Phase Transitions in Perovskitelike Compounds of the Rare Earths

Abstract

High-temperature powder x-ray diffraction photography and differential thermal analysis give 1643 K for the rhombohedral-to-cubic transition temperature of PrAl${\mathrm{O}}_{3}$; for NdAl${\mathrm{O}}_{3}$, this transition temperature is estimated to be 2020 K. The 800 K transition temperature of LaAl${\mathrm{O}}_{3}$ is further confirmed. A rhombohedral-to-cubic transition temperature of 1230 K is predicted for CeAl${\mathrm{O}}_{3}$. The orthorhombic-to-rhombohedral transition temperatures of LaFe${\mathrm{O}}_{3}$, LaGa${\mathrm{O}}_{3}$, and SmAl${\mathrm{O}}_{3}$ have been corroborated; these are 1260, 1150, and 1075 K, respectively. Ruiz et al. have reported an orthorhombic-to-rhombohedral transition in LaCr${\mathrm{O}}_{3}$ at 550 K, and the rhombohedral-to-cubic transition near 1300 K. The former occurs actually at 533\ifmmode\pm\else\textpm\fi{}3 K, the latter above 1873 K and probably near 2000 K.