Bond properties of the chalcopyrite and stannite phases in the Cu-(In,Ga)-Se system

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-Xalpha method. Bond stretching force constant a 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)-Se-8i and In(Ga)(2b)-Se-8i bonds are estimated to be tighter by 23-25 and 8-9%, respectively, than In(Ga)(4b)-Se-8d bond in the chalcopyrite structure. The Gamma(1) 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)(3)Se-5 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. (C) 2003 Elsevier Ltd. All rights reserved.