Phase Stability and Ordering in Rocksalt-based ABX2 and (Pb/Sn)X-ABX2 Thermoelectrics*

Using first-principles density functional theory (DFT) calculations, we study the phase stability and ordering in ABX2 and (Pb/Sn)X-ABX2 (A=Na, Ag; B=Sb; X=S, Se, Te) systems that crystallize in rocksalt-based lattices. We use the cluster expansion method to calculate the T=0 K energies of ordered arrangements, and special quasirandom structures (SQS) to estimate the energetics of structures with A/B and (Pb/Sn)/AB cation disorder. From the DFT-calculated energetics, we: (a) predict the miscibility and ordering tendency in each system, (b) find that phase separation is favored in all (Pb/Sn)X-ABX2 systems while compound-forming is favored in most ABX2 systems, (c) predict solubility boundaries for phase-separating systems, and (d) identify that cation ordering in the L11 structure type is often the lowest-energy one in ABX2 compounds, and discuss why L11 is favored.