Lattice dynamics of the ferroelectric Li₂Ge₇O₁₅ crystal

Li₂Ge₇O₁₅ is a ferroelectric and acousto-optical crystal. The lattice dynamics of the ferroelectric and paraelectric phases of the Li₂Ge₇O₁₅ crystal have been investigated using density functional theory calculations and Raman spectroscopy. The symmetry analysis of the phonon modes at the center of the Brillouin zone of both phases was performed. The phonon dispersions of both phases of the Li₂Ge₇O₁₅ crystal were simulated. The unstable modes in the paraelectric phase were determined. Temperature studies of the low-frequency region of the Raman spectra were repeated using a dynamic regime. The behavioral features of the soft mode were demonstrated, clearly showing how they depend on the orientation of the sample. The experimental behavior of the low-frequency modes has been explained. The theoretical data have been analyzed in comparison with our experimental data and the data of other authors. This study significantly improved the understanding of the lattice dynamics of both phases of the Li₂Ge₇O₁₅ crystal.