The pressure dependence of the optical-phonon frequencies in berlinite (${\mathrm{AlPO}}_4$) and α-quartz has been investigated by high-pressure Raman scattering experiments, using a diamond cell; berlinite was studied up to 12 GPa and α-quartz to 20 GPa. The $A_1$ soft phonon mode at 216 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{AlPO}}_4$ and the corresponding mode in α-quartz at 206 ${\mathrm{cm}}^{\mathrm{-}1}$ 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 $A_1$ soft mode phonon with the two-phonon (acoustic) states in α-quartz and with one-phonon (optical) and two-phonon (acoustic) states in ${\mathrm{AlPO}}_4$. Pressure experiments help in distinguishing the relative strength of these interactions in the two materials. The mode-Grüneisen parameters have been obtained and, again, the $A_1$ soft mode has a large ${\gamma }_i$, namely, 3.45 in ${\mathrm{AlPO}}_4$ and 4.32 in α-quartz. Using refractive-index matching with α-quartz and ${\mathrm{AlPO}}_4$, the n(P) of a 4:1 methanol-ethanol mixture has been evaluated.

• Received 10 October 1986

DOI:https://doi.org/10.1103/PhysRevB.35.8316