Analysis of crystal structure and phase transition of LaCrO₃ by various diffraction measurements

Crystal structure and structural phase transition of LaCrO₃ have been investigated by using various diffraction technique. X-ray diffraction at high temperature revealed that the crystal structure of LaCrO₃ below 240 °C and above 280 °C is orthorhombic- and rhombohedral-distorted perovskite, respectively. The discrete volume compression was observed at the phase transition from orthorhombic to rhombohedral phase. The space group of LaCrO₃ below 240 °C was determined to be Pbnm (No. 62), whereas that above 280 °C was R3̄c (No. 167) by using selected area electron diffraction and convergent-beam electron diffraction. The Rietveld refinement of neutron diffraction patterns of LaCrO₃ at various temperatures was carried out using the space group determined by the electron diffraction. It was revealed that the volume compression at the phase transition was principally due to the shrinkage of the [CrO₆] octahedra. DSC-XRD simultaneous measurements indicated that the phase transition started at about 250 °C and was completed at about 268 °C with heating rate of 5 °C/min involving absorption of heat. No intermediate phase other than orthorhombic or rhombohedral was observed at the phase transition.