Structural Phase Transition in (NH4)3GeF7–Raman Spectroscopy Data

Abstract

We obtained Raman spectra of (NH4)3GeF7 crystals with a wide range of frequencies (10–3400 cm−1) and temperatures (8–300 K), including phase transition, which was accompanied by a symmetry increase with a temperature decrease. The internal vibrations of the GeF62− group were classified by the positional symmetry method. Considerable transformations of the Raman spectra were observed at the lower frequency range of lattice vibrations and the ranges of the internal vibrations of the ammonium ions. In contrast, the internal modes of the GeF62− groups changed only slightly due to their resonance splitting in the multiplied unit cell, which agreed well with the proposed phase transition mechanism induced by ammonium group ordering.