Raman investigation of vibrational states in quartz-type GaPO4 at various temperatures

The Raman-active lattice modes in quartz-homeotypic crystal GaPO4 at low an d room temperature were investigated using a Raman spectrometer of limiting resolution 0.005 cm(-1). Particular attention was paid to the comparison o f the LO-TO splittings of the low-lying energy E modes in the series alpha- SiO2, alpha-AlPO4 and GaPO4. The linewidths and Raman shifts of the A, (ca. 460 cm(-1)) and E (TO) (ca. 117 cm(-1)) modes in GaPO4 were measured in th e temperature range 8-300 K. These data were compared with those obtained f or a-quartz and berlinite (alpha-AlPO4) for the corresponding A, and E mode s. The phonon dampings are discussed in terms of simple cubic and quartic a nharmonic processes. In particular; a dephasing process involving two acous tic phonons of mean wavenumber ca, 60 cm(-1) was assumed. The residual line widths at T --> 0 K are interpreted in terms of a random distribution of im purity sites in the crystal. (C) 1998 John Wiley & Sons, Ltd.