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

Abstract

Two-probe D.C. conductivity measurements are reported for the three series of hexachloroantimonates(III,V) whose optical properties were examined in the preceding paper: A₂^ISb_xSn_1-xCl₆(A^I= NH₄⁺, Cs⁺), Cs₂ln^III_½ySb^III_y^– Sb^V_½Cl₆. For values of x and y greater than about 0·1, the compounds behave as ohmic semiconductors. The specific conductivities at room temperature are proportional to x² and y respectively, while the activation energies are independent of the antimony concentration. Seebeck coefficients, determined at room temperature, indicate that for x and y > 0·1 the majority carriers are holes, but below 0·1, electrons. It is suggested that the conductivity of the compounds containing sufficient antimony to form a continuous path through the lattice is electronic while that of the dilute materials and single-valence host lattices is ionic. In the former, the magnitude of the conductivity is consistent with a diffusion mechanism in which carrier formation by electron transfer between SbCl₆^3– and SbCl₆^– is followed by migration of the resulting SbCl₆^2– among the SbCl₆^3–. The charge-carrier formation step is the adiabatic analogue of the Franck–Condon intermolecular charge transfer process studied in the preceding paper. Relaxation frequencies calculated from the observed conductivity by use of the diffusion model are in the range of Sb–Cl vibrations. The relation between the semiconductor activation energy and the optical chargetransfer energy is discussed.