Effects of the presence of valence-shell non-bonding electron pairs on the properties and structures of caesium tin(II) bromides and of related antimony and tellurium compounds

Abstract

The ideal perovskite structure of CsSn^IIBr₃ at room temperature has been confirmed by a single-crystal X-ray diffraction study. It is proposed that the high-symmetry environment for the Sn^II in this compound arises because the distorting effect of the non-bonding electrons is reduced by their populating an empty low-energy band in the solid, thus giving rise to the black colour and metallic-conducting properties. The high-temperature phases of CsSn₂^IIBr₅, Cs₄Sn^IIBr₆, and of compositions from the CsSn^II₂Br₅–CsSn^II₂Cl₅ system show similar properties. Colour can be introduced into the Cs₂Sn^IVBr₆ system by formation of mixed phases with CsSn^IIBr₃ which has a closely related structure. The colour and electrical properties of the mixed Sn^II–Sn^IV material can be explained by population of the low-energy delocalised solid-state bands by the Sn^II non-bonding electrons without recourse to intervalence-transfer-absorption ideas. Comparison of the complex bromides and chlorides of Sn^II and Te^IV with the mixed-valence Sb^III–Sb^V compounds suggests that direct population of bands rather than intervalence charge transfer may also be the dominant process in determining the properties of the antimony derivatives, l.r. data for the mixed-valence antimony compounds are consistent with the direct-population explanation.