${\mathrm{Mg}}_2\mathrm{Si}$ and ${\mathrm{Mg}}_2\mathrm{Sn}$ are indirect band gap semiconductors with two low-lying conduction bands (the lower mass and higher mass bands) that have their respective band edges reversed in the two compounds. Consequently, for some composition $x$, ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ solid solutions must display a convergence in energy of the two conduction bands. Since ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ solid solutions are among the most prospective of the novel thermoelectric materials, we aim on exploring the influence of such a band convergence (valley degeneracy) on the Seebeck coefficient and thermoelectric properties in a series of ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ solid solutions uniformly doped with Sb. Transport measurements carried out from 4 to 800 K reveal a progressively increasing Seebeck coefficient that peaks at $x=0.7$. At this concentration the thermoelectric figure of merit $ZT$ reaches exceptionally large values of 1.3 near 700 K. Our first principles calculations confirm that at the Sn content $x\approx 0.7$ the two conduction bands coincide in energy. We explain the high Seebeck coefficient and $ZT$ values as originating from an enhanced density-of-states effective mass brought about by the increased valley degeneracy as the two conduction bands cross over. We corroborate the increase in the density-of-states effective mass by measurements of the low temperature specific heat. The research suggests that striving to achieve band degeneracy by means of compositional variations is an effective strategy for enhancing the thermoelectric properties of these materials.

• Received 16 September 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.166601