Charge transfer in mixed-valence solids. Part IV. Electronic spectra of hexachloroantimonates(III,V)

Abstract

Four series of hexachloroantimonate(III,V) salts, A^I ₂Sb_xSn_1–xCl₆(A^I= NH₄ ⁺, Cs⁺, and MeNH₃ ⁺), and Cs₂Sb^III _y-In^III _½–ySb^V _½Cl₆ have been prepared and characterised by chemical analysis, X-ray powder diffraction, and far-i.r. measurements. Visible and u.v. spectra of all the compounds were measured at room temperature by diffuse reflectance and, in the case of the MeNH₃ ⁺ salts, by single-crystal transmission between room temperature and 6°K. The concentration-dependence of the spectra showed that the intensity of the visible mixed-valence band was proportional to the concentration of Sb^III–Sb^v pairs in the hexachlorostannate(IV) lattice. From the absolute intensity of the mixed-valence transition it was found that delocalisation of the optical (5s) electrons of SbCl₆ ^3– on to the neighbouring SbCl₆ ^– was less than 0·1% in the ground state, in agreement with the far-i.r. results. The band, therefore, is almost purely charge-transfer in character. The temperature-dependence of the half-width of the band between 300 and 6°K could be accounted for in terms of broadening by a single effective frequency of 210 cm.^–1. The relation between this optically derived frequency and the ground-state vibrational frequencies of SbCl₆ ^3– and SbCl₆ ^– is considered.