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

Abstract

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\ifmmode^\circ\else\textdegree\fi{}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\ifmmode\pm\else\textpm\fi{}20)\ifmmode^\circ\else\textdegree\fi{}K] and NdAl${\mathrm{O}}_{3}$ [at (1640\ifmmode\pm\else\textpm\fi{}30) \ifmmode^\circ\else\textdegree\fi{}K] is presented here for the first time. In each material, the phase transition is due to the collapse of the ${\ensuremath{\Gamma}}_{25}$ mode at the $R$ point of the high-temperature cubic Brillouin zone. Modes associated with ${\ensuremath{\Gamma}}_{25}$ and ${\ensuremath{\Gamma}}_{15}$ are observed in the trigonal phase in each crystal.