Controlled Reduction of Sn4+ in the Complex Iodide Cs2SnI6 with Metallic Gallium

Metal gallium as a low-melting solid was applied in a mixture with elemental iodine to substitute tin(IV) in a promising light-harvesting phase of Cs2SnI6 by a reactive sintering method. The reducing power of gallium was applied to influence the optoelectronic properties of the Cs2SnI6 phase via partial reduction of tin(IV) and, very likely, substitute partially Sn4+ by Ga3+. The reduction of Sn4+ to Sn2+ in the Cs2SnI6 phase contributes to the switching from p-type conductivity to n-type, thereby improving the total concentration and mobility of negative-charge carriers. The phase composition of the samples obtained was studied by X-ray diffraction (XRD) and 119Sn Mössbauer spectroscopy (MS). It is shown that the excess of metal gallium in a reaction melt leads to the two-phase product containing Cs2SnI6 with Sn4+ and β-CsSnI3 with Sn2+. UV–visible absorption spectroscopy shows a high absorption coefficient of the composite material.