Prediction of New Stable Compounds and Promising Thermoelectrics in the Cu–Sb–Se System

We study the phase stability and predict as-yet-unreported compounds in the thermoelectric Cu–Sb–Se ternary system. We use a combination of total energies obtained from density-functional-theory-based (DFT) calculations with vibrational entropies from phonon calculations (within the harmonic approximation) and configurational entropies, treated with cluster expansions (CE). The Cu–Sb–Se ternary phase diagram is determined (treating all phases as line compounds) using the grand-canonical linear programming method. We find the following results: (1) we predict the stability of a new previously unknown, zinc blende-based Cu 4 SbSe 5 compound but find that it is thermodynamically stable up to only ∼300 K; (2) we also predict that a Cu 12 Sb 4 Se 13 phase (isostructural with Cu 12 Sb 4 S 13, but unreported in the Cu–Sb–Se system) appears in the phase diagram at high temperatures (but below the temperatures where the observed Cu 3 SbSe 3 phase is stable); (3) based on quasi-harmonic phonon and band structure calculations, we find that Cu 12 Sb 4 Se 13 has thermal conductivity and an electronic structure that suggests it as a promising thermoelectric material.