Structure, Magnetism, and Superconductivity of the Layered Iron Arsenides Sr_1-<i>x</i>Na_<i>x</i>Fe₂As₂

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

Two New Structurally Related Strontium Gallium Nitrides: Sr₄GaN₃O and Sr₄GaN₃(CN₂)

The strontium gallium oxynitride Sr4GaN3O and nitride−carbodiimide Sr4GaN3(CN2) are reported, synthesized as single crystals from molten sodium at 900 °C. Red Sr4GaN3O crystallizes in space group Pbca (No. 61) with a = 7.4002(1) Å, b = 24.3378(5) Å, c = 7.4038(1) Å, and Z = 8, as determined from single-crystal X-ray diffraction measurements at 150 K. The structure may be viewed as consisting of slabs [Sr4GaN3]2+ containing double layers of isolated [GaN3]6- triangular anions arranged in a “herringbone” fashion, and these slabs are separated by O2- anions. Brown Sr4GaN3(CN2) has a closely related structure in which the oxide anions in the Sr4GaN3O structure are replaced by almost linear carbodiimide [CN2]2- anions [Sr4GaN3(CN2):  space group P21/c (No. 14), a = 13.4778(2) Å, b = 7.4140(1) Å, c = 7.4440(1) Å, β = 98.233(1)°, and Z = 4]

Topotactic Oxidative and Reductive Control of the Structures and Properties of Layered Manganese Oxychalcogenides

Topotactic modification, by both oxidation and reduction, of the composition, structures, and magnetic properties of the layered oxychalcogenides Sr4Mn3O7.5Cu2Ch2 (Ch = S, Se) is described. These Mn3+ compounds are composed of alternating perovskite-type strontium manganese oxide slabs separated by anti-fluorite-type copper chalcogenide layers and are intrinsically oxide deficient in the central layer of the perovskite slabs. The systems are unusual examples of perovskite-related compounds that may topotactically be both oxidized by fluorination and reduced by deintercalation of oxygen from the oxide-deficient part of the structure. The compounds exhibit antiferromagnetic ordering of the manganese magnetic moments in the outer layers of the perovskite slabs, while the other moments, in the central layers, exhibit spin-glass-like behavior. Fluorination has the effect of increasing the antiferromagnetic ordering temperature and the size of the ordered moment, whereas reduction destroys magnetic long-range order by introducing chemical disorder which leads to both further disorder and frustration of the magnetic interactions in the manganese oxide slab

Two New Structurally Related Strontium Gallium Nitrides: Sr₄GaN₃O and Sr₄GaN₃(CN₂)

The strontium gallium oxynitride Sr4GaN3O and nitride−carbodiimide Sr4GaN3(CN2) are reported, synthesized as single crystals from molten sodium at 900 °C. Red Sr4GaN3O crystallizes in space group Pbca (No. 61) with a = 7.4002(1) Å, b = 24.3378(5) Å, c = 7.4038(1) Å, and Z = 8, as determined from single-crystal X-ray diffraction measurements at 150 K. The structure may be viewed as consisting of slabs [Sr4GaN3]2+ containing double layers of isolated [GaN3]6- triangular anions arranged in a “herringbone” fashion, and these slabs are separated by O2- anions. Brown Sr4GaN3(CN2) has a closely related structure in which the oxide anions in the Sr4GaN3O structure are replaced by almost linear carbodiimide [CN2]2- anions [Sr4GaN3(CN2):  space group P21/c (No. 14), a = 13.4778(2) Å, b = 7.4140(1) Å, c = 7.4440(1) Å, β = 98.233(1)°, and Z = 4]

Crystal and Magnetic Structures of the Oxide Sulfides CaCoSO and BaCoSO

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 <i>P</i>6₃<i>mc</i> (No. 186), and the lattice parameters are <i>a</i> = 3.7524(9) Å and <i>c</i> = 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 <i>k</i> = (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 <i>C</i>_<i>c</i><i>c</i> (space group 9.40 in the Belov, Neronova, and Smirnova (BNS) scheme) with Co²⁺ moments of 2.72(5) μ_B. 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) μ_B) are found by experiment to be similar to those predicted computationally

Topotactic Oxidative and Reductive Control of the Structures and Properties of Layered Manganese Oxychalcogenides

Topotactic modification, by both oxidation and reduction, of the composition, structures, and magnetic properties of the layered oxychalcogenides Sr4Mn3O7.5Cu2Ch2 (Ch = S, Se) is described. These Mn3+ compounds are composed of alternating perovskite-type strontium manganese oxide slabs separated by anti-fluorite-type copper chalcogenide layers and are intrinsically oxide deficient in the central layer of the perovskite slabs. The systems are unusual examples of perovskite-related compounds that may topotactically be both oxidized by fluorination and reduced by deintercalation of oxygen from the oxide-deficient part of the structure. The compounds exhibit antiferromagnetic ordering of the manganese magnetic moments in the outer layers of the perovskite slabs, while the other moments, in the central layers, exhibit spin-glass-like behavior. Fluorination has the effect of increasing the antiferromagnetic ordering temperature and the size of the ordered moment, whereas reduction destroys magnetic long-range order by introducing chemical disorder which leads to both further disorder and frustration of the magnetic interactions in the manganese oxide slab

Fragmentation of an Infinite ZnO₂ Square Plane into Discrete [ZnO₂]²⁻ Linear Units in the Oxyselenide Ba₂ZnO₂Ag₂Se₂

Fragmentation of an Infinite ZnO2 Square Plane into Discrete [ZnO2]2− Linear Units in the Oxyselenide Ba2ZnO2Ag2Se2</sub

Fragmentation of an Infinite ZnO₂ Square Plane into Discrete [ZnO₂]²⁻ Linear Units in the Oxyselenide Ba₂ZnO₂Ag₂Se₂

Fragmentation of an Infinite ZnO2 Square Plane into Discrete [ZnO2]2− Linear Units in the Oxyselenide Ba2ZnO2Ag2Se2</sub

Ba₂Mn₂O₄Cu_0.9S: A layered Oxysulfide with a New Perovskite-Related Manganese Oxide Fragment

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

Synthesis and Structure of Alkaline Earth Silicon Nitrides: BaSiN₂, SrSiN₂, and CaSiN₂

The alkaline earth silicon nitrides AESiN2 (AE = Ca, Sr, Ba) are reported, synthesized as clear, colorless, single crystals from molten sodium at 900−1100 °C or, in the cases of BaSiN2 and SrSiN2, as white powders by reacting powdered intermetallics AESi with flowing anhydrous ammonia at 550−1000 °C. Structures were determined from single-crystal X-ray diffraction measurements at 150 K:  BaSiN2 crystallizes in space group Cmca (No. 64) with a = 5.6046(1) Å, b = 11.3605(3) Å, c = 7.5851(2) Å, and Z = 8. The structure consists of pairs of SiN4 tetrahedra edge-linked to form bow-tie-shaped Si2N6 dimers which share vertexes to form layers and has no analogue in oxide chemistry. SrSiN2 has a distorted form of this structure (SrSiN2:  space group P21/c (No. 14), a = 5.9750(5) Å, b = 7.2826(7) Å, c = 5.4969(4) Å, β = 113.496(4)°, Z = 4). The structure of CaSiN2 contains only vertex-sharing SiN4 tetrahedra, linked to form a three-dimensional stuffed-cristobalite type framework isostructural with KGaO2 (CaSiN2:  space group Pbca (No. 61), a = 5.1229(3) Å, b = 10.2074(6) Å, c = 14.8233(9) Å, Z = 16)