Raman and far-infrared studies of lattice modes of piezoelectric crystal of trioxane

Abstract

Optically active lattice vibrations of trioxane (CH2O)3 crystal with the piezoelectric space group of R3c-C63v were investigated by means of Raman and far-infrared spectroscopy. As predicted by the group theory, one A1 [referred to as (A1)L, at 70 cm−1] and three E(EL1, EL2, and EL3 at 95, 90, and 65 cm−1, respectively) bands appeared with distinct dichroism in the polarized far-infrared spectra taken on a uniaxially oriented thin film cooled at liquid nitrogen temperature. Right-angle Raman scattering was measured on single-crystal specimens of various orientations at temperatures from 280 to 40 K. The lowest EL3 mode exhibited clear LO–TO doubling with a frequency gap of 6 cm−1, owing to the piezoelectric or polar character of the crystal. The angular dependence of the Raman frequency was also observed. The broad Raman band around 90 cm−1 was found to separate into two components of different polarization, assignable to the EL1 and EL2 modes. At a temperature as low as 60 K, one component (EL2) split further into a doublet due to the LO–TO splitting with a frequency gap of 4.1 cm−1.