Effect of liquidlike cations on electronic and defect properties of solid solutions of Cu2Te and Ag2Te

Abstract

${\mathrm{Cu}}_{2}\mathrm{Te}$ was recently shown to be an excellent thermoelectric material with a figure of merit approaching 2. Ag alloying was found to be effective for tuning the electrical conductivity of ${\mathrm{Cu}}_{2}\mathrm{Te}$ toward high thermoelectric performance. The liquidlike behavior of cations is well-known to yield ultralow thermal conductivity in the cuprous chalcogenide materials. Here, we study the effect of the liquidlike behavior on the electronic and defect properties of the solid solutions ${\mathrm{Cu}}_{2\ensuremath{-}x}{\mathrm{Ag}}_{x}\mathrm{Te}$. Using first-principles molecular dynamics (MD) simulation, we found that the band-gap variation with increasing $x$ as obtained from MD ensemble averages is much slower than that from direct calculation using the high-symmetry ordered structures. The solid solutions within the whole range of $x$ exhibit significant band gaps. The liquidlike behavior also results in flattened top valence bands, which reduce the Seebeck coefficient and electrical conductivity. Regarding the defect properties, we found that the liquidlike behavior can fail the standard defect models employing the high-symmetry ordered structure. Again, MD simulation would be necessary for sampling the configuration space in defect calculations. However, converging the defect formation energy for the diffusive system would require much longer simulation time than converging the band gap. Our present results suggest that Ag alloying could slightly increase the formation energy of Cu vacancy.