Infrared Absorption Spectrum of the Silicate Ion in the Garnet Structure

Abstract

Infrared absorption bands at 912 and 861 cm−1 exhibited by some crystals of yttrium iron garnet and similar bands exhibited by some rare earth iron garnet crystals have been identified as vibrational fundamentals of a silicate ion impurity. The bands are assigned to split components of a triply degenerate vibration (ν3) of the tetrahedral SiO4 ion. The splitting of this vibration into a nondegenerate and a doubly degenerate vibration is attributed to distortion of the silicate ion along an S4 axis by the garnet structure. Integrated absorption measurements identify the 912 cm−1 band as the nondegenerate component. Information obtained from the isolated silicate ion in yttrium iron garnet is used to interpret the spectra of the silicate garnets. Complete removal of the degeneracy of vibrations arising from ν3 is observed in the silicate garnets, suggesting that the majority of the silicate ions possess a local symmetry lower than that predicted by the assigned space group Ia3d—Oh10.