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

Herein we present a detailed study of single crystalline samples of Ag7+x(P1-xGex)S-6 (x = 0, 0.1, 0.25, 0.33, 0.5, 0.75, 1) solid solutions. The Ag7+x(P1-xGex)S-6 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 similar to 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 ln alpha 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 E-g* and Urbach energy E-U were determined. Increasing the Ge content in solid solutions causes a decrease in E-g* (1.971-1.466 eV) and E-U (179.46-21.90 meV) values. Revealed E-g* values suggest that single crystalline Ag7+x(P1-xGex)S-6 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 E-g* values. The changes in optical parameters are explained based on the structural and microstructural parameters of the studied solid solutions.

Automated summary

In this study, single crystalline samples of Ag7+x(P1-xGex)S-6 (x = 0, 0.1, 0.25, 0.33, 0.5, 0.75, 1) solid solutions were investigated. The optical parameters and structural parameters of the solid solutions were analyzed, and it was found that increasing the Ge content can decrease the pseudo-gap energy and Urbach energy of the materials. This suggests that the single crystalline Ag7+x(P1-xGex)S-6 has potential applications in solar harvesting.