INTERPRETATION OF IONIC TRANSPORT-PROPERTIES OF SOME SILVER CHALCOGENIDES

An analysis of the composition dependence of ionic transport properties is reported which concentrates on the alpha- and beta- silver chalcogenides. The numerous assumptions inherent in the evaluation of the ionic caterpillar-type motion in structurally disordered Ag2+delta-Te are stated and it is shown that the Haven ratio measured need not necessarily originate from ionic motion by the caterpillar mechanism. General relations for coulometric titration curves are derived which allow for the contributions of the electrons and ions to the electromotive force. For compounds of structural cation disorder a procedure for calculating the composition dependence of the chemical potential of the cations, mu-i, is proposed. It results from an estimation of the configurational free energy. In the case of silver telluride at 250-degrees-C it yields d-mu-i/d-delta = 1.08 kT, thus confirming the negligibility of the ionic contribution to the emf. Relations describing the composition dependence of the chemical diffusion coefficient are given and supplemented with vivid interpretation. The maximum of the chemical diffusion coefficient of structurally disordered silver telluride near the stoichiometric composition and the absence of such a maximum for Frenkel-disordered silver selenide are shown to conform with theory. The theoretical description of diffusion in compounds of several mobile components is reconsidered.