Charge transfer in mixed-valence solids. Part VI. Spectra and semiconductivity of mixed crystals of hexachloroplumbates(II,IV) and hexachloroindates(III)

Abstract

A series of hexachloroplumbate(II,IV) salts, Co(NH₃)₆Pb_xIn_1–xCl₆, has been prepared and characterised by chemical analysis and X-ray powder diffraction. Diffuse reflectance spectra of all the compounds have been measured, and the mixed-valence absorption band identified. Two-probe D.C. conductivity measurements are reported. All the compounds with x > 0·1 are ohmic semiconductors; the specific conductivities at room temperature are proportional to x², indicating that carrier formation requires pairs of PbCl₆^2– and PbCl₆^4–. The semiconduction activation energy is independent of concentration for x > 0·2 and equal to 0·73 eV. The Seebeck coefficient of Co(NH₃)₆PbCl₆ is + 0·9 mV/°C at room temperature. The optical and conductivity data are discussed in the light of the model put forward previously to explain the comparable properties of hexachloroantimonates(III,V).