Lattice dynamics and electronic properties of a BaGa$_{2}$GeS$_{6}$ nonlinear crystal: Raman scattering, IR reflection, and ab initio calculations

We have measured the polarized Raman scattering and IR reflection spectra of a BaGa$_{2}$GeS$_{6}$ crystal, the Ga and Ge atoms of which are disordered, randomly occupying crystallographic positions of the same kind. The IR reflection spectra have been processed by the dispersion analysis method. The dispersion of phonons, the density of phonon states, the electronic band structure, and the partial densities of electronic states have been calculated in the DFT approximation using the alchemical potential method for disordered Ga and Ge atoms. It has been found that the stretching vibrations in the vibrational spectra are separated from the bending modes by a gap of about 100 cm$^{-1}$ and that the band structure of the crystal has an indirect energy gap.