EVIDENCE FOR THE DIRECT POPULATION OF SOLID-STATE BANDS BY NONBONDING ELECTRON PAIRS IN COMPOUNDS OF THE TYPE CSM(II)X3(M(II)=GE,SN,PB, X=CL,BR,I)

The optical and electrical conductivity behaviour of phases in the systems CsM(II)X(3) (M(II)=Ge, Sn, Pb; X=Cl, Br, I) are explained by donation of electron density from non-bonding electron pairs on the M(II) metal into a conduction band formed by the overlap of halogen d states. The greater ionic character of the Pb-II reduces the electron density donation and dominates the optical spectra fo the Pb-rich phases. The optical properties of the CsM(II)I(3) phases (M(II)=Ge, Sn, Pb) show that the donation of non-bonding electron-pair electrons to iodine d bands is favoured over the self-population of the bands by iodine electrons. Mossbauer data for the mixed systems CsSn(1-x)Pb(x)X(3) (X=Br,I) and CsSn1-xCdxBr3 are consistent with the band structure properties for these phases.