Bond properties of the chalcopyrite and stannite phases in the Cu–(In,Ga)–Se system Original Research Article

Bond properties of the chalcopyrite and (defect) stannite phases in the Cu–(In,Ga)–Se system are compared in view of the bond overlap population calculated by the molecular orbital calculation of the DV-Xα method. Bond stretching force constant α is estimated for the stannite phases through the bond Ovlp. The Cu–Se and In(Ga)–Se bonds in defect stannite structure are considered to be mechanically weakened by the 2b-site vacancies. We estimate the weakened force constants to be 60–70% of those in the chalcopyrite structure. On the other hand, in In(Ga)-rich stannite, In(Ga)4d–Se8i and In(Ga)2b–Se8i bonds are estimated to be tighter by 23–25 and 8–9%, respectively, than In(Ga)4b–Se8d bond in the chalcopyrite structure. The Γ1 frequencies of the stannite phases are also calculated using the estimated force constants. Characteristic Raman signals peaked at 160–175 cm−1 observed for the Cu(In1−xGax)3Se5 system are explained by the Cu-rich phase for the Cu–In–Se system, and the phase combination of Cu-rich and Ga2aV2b types for the Cu–Ga–Se system from these calculations.