Band structure and optical properties of low temperature modification of Ag 7 PS 6 single crystal

The Ag 7 PS 6 single crystal were grown by directional crystallization from the melt. Grown Ag 7 PS 6 crystallize in low temperature modification: space group P2 1 3 with cell parameters a = 10.3917 Å, Z = 4. The single crystalline sample was investigated by the means of optical ellipsometry (298 K) and spectroscopy (77–300 K). The refractive index n is characterized by increasing values ( n max at hν = 1.92 eV) when approaching the optical absorption edge, and the extinction coefficient k is characterized by the presence of an anomaly in the region of the optical absorption edge. Temperature studies of the optical absorption edge have shown an exponential form of the absorption coefficient α that obeys Urbach’s rule at all temperatures. The optical pseudo-gap E g * and the Urbach energy E U were calculated from the obtained experimental data. It is shown that E g * and E U decrease linearly with increasing temperature E g * = 2.13–1.92 eV and E U = 101–185 meV. The atypical behavior of the Urbach energy is associated with disordering at the microstructural level in Ag 7 PS 6 crystals. The experimental results are accompanied by theoretical calculation using the local-density approximation with the Hubbard correction LDA+ U. The calculated data are in good agreement with the experimental results.