Stabilization of ternary compounds via ordered arrays of defect pairs

First-principles calculations show that the defect pair (2V(Cu)(-) + In-Cu(+)) in CuInSe2 has an unusually low formation energy, due both to the relative ease of forming Cu vacancies (V-Cu) and to the attractive interactions between V(Cu)(-) and In-Cu(2+). The defect pair is predicted to be electrically inactive. This explains the surprising electrical tolerance of CuInSe2 to its huge (similar to 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.