Structure and superconductivity of LiFeAs

The lithium ions in Lithium iron arsenide phases with compositions close to LiFeAs have been located using powder neutron diffraction. These phases exhibit superconductivity at temperatures at least as high as 16 K demonstrating that superconductivity in compounds with [FeAs]- anti-PbO-type anionic layers occurs in compounds with at least three different structure types and occurs for a wide range of As-Fe-As bond angles.Comment: 3 pages, 3 figures, 3 table

Magnetic ordering in the layered oxyselenides Sr2CoO2Ag2Se2 and Ba2CoO2Ag2Se2

The antiferromagnetic structures of Sr2CoO2Ag2Se2 and Ba2CoO2Ag2Se2 are solved using powder neutron diffraction. Both compounds adopt the same magnetic structure, based on a √2a × √2a × c expansion of the nuclear cell with magnetic space group PC42/n (86.72 in the Belov-Neronova-Smirnova notation). This structure is adopted as a result of nearest-neighbour antiferromagnetic interactions within the CoO2 planes. The refined long-range-ordered magnetic moments of Sr2CoO2Ag2Se2 and Ba2CoO2Ag2Se2 are 3.7(1) and 3.97(3) μB per Co ion respectively. The refined moments are significantly greater than the value predicted from just considering the spin (3 μB); this is attributed to a significant orbital contribution to the magnetic moment in an analogous manner to that previously observed for Sr2CoO2Cu2S2 and the values conform to a relationship between the shape of the distended CoO4Ch2 (Ch = S, Se) octahedron and the size of the ordered moment established for a series of related compounds

Sr2Mn2O4Se: A New Oxychalcogenide with Antiferromagnetic Chains

The synthesis, crystal structure and magnetic properties of the brown oxide selenide Sr2Mn2O4Se are described. The compound crystallises in space group Pnma with room temperature lattice parameters a = 9.4825(1), b = 10.3356(1) and c = 6.23895(8) Å. The structure contains Mn3+ ions in highly distended MnO 4Se2 octahedra with selenide ions in the axial positions. These octahedra share two of their faces with neighbouring octahedra with the result that all the manganese and oxide ions are involved in the formation of undulating 1∞[MnO4/2]- chains directed along the a axis, which form corrugated chains along the c axis by sharing the selenide vertices of the octahedra. These chains are isolated from one another along the b axis. Magnetic susceptibility measurements show that the compound does not obey the Curie-Weiss law below room temperature and there is a magnetic transition at 160(5) K associated with antiferromagnetic ordering of the Mn 3+ moments. The magnetic structure measured at 5 K is described by a k = (0, 0, 0) propagation vector and nearest neighbour moments are coupled antiferromagnetically. Copyright © 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

Fragmentation of an infinite ZnO2 square plane into discrete [ZnO2]2- linear units in the oxyselenide Ba2ZnO2Ag2Se2.

Analysis of single crystal X-ray diffraction, neutron powder diffraction, electron diffraction and Zn-K-edge EXAFS data show that Ba2ZnO2Ag2Se2 contains unusual isolated [ZnO2]2- moieties resulting from fragmentation of a ZnO2 infinite plane placed under tension

Structures, physical properties, and chemistry of layered oxychalcogenides and oxypnictides.

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 Sr 2MO 2Cu 2-deltaS 2 (M = Mn, Co, Ni), and redox properties exhibited by the materials Sr 2MnO 2Cu 2 m-0.5 S m+1 ( m = 1-3) and Sr 4Mn 3O 7.5Cu 2Ch 2 (Ch = S, Se). Recent results in the oxychalcogenide area are reviewed, and some new results on the intriguing series of compounds Sr 2MO 2Cu 2-deltaS 2 (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