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)6PbxIn1–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 PbCl62– and PbCl64–. 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).