Guo Liu

Neutron diffraction studies of the crystal and magnetic structure of Nd1.5Sr0.5NiO4

The crystal structure and magnetic ordering of Nd1.5Sr0.5NiO4 which shows a susceptibility maximum at 17 K have been investigated by neutron diffraction. A tetragonal K2NiF4-type structure is confirmed at room temperature with a = 3.7756(3) A and c = 12.485(1) A. A mean NiO distance of 1.986(5) A reflects the presence of equal concentrations of Ni2+ and Ni3+. Low temperature neutron diffraction data to 11 K reveal features not apparent in the bulk susceptibility. Below 50 K a broad, diffuse reflection at Q = 0.835 A−1 is found which can be indexed as (1200) relative to the chemical cell. The intensity of this peak has an unusual temperature dependence with abrupt increases between 25 and 21 K and between 14 and 11 K. The correlation length derived from the peak width decreases from a constant value of 20(4) A between 14 and 21 K to 12(2) A at 11 K. Also at 11 K a resolution-limited reflection appears which can be indexed as (101) on a magnetic cell with aM = √2aCH which is typical for long-range antiferromagnetic order in K2NiF4 structure compounds. The diffuse (1200) reflection is ascribed to short-range magnetic order on the Ni sublattice while the long-range order below 11 K involves most likely NdNi intersublattice coupling. Both forms of order coexist at 11 K.

Crystal structure and magnetic properties of orthorhombic Sr2VO4 with tetrahedral vanadium(IV)

Abstract A new crystalline modification of Sr2VO4, isostructural with Sr2CrO4, has been prepared by both low and high temperature methods. The space group appears to be Pna21 with a = 14.092(4) A, b = 5.806(2) A, and c = 10.106(3) A. Atomic positions have been refined by the Rietveld method from powder neutron and X-ray diffraction data. The structure is a supercell of the K2SO4-type and thus appears to be the first example of an oxide containing VO−44 polyhedra. This phase is apparently a high temperature modification stable above about 1100°C. Orthorhombic Sr2VO4 can also be prepared at low temperature (about 500°C) by reduction of Sr2VO4Cl with Na metal. Surprisingly, the magnetic susceptibility shows a broad maximum at 60 K and the data can be understood very well on a S = 1 2 dimer model. These results are compared with similar ones for electronically related Cu(II) salts.