Effect of structural site disorder on the optical properties of Ag 6+ x (P 1− x Ge x )S 5 I solid solutions

This paper presents a study of the spectral and temperature dependences of the Ag 6+ x (P 1− x Ge x )S 5 I ( x = 0.25; 0.5; 0.75) single crystals optical properties. The refractive index ( n ) and extinction coefficient ( k ) were investigated by the spectral ellipsometry method. The spectral dependences of the n and k are characterized by the presence of anomalies in the region of the optical absorption edge. The temperature (77–300 K) behavior of the optical absorption edge was studied by optical spectroscopy. The spectral dependences of the absorption coefficient have an exponential form and obey to Urbach’s rule. The corresponding values of pseudogap E g * , steepness parameter σ, and electron–phonon interaction parameter σ 0 are calculated. An increase in temperature causes a decrease in the E g * values and a monotonic increase in the parameter σ . The compositional dependences of E U and σ 0 are characterized by a presence of extreme point for Ag 6.75 (P 0.25 Ge 0.75 )S 5 I composition. Using Dow–Redfield theory, the mechanism of electron–phonon interaction (EPI) is described. Urbach’s total energy was divided into contributions of temperature-independent ( E U ) X,C and temperature-dependent ( E U ) T types of disorder using the Einstein model. It is established that in general, the heterovalent substitution P 5+ → Ge 4+ leads to a weakening of the EPI in solid solutions. The exception is the Ag 6.75 (P 0.25 Ge 0.75 )S 5 I composition, for which there is a slight increase in EPI. Peculiarities of E U and σ 0 changes in Ag 6+ x (P 1− x Ge x )S 5 I solid solutions were explained using structural descriptors. It was established that decrease in the volume of [(PGe)S 4 ] tetrahedron, for Ag 6.75 (P 0.25 Ge 0.75 )S 5 I solid solution, is one of the factors enhancing the EPI.