High-pressure Raman study of the vibrational modes in AlPO<SUB>4</SUB> and SiO<SUB>2</SUB> (&#945;-quartz)

The pressure dependence of the optical-phonon frequencies in berlinite (AlPO4) and &#945;-quartz has been investigated by high-pressure Raman scattering experiments, using a diamond cell; berlinite was studied up to 12 GPa and &#945;-quartz to 20 GPa. The A1 soft phonon mode at 216 cm-1 in AlPO4 and the corresponding mode in &#945;-quartz at 206 cm-1 exhibit similar but anomalously large and nonlinear pressure dependences. Further, these two modes exhibit striking decreases in their widths with increasing pressure. These changes appear to be connected with the decoupling of the strong anharmonic interactions of the A1 soft mode phonon with the two-phonon (acoustic) states in &#945;-quartz and with one-phonon (optical) and two-phonon (acoustic) states in AlPO4. Pressure experiments help in distinguishing the relative strength of these interactions in the two materials. The mode-Gruneisen parameters have been obtained and, again, the A1 soft mode has a large &#947;i, namely, 3.45 in AlPO4 and 4.32 in &#945;-quartz. Using refractive-index matching with &#945;-quartz and AlPO4, the n(P) of a 4:1 methanol-ethanol mixture has been evaluated