the Mn moments in the MnO2 sheets are coupled ferromagnetically and long-range antiferromagnetic order results from antiferromagnetic coupling between planes. In the region of the transition between the two different structural and magnetic long-range ordering schemes (0.1 < x < 0.2) the two magnetic structures coexist in the same sample. The evolution of the competition between magnetic ordering schemes and the length scale of the structural order with composition in Sr2MnO2Cu1.5(S1–xSex)2 suggest that the changes in magnetic and structural order are related consequences of the introduction of chemical disorder.Keywords: magnetic order; manganite; oxychalcogenide; vacancy order;

The antiferromagnetic structures of the layered oxychalcogenides (Sr1−xBax)2CoO2Cu2S2 (0 ≤ x ≤ 1) have been determined by powder neutron diffraction. In these compounds Co2+ is coordinated by four oxide ions in a square plane and two sulfide ions at the apexes of an extremely tetragonally elongated octahedron; the polyhedra share oxide vertexes. The magnetic reflections present in the diffraction patterns can in all cases be indexed using a √2a × √2a × c expansion of the nuclear cell, and nearest-neighbor Co2+ moments couple antiferromagnetically within the CoO2 planes. The ordered magnetic moment of Co2+ in Sr2CoO2Cu2S2 (x = 0) is 3.8(1) μB at 5 K, consistent with high-spin Co2+ ions carrying three unpaired electrons and with an additional significant unquenched orbital component. Exposure of this compound to moist air is shown to result in copper deficiency and a decrease in the size of the ordered moment to about 2.5 μB; there is a strong correlation between the size of the long-range ordered moment and the occupancy of the Cu site. Both the tetragonal elongation of the CoO4S2 polyhedron and the ordered moment in (Sr1−xBax)2CoO2Cu2S2 increase with increasing Ba content, and in Ba2CoO2Cu2S2, which has Co2+ in an environment that is close to purely square planar, the ordered moment of 4.5(1) μB at 5 K is over 0.7 μB larger than that in Sr2CoO2Cu2S2, so the unquenched orbital component in this case is even larger than that observed in octahedral Co2+ systems such as CoO. The experimental observations of antiferromagnetic ground states and the changes in properties resulting from replacement of Sr by Ba are supported by ab initio calculations on Sr2CoO2Cu2S2 and Ba2CoO2Cu2S2. The large orbital moments in these systems apparently result from spin−orbit mixing of the unequally populated dxz, dyz, and dz2 orbitals, which are reckoned to be almost degenerate when the CoO4S2 polyhedron reaches its maximum elongation. The magnitudes of the ordered moments in high-spin Co2+ oxide, oxychalcogenide, and oxyhalide systems are shown to correlate well with the tetragonal elongation of the coordination environment. The large orbital moments lead to an apparently magnetostrictive distortion of the crystal structures below the Neél temperature, with the symmetry lowered from tetragonal I4/mmm to orthorhombic Immm and the size of the distortion correlating well with the size of the long-range ordered moment for all compositions and for temperature-dependent data gathered on Ba2CoO2Cu2S2.

The synthesis and evolution of the structure, magnetic ordering, and superconductivity of the layered iron arsenides Sr1-xNaxFe2As2 is reported. In the Sr1-xNaxFe2As2 solid solution, the limiting Na-rich composition in samples made using conventional solid state synthesis at elevated temperatures occurs at an unusually small value (x = 0.4) compared with other alkali-metal-doped alkaline earth iron arsenides. Above this limiting value of the sodium content, competing phases are formed: for x = 0.42, an elemental iron impurity is evident, and additional impurities appear for x > 0.42. Superconductivity is detected in the compositions approaching the phase limit (Tc = 26 K for x = 0.4) in line with analogous isoelectronic materials. However, the magnetically ordered state which competes with the superconducting state appears not to be completely suppressed even at the limiting composition. The Na doping of SrFe2As2 is contrasted with the K-doping of SrFe2As2 and Na-doping of BaFe2As2 and other “122” iron arsenide compounds.

The response of the superconducting state and crystal structure of LiFeAs to chemical substitutions on both the Li and the Fe sites has been probed using high-resolution X-ray and neutron diffraction measurements, magnetometry, and muon-spin rotation spectroscopy. The superconductivity is extremely sensitive to composition: Li-deficient materials (Li1−yFe1+yAs with Fe substituting for Li) show a very rapid suppression of the superconducting state, which is destroyed when y exceeds 0.02, echoing the behavior of the Fe1+ySe system. Substitution of Fe by small amounts of Co or Ni results in monotonic lowering of the superconducting transition temperature, Tc, and the superfluid stiffness, ρs, as the electron count increases. Tc is lowered monotonically at a rate of 10 K per 0.1 electrons added per formula unit irrespective of whether the dopant is Co and Ni, and at higher doping levels superconductivity is completely suppressed. These results and the demonstration that the superfluid stiffness in these LiFeAs-derived compounds is higher than in all of the iron pnictide materials underlines the unique position that LiFeAs occupies in this class.

The evolution of the structure, magnetic ordering, and superconductivity in the series Ba1−xNaxFe2As2 is reported up to the limiting Na-rich composition with x = 0.6; the more Na-rich compositions are unstable at high temperatures with respect to competing phases. The magnetic and superconducting behaviors of the Ba1−xNaxFe2As2 members are similar to those of the better-investigated Ba1−xKxFe2As2 analogues. This is evidently a consequence of the quantitatively similar evolution of the structure of the FeAs layers in the two series. In Ba1−xNaxFe2As2 antiferromagnetic order and an associated structural distortion are evident for x ≤ 0.35 and superconductivity is evident when x exceeds 0.2. For 0.4 ≤ x ≤ 0.6 bulk superconductivity is evident, and the long-range antiferromagnetically ordered state is completely suppressed. The maximum Tc in the Ba1−xNaxFe2As2 series, as judged by the onset of diamagnetism, is 34 K in Ba0.6Na0.4Fe2As2. Despite the large mis-match in sizes between the two electropositive cations which separate the FeAs layers, there is no evidence for ordering of these cations on the length scale probed by electron diffraction.

A new layered iron arsenide NaFeAs isostructural with the superconducting lithium analogue displays evidence for the coexistence of superconductivity and magnetic ordering.

The reported anomalously small cell volume and magnetic moment of the layered oxysulfide CeCuOS are apparently the results of copper deficiency arising from exposure to ambient moist air; the stoichiometric compound does exist and is a well-behaved CeIII compound.

The oxysulfides Ba2Mn2O4Cu0.9S and Ba1.3Sr0.7Mn2O4CuS are reported. The compounds crystallize in space group P4/nmm with 2 formula units in the unit cell and lattice parameters a = 3.9916(1) Å and c = 19.5628(5) Å for a refined composition of Ba2Mn2O4Cu0.89(1)S and a = 3.9595(1) Å and c = 19.2227(3) Å for Ba1.32(3)Sr0.68(4)Mn2O4Cu0.98(2)S determined, respectively, from powder neutron and X-ray single-crystal diffraction measurements. The structure consists of alkaline earth Manganite slabs composed of MnO5 square-based pyramids sharing edges and vertexes, which are separated by CuS antifluorite-type puckered layers. The central part of the alkaline earth manganite slab is composed of MnO5 square-based pyramidal units sharing all four of their basal edges with neighboring units in the basal plane and sharing their apical vertices with those of square-based MnO5 pyramids in the outer part of the alkaline earth manganite slab. This is similar to a structural fragment in the as-yet-unrealized strontium iron oxide Sr4Fe6O12, which is the oxide-poor relative of the anion-conducting Sr4Fe6O13±δ phases. The intrinsic copper deficiency in semiconducting Ba2Mn2O4Cu0.9S suggests a mean manganese oxidation state of +2.55. The results of magnetic susceptibility measurements and low-temperature neutron diffraction results suggest that some moments participate in long-range antiferromagnetic order, whereas others give rise to spin-glass-like behavior.

Lithium iron arsenide phases with compositions close to LiFeAs exhibit superconductivity at temperatures at least as high as 16 K, demonstrating that superconducting [FeAs]− anionic layers with the anti-PbO structure type occur in at least three different structure types and with a wide range of As–Fe–As bond angles.

A series of layered oxychalcogenide and oxypnictide solids is described that contain oxide layers separated by distinct layers, which contain the softer chalcogenide (S, Se, Te) or pnictide (P, As, Sb, Bi) anions. The relationships between the crystal structures adopted by these compounds are described, and the physical and chemical properties of these materials are related to the structures and the properties of the elements. The properties exhibited by the oxychalcogenide materials include semiconductor properties, for example, in LaOCuCh (Ch = chalcogenide) and derivatives, unusual magnetic properties exhibited by the class Sr2MO2Cu2−δS2 (M = Mn, Co, Ni), and redox properties exhibited by the materials Sr2MnO2Cu2m−0.5Sm+1 (m = 1−3) and Sr4Mn3O7.5Cu2Ch2 (Ch = S, Se). Recent results in the oxychalcogenide area are reviewed, and some new results on the intriguing series of compounds Sr2MO2Cu2−δS2 (M = Mn, Co, Ni) are reported. Oxypnictides have received less recent attention, but this is changing: a new frenzy of research is underway following the discovery of high-temperature superconductivity (>40 K) in derivatives of the layered oxyarsenide LaOFeAs. The early results in this exciting new area will be reviewed.

All of the copper in the layered oxysulfides Sr2MnO2Cu1.5S2 and Sr2MnO2Cu3.5S3 may be extruded as the element and the copper ions replaced quasi-reversibly by lithium ions in reductive topotactic ion exchange reactions; dramatic changes in magnetic properties result.

NaSnN with the non-centrosymmetric layered structure type of KSnAs and featuring the new layered Zintl ion [SnN]− is the first example of a ternary nitride containing Sn–N bonds, and the first example of a nitride containing formally divalent tin.

The layered nitride Sr11Ge4N6 contains Ge4− Zintl anions in both [Sr4Ge]4+ layers and [GeN2Sr7]4+ antiperovskite-type slabs which are separated by sheets of bent [GeIIN2]4− ions; the observed range of formal germanium oxidation states in nitrides thus extends between +4 and −4.

Sr4Mn2Cu5O4S5 contains manganese oxide sheets separated by unusual antifluorite-type Cu3S3 layers in which copper(I) ions are distributed over three- and four-coordinate sites in a similar fashion to in α-Cu2−xS and suggestive of high two-dimensional copper ion mobility.

Sodium may be topotactically inserted into the perovskite layers (under thermodynamic control) or the rock-salt layers (under kinetic control) of the cation-deficient n = 2 Ruddlesden–Popper oxysulfides Ln2Ti2O5S2 with concomitant reduction of TiIV.

Lithium iron arsenide phases with compositions close to LiFeAs exhibit superconductivity at temperatures at least as high as 16 K, demonstrating that superconducting [FeAs]− anionic layers with the anti-PbO structure type occur in at least three different structure types and with a wide range of As–Fe–As bond angles.

A new layered iron arsenide NaFeAs isostructural with the superconducting lithium analogue displays evidence for the coexistence of superconductivity and magnetic ordering.