Structural effects of Sr substitution in La2-xSrxNiO4+δ

Structures within the La2-xSrxNiO4+delta system (0 < x less than or equal to 1) were refined by Rietveld analysis of powder x-ray-diffraction data. Our results reveal three distinct regimes that parallel energetic transformations within the series. For 0 less than or equal to x < 0.2, enthalpies of oxidation are strongly exothermic and the compounds are monophasic and orthorhombic. Hole saturation occurs for 0.2 < x < 0.6, and the symmetry decreases from orthorhumbic to monoclinic. For 0.6 < x < 1.0, diffraction patterns suggest phase immiscibility between a Sr-poor, hole-saturated compound and a Sr-rich phase for which oxidation is strongly favored. Fourier synthesis maps for all compositions revealed evidence for microtwinning and positional disorder among apical oxygen atoms. [S0163-1829(98)06118-9].