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 formatioin energy, due both to the relative ease of forming Cu vacancies (Vcu) and to the attractive interactions between V-Cu and In2+Cu. The defect pair is predicted to be electrically inactive. This explains the surprising electrical tolerance of CuInSe2 to its huge (.apprx.1%) concentrationof native defects. An attractive interaction among the defect pairs is further predicted to lead to a crystallogrphic ordering of the pairs, explaining the observed, but hitherto surprising, structures CuIn5Se8, CuIn3Se5, Cu2In4Se7, etc.