Polarization dependence of the first-order Raman spectrum of α-${\mathrm{AlPO}}_4$

Investigating in great detail the polarization dependence of the first-order Raman spectrum of α-${\mathrm{AlPO}}_4$ (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 β-${\mathrm{SiO}}_2$, α-${\mathrm{SiO}}_2$, α-${\mathrm{AlPO}}_4$. This demonstrates a strong molecular character which is well accounted for by a simple model calculation in which one links, progressively, three decoupled ${\mathrm{SiO}}_4$ tetrahedra to build a quartzlike unit cell.