Anionic framework descriptors and microstructure affects on optical parameters of Ag7+x(P1-xGex)S6 single crystals

Abstract

Herein we present a detailed study of single crystalline samples of Ag7+x(P1-xGex)S6 (x = 0, 0.1, 0.25, 0.33, 0.5, 0.75, 1) solid solutions. The Ag7+x(P1-xGex)S6 crystals were grown by directional crystallization from melt and characterized by techniques of XRD, Raman scattering, optical transmission measurements, spectral ellipsometry and electron microscopy. XRD and Raman scattering indicates a formation of solid solutions. The spectral dependences of refractive index are nonlinear with a maximum at the spectral range of ∼645–820 nm. It has been established, that P5+→Ge4+ cationic substitution leads to a monotonic nonlinear increase in the refractive index n from 2.58 (x = 0) to 2.75 (x = 1). The obtained values of refractive index were used to describe the optical parameters by the Wemple-DiDomenico equation. The spectral dependences of lnα at all studied temperatures (77–300 K) have an exponential shape near the absorption edge, indicating that they obey Urbach's rule. The corresponding value of the pseudo-gap Eg* and Urbach energy EU were determined. Increasing the Ge content in solid solutions causes a decrease in Eg* (1.971–1.466 eV) and EU (179.46–21.90 meV) values. Revealed Eg* values suggest that single crystalline Ag7+x(P1-xGex)S6 might be used as perspective solar harvesting materials. The practical application of these materials can be further enhanced with the results of temperature studies of variations of Eg* values. The changes in optical parameters are explained based on the structural and microstructural parameters of the studied solid solutions.