Evidence for the direct population of solid-state bands by non-bonding electron pairs in compounds of the type CsMIIX3(MII= Ge, Sn, Pb; X = Cl, Br, I)

Abstract

The optical and electrical conductivity behaviour of phases in the systems CsM^IIX₃(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^III₃ 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. Mössbauer data for the mixed systems CsSn_{1 –x}Pb_xX₃(X = Br,I) and CsSn_{1 –x}Cd_xBr₃ are consistent with the band structure properties for these phases.