Raman Study of Trigonal-Cubic Phase Transitions in Rare-Earth Aluminates

The Raman spectra of LaAl${\mathrm{O}}_3$, PrAl${\mathrm{O}}_3$, and NdAl${\mathrm{O}}_3$ have been observed at temperatures from 10 to 1135°K. Each of these materials evidences a nominally second-order trigonal cubic phase transition; that in LaAl${\mathrm{O}}_3$ has previously been analyzed by other workers on the basis of x-ray, EPR, and inelastic neutron scattering; however, evidence for the transitions in PrAl${\mathrm{O}}_3$ [at (1320±20)°K] and NdAl${\mathrm{O}}_3$ [at (1640±30) °K] is presented here for the first time. In each material, the phase transition is due to the collapse of the ${\Gamma }_{25}$ mode at the $R$ point of the high-temperature cubic Brillouin zone. Modes associated with ${\Gamma }_{25}$ and ${\Gamma }_{15}$ are observed in the trigonal phase in each crystal.