Impact of Structure on the Vibrational Properties of Ag8SnSe6 Crystal: Experimental and Theoretical Studies

The crystal structure of Ag8SnSe6 argyrodite, grown by the direct melting from the elemental Ag, Sn, and Se in the ampoule, was determined by the powder X-ray diffraction and shown to be the orthorhombic structure with the space group Pmn21 (no. 31). Structure data, atomic coordinates, and interatomic distances have been obtained by the Rietveld method. Lattice dynamics of Ag8SnSe6 argyrodite are studied by the Raman scattering excited at high absorbance spectral range and first-principles calculations. Based on the comparison of frequencies from results of density functional lattice dynamics calculations and polarization properties observed in the spectra the vibrational modes are assigned to the lattice eigenmodes. A satisfactory agreement between theoretical calculations and experimental measurements is established. The SnSe4 tetrahedral groups were found to be responsible for the main (highest intensity) phonon vibration.