Electronic Structure and Thermal Study of Ag3PS4 and Ag5PS4Cl2 from Direct to Indirect Energy Gap

Reshak AH and Sikander Azam

Abstract

The band structure and density of states of the orthorhombic Ag3PS4 and Ag5PS4Cl2 compounds has been investigated. The calculations are derived from the total-energy calculations using the Full Potential-Linearized Augmented Plane Wave (FPLAPW) method. The exchange and correlation potential is treated by the Local Density Approximation (LDA), Generalized Gradient Approximation (GGA) and Engel Vosko GGA. From the study of the band structure we confirmed that the Valence Band Maximum (VBM) and Conduction Band Minimum (CBM) of Ag3PS4 compounds situated at Г point, indicating a direct band gap. While for Ag5PS4Cl2 compound the VBM occurs along the Z–S direction and CBM at Г point exhibiting indirect band gap. The partial density of states confirm that the lower part of the valence band is dominated by S-s orbital, the mid part by P-s orbital and the VBM by the Ag-d orbital. The conduction band consists the major contribution of Ag-s/p orbital. Furthermore, the thermoelectric properties of both compounds were investigated by means of BoltzTraP code. The electrical conductivity, electrical resistivity, the power factor and Seebeck coefficient were investigated in the temperature range from 300 to 800 K. The electrical conductivity was enhanced by increasing the temperature, which leads to a small electrical resistivity and a large Seebeck coefficient. Consequently, large values of about 3.75×1011 and 1.2×1011 W/m K2 for Ag3PS4 and Ag5PS4Cl2 compounds were obtained. Therefore, the thermoelectric property of Ag3PS4 is not as good as that of Ag5PS4Cl2. Therefore, it can be concluded that Ag5PS4Cl2 is suitable for high performance thermoelectric application than Ag3PS4 at higher temperature.