Determination of the optical pseudogap in disordered phases with argyrodite structure

This paper is aimed to the determination of the optical pseudogap values in phases with argyrodite structure by a simple method of diffuse reflectance spectroscopy. Samples of (Cu1-xAgx)7PSe6 (x = 0, 0.2, 0.3, 0.4, 0.6, 0.7, 0.9, 1.0) solid solutions were obtained in the form of microcrystalline powders by grinding in agate mortar. The diffuse reflectance spectra of the obtained (Cu1-xAgx)7PSe6 samples were studied in the spectral range 220–1400 nm (293 K) and analyzed using the Kubelka-Munk function. It was found that for (Cu1-xAgx)7PSe6 solid solutions the increase in the copper amount causes a shift of the beginning of the absorption edge to the region of higher energies. The optical pseudogap of (Cu1-xAgx)7PSe6 solid solutions was estimated by combining the Kubelka-Munch theory and the Tauc equation. It is established that the isovalent cationic substitution of Ag+ → Cu+ leads to a monotonic decrease in the values of the optical pseudogap, as a result, the compositional dependence of which is close to linear. This behavior of the optical pseudogap can only be associated with a decrease in the lattice parameters and characterizes the formation of solid solutions.