Phase Transitions in Perovskitelike Compounds of the Rare Earths

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±3 K, the latter above 1873 K and probably near 2000 K.