Electronic band structure, stability and thermoelectric performances of two phases of Cu2S: ab initio PAW approach

Abstract

The electronic band structure calculations for β- and γ-phases of copper sulfide Cu₂S were performed by using the LDA + U and hybrid exchange-correlation potential approximations. The mechanical stability of both phases have been confirmed via calculations of elasticity constants. Based on the phonon spectra calculations it has been shown that the β-phase is dynamically unstable. This favours the transitions of copper atoms from positions inside the Cu₂S layers into the interlayer spacing. Within the framework of the Boltzmann-Onsager theory, considering the dependence of the electron relaxation time on the electron energy and wave vector, the calculations were performed for the electric conductivity, Seebeck coefficient, power factor and figure-of-merit. The concentration dependence of the figure-of-merit at a high amount of p-type carriers is shaped as an extremum curve because of the balance between a temperature-caused increase in the electrical conductivity and a decrease in the Seebeck coefficient. The maximum of figure-of-merit can be reached at a carrier concentration of 10²¹ 1/cm³ approximately.