Raman study of theα−βcristobalite phase transition in AIPO4

Abstract

The Raman spectra of small single crystals of the cristobalite form of aluminum phosphate (AIP${\mathrm{O}}_{4}$) have been obtained in both the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phase. Particular emphasis has been placed on the behavior of the phonons near the phase-transition temperature. The transition is first order, but the latent heat and discontinuity in lattice parameters are small enough to permit cycling of the sample through the transition without damage. A soft mode is observed which has energy 102 ${\mathrm{cm}}^{\ensuremath{-}1}$ at ambient temperatures. The observed spectra confirm the assignments of space groups $C222 ({D}_{2}^{5})$ and $I\overline{4}m2 ({D}_{2d}^{9})$ for the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases, respectively. Phonon comparisons are made with isomorphic BP${\mathrm{O}}_{4}$ and Si${\mathrm{O}}_{2}$.