Evolution of the structure of the K2NiF4 phases La2-xSrxNiO4+δ with oxidation state: Octahedral distortion and phase separation (0.2 ≤ x ≤ 1.0)

The structural properties of the La2-xSrxNiO4+δ 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+δ 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