Highly effective Mg$_{\mathrm{9}}$Si$_{\mathrm{5\thinspace }}$thermoelectric for mid temperature applications

Commercial acceptance of a thermoelectric device relies not only on its figure of merit (ZT), but also on its cost and environmental friendliness. In this regard, Mg$_{\mathrm{2}}$Si is a potential candidate system. However, the low solubility of substituents in Mg$_{\mathrm{2}}$Si severely restricts its optimization and applicability in the energy. Recently a new compound, Mg$_{\mathrm{9}}$Si$_{\mathrm{5}}$, had been synthesized. The material accommodates a variety of dopants with varying doping concentration. Using density functional theory based calculations~and Boltzmann~transport theory we study the electronic structure and transport properties of Mg$_{\mathrm{9}}$Si$_{\mathrm{5}}$. We find Mg$_{\mathrm{9}}$Si$_{\mathrm{5}}$ is a 0.17 eV semiconductor exhibiting appreciable characteristic properties of a mid-temperature thermoelectric. Based on an empirical estimate, we find its ZT to be approximately 1.1, at an operable temperature of 600 K.