Raman and Infrared Phonon Spectra of Novel Nonlinear Optical Materials PbGa2GeS6 and PbGa2GeSe6: Experiment and Theory

Phonon spectra of two novel nonlinear optical crystals, PbGa2GeSe6 and PbGa2GeS6, are studied experimentally by Raman and infrared reflection spectroscopy and theoretically by density functional theory (DFT) lattice dynamics calculations. The experimental peak frequencies are compared with calculated vibration frequencies, and a very reasonable agreement is established. A comparative analysis of results for PbGa2GeSe6 and PbGa2GeS6 with earlier reported data on relevant binary and ternary metal chalcogenides clearly indicates that the similarity of their phonon spectra stems from the presence of GeS4 and GeSe4 tetrahedra as structural building blocks. The phonon spectra of novel nonlinear optical crystals PbGa2GeSe6 and PbGa2GeS6 are studied by Raman and infrared spectroscopy and density functional theory (DFT) calculations. Based on a good agreement between experimental and calculated peak frequencies and comparison with reported data on related binary and ternary chalcogenides, it is concluded that the phonon spectra are determined by GeS4 and GeSe4 tetrahedra as structural building blocks.