Two structural phase transitions in PrAl${\mathrm{O}}_3$ have been observed at 205 and 151°K, and determined to be of first and second order, respectively. The electric and magnetic susceptibilities, measured as a function of temperature, exhibit a step discontinuity at the 205°K transition, while only a change in slope is observed at the 151°K transition. Spectroscopic studies of single-crystal PrAl${\mathrm{O}}_3$ in the region 4.2-300°K indicate a strong dependence of the crystalline energy levels on temperature. The observed variation in the magnetic susceptibility with temperature is consistent with that of the energy levels. There are indications that the crystal instability is related to the ${\mathrm{Pr}}^{3+}$-${\mathrm{Pr}}^{3+}$ interaction.

• Received 31 December 1968

DOI:https://doi.org/10.1103/PhysRev.186.476