Investigation of the thermoelectric properties of the Zintl RbZn4As3 material from first principles

Today, when the return to renewable energy systems has begun, the conversion of waste heat energy into electrical energy by thermoelectric materials can also be considered as renewable energy. In order to develop these systems, research is being conducted to increase the efficiency of thermoelectric materials. The performance of a thermoelectric material is described by ZT. This value is directly related to the Seebeck coefficient and electrical conductivity and inversely related to the thermal conductivity. The ability of some newly discovered Zintl phase compounds to have low thermal conductivity and high electrical conductivity has increased the interest in research in this direction. In this work, the tetragonal RbZn4As3 material with KCu4As3 prototype from the Zintl phase family is theoretically investigated. The structural, electronic and lattice dynamical properties of the tetragonal RbZn4As3 material are calculated on the basis of Density Functional Theory (DFT) using the generalized gradient approximation (GGA) and Hubbard correction (GGA+U). Equilibrium lattice parameters, bulk modulus, electronic band structures, electronic density of states, phonon dispersion curves and phonon density of states were obtained and compared with other existing experimental and theoretical studies. The material was found to have the direct band gap, with the GGA approximation giving a negative band gap, the GGA+U approximation giving a band gap of 0,15 eV and the hybrid functionals giving a band gap of 0,63 eV. Thermal transport properties were obtained by solving the phonon Boltzmann transport equation and it was found that strong acoustic-optical phonon-phonon interactions lead to low lattice thermal conductivity. It is also found that the lattice thermal conductivity of the material is direction dependent. The thermoelectric properties were calculated using the semi-classical Boltzmann transport equation under the constant relaxation time approximation. As a result of the calculations, the Seebeck coefficients obtained with p-type doping were significantly higher than those obtained with n-type doping. Although n-type doping exhibits higher electrical conductivity, its thermoelectric performance was lower than that of p-type doping due to low Seebeck coefficient and high electronic thermal conductivity. In this study, Zintl RbZn4As3 material was found to have low thermal conductivity and high Seebeck coefficient for p-type doping, resulting in high performance ZT values at moderate and high temperatures. The obtained results show that it can be a pioneer for promising thermoelectric materials.