Neutron diffraction study of the crystal structure and structural phase transition of La0.7Ca0.3-xSrxCrO3 (0<x<0.3). The relationship between thermodynamic behavior and crystal structure changes at the phase transition

The crystal structure of the La0.7Ca0.3-xSrxCrO3 series, including the compositional and temperature dependence of the structural parameters, has been Studied by variable temperature neutron diffraction measurements. The extent of the distortions from the ideal cubic perovskite structure has been evaluated quantitatively using the average bond lengths and the mean volumes of the [CrO6] octahedron and [(La/Ca/Sr)O-12] polyhedron, and has been shown to decrease with increase of Sr content or temperature. At the structural phase transition from the orthorhombic (Pnma) structure to the rhombohedral (R (3) over barc) one, the volume of the [CrO6] octahedron decreases whereas that of the [(La/Ca/Sr)O-12] polyhedron shows little difference, resulting in an overall decrease in the level of distortion. The change in the degree of distortion at the phase transition decreases with increase of Sr content, in agreement with the smaller variation of the enthalpy and volume for the specimens with higher Sr content.