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

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Abstract
The ideal perovskite structure of CsSnIIBr3 at room temperature has been confirmed by a single-crystal X-ray diffraction study. It is proposed that the high-symmetry environment for the SnII 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, Cs4SnIIBr6, and of compositions from the CsSnII 2Br5–CsSnII 2Cl5 system show similar properties. Colour can be introduced into the Cs2SnIVBr6 system by formation of mixed phases with CsSnIIBr3 which has a closely related structure. The colour and electrical properties of the mixed SnII–SnIV material can be explained by population of the low-energy delocalised solid-state bands by the SnII non-bonding electrons without recourse to intervalence-transfer-absorption ideas. Comparison of the complex bromides and chlorides of SnII and TeIV with the mixed-valence SbIII–SbV 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.

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