Single-crystal vibrational spectrum of benitoite, a non-centrosymmetric silicate crystal, and some comments on beryl

Abstract

Benitoite, BaTiSi₃O₉, is a non-centrosymmetric crystal showing pronounced LO–TO splitting. An almost complete assignment was deduced from single-crystal i.r. reflectance and Raman data. For the E′ species excellent agreement was found between i.r. and Raman observations, which were complementary. LO modes were observed directly in Raman back-scattering experiments. It is shown that the complex vibrational spectra of both benitoite and beryl are constructed from pairs of coupled ring frequencies (there are two silicate rings in each unit cell) together with Ti–O, Al–O, and Be–O stretches (as appropriate) at frequencies as high as 800 cm^–1.