Ab initio calculations of conduction band effective mass parameters of thermoelectric Mg 2X 1-xY x (X, Y = Si, Ge, Sn) alloys

Abstract

Since there are still research interests in the physical properties of quasi-binary thermoelectric Mg 2X 1-xY x alloys, with X, Y = Si, Ge, Sn, we present an ab initio analysis that yields the relative formation energy and effective masses of the conduction bands, in the whole compositional range x. We base our calculations on the full-relativistic Korringa, Kohn and Rostocker (KKR) Green’s functions formalism within the coherent potential approximation (CPA). Formation energies, measured relative to the end Mg 2X compounds, show no excess energy for the Mg 2Si - Mg 2Ge substitution thus indicating a complete solubility. In contrast, concave and asymmetric formation energies for intermediate compositions in the Mg 2X - Mg 2Sn alloys manifest a miscibility gap. With this basis, we compute and discuss the crossing of the conduction bands observed in n-type Mg 2X 1-xSn x materials. We present direction- and band-dependent effective masses using a generalized single parabolic band effective mass approximation to discuss anisotropic effects, to interpret available experimental and theoretical data, and to predict intermediate and not yet published transport parameters on these alloys.