Polarization dependence of the first-order Raman spectrum of -AlPO4

Investigating in great detail the polarization dependence of the first-order Raman spectrum of -AlPO4 (berlinite), we discuss the experimental results in terms of first-order (quartzlike) and folded-zone frequencies. We identify all Raman-active modes predicted by group-theory arguments and find, in many cases, a small renormalization of experimental values through the isoelectronic sequence -SiO2, -SiO2, -AlPO4. This demonstrates a strong molecular character which is well accounted for by a simple model calculation in which one links, progressively, three decoupled SiO4 tetrahedra to build a quartzlike unit cell. © 1991 The American Physical Society.