Phase Stability and Ordering in Rock Salt-Based Thermoelectrics: NaSbX2, AgSbX2, and Their Alloys with PbX and SnX (X = S, Se, Te)

Using first-principles density functional calculations, we study the phase stability and cation ordering behavior in ABX2, PbX-ABX2, and SnTe-ABTe2 (A = Na, Ag; B = Sb; X = S, Se, Te) thermoelectrics that crystallize in rock salt-based lattices. We construct, separately for each ABX2 system, cluster expansions fitted to T = 0 K energies of cation-ordered arrangements in order to identify the respective ground-state structures. We calculate the mixing energetics of A/B and {Pb, Sn}/AB cation-disordered solid solutions in ABX2 and {Pb, Sn}X-ABX2 systems, respectively, using special quasirandom structures. We find that (i) L11, a rock salt-based superlattice with A1B1 stacking of cations along [111], is often the most favored cation ordering type across ABX2 systems due to the ability of this superlattice structure to accommodate large size mismatches between A and B; (ii) A/B cation ordering is only weakly preferred over disorder in all ABX2 systems, by <34 meV/cation, consistent with experimental observati...