EVOLUTION OF THE STRUCTURE OF THE K2NiF4 PHASES La2-xSrxNiO4+delta WITH OXIDATION STATE: OCTAHEDRAL DISTORTION AND PHASE SEPARATION (0.2 less than or equal to x less than or equal to 1.0)

摘要

The structural properties of the La2-xSrxNiO4+delta solid solution have been investigated by high resolution powder neutron diffraction as a function of Sr dopant concentration. There is a significant change in the evolution of the shape of the NiO6 octahedron beyond x=0.6, where previous transport measurements have indicated a transition in the electronic structure. For the x=1 member of the series prepared under conditions often adopted in the literature, a subtle inhomogeneity is visible in the high-resolution powder diffraction patterns and interpreted as arising from a distribution of La/Sr ratios throughout the sample under these widely used preparative conditions. The refined model, incorporating both strain broadening and Bragg peak displacement, can be interpreted as a continuum of phases with differing c, but similar a, lattice parameters. Preparation of LaSrNiO4+delta at 1450 C appears to overcome this effect but Rietveld refinement is only possible with the aid of hkl dependent line broadening analysis, in contrast to the other members of the series studied. 57 refs.