Stabilization of Ternary Compounds via Ordered Arrays of Defect Pairs

First-principles calculations show that the defect pair (2VCu^-+InCu⁺) in CuInSe₂ has an unusually low formation energy, due both to the relative ease of forming Cu vacancies (VCu) and to the attractive interactions between VCu^- and InCu²⁺. The defect pair is predicted to be electrically inactive. This explains the surprising electrical tolerance of CuInSe₂ to its huge (~1%) concentration of native defects. An attractive interaction among the defect pairs is further predicted to lead to a crystallographic ordering of the pairs, explaining the observed, but hitherto surprising, structures CuIn₅Se₈, CuIn₃Se₅, Cu₂In₄Se₇, etc.