High-Temperature Raman Study of Samarium Aluminate

Raman spectra of an oriented single crystal of SmAl${\mathrm{O}}_3$ have been obtained at temperatures from 10 to 970°K. The temperature dependences of phonon frequencies and scattering cross sections are correlated with structural changes occurring at elevated temperatures. Three phonon modes are observed to soften (decrease) in frequency as the temperature is increased. The hypothesis is put forth that these soft modes are not dynamically associated with the known ${V_h}^{16}\to D_{3d}$ phase transition at ∼1100°K, but rather with a previously unsuspected ${D_{3d}}^6\to {O_h}^1$ phase transition near the melting point. The soft-mode frequencies are discussed in terms of an anharmonic potential, and it is shown that the sign of a quartic term is opposite that of LaAl${\mathrm{O}}_3$, PrAl${\mathrm{O}}_3$, and NdAl${\mathrm{O}}_3$.