IONIC-CONDUCTIVITY OF ZRO2-12 MOL PERCENT-Y2O3 SINGLE-CRYSTALS

Fast ionic conductors are important to study because of their use in t he construction of technologically useful devices such as electrochemi cal cells, oxygen monitors, and the high-temperature fuel cell. Oxygen -ion conductors form a major subgroup of these materials, and, in part icular, stabilized zirconia is one of the more important solid electro lytes. However, the ionic conductivity of this material is still only rather poorly understood. The aim of the present work is to describe, by means of a method of local fits (LF's) to Arrhenius's law, the expe rimental values of the ionic conductivity of ZrO2-12 MOI % Y2O3 single crystals in the temperature range from 200-degrees-C to 1600-degrees- C. This method yields two sets of data: the preexponential factor, A(L F)i, and the activation enthalpy, DELTAH(LF)i. The In A(LF)i versus DE LTAS(T)/k plot [where DELTAS(T) is the entropy change in the process] is a very good test of the accuracy of the LF method. The DELTAH(LF)i values are fitted by a least-squares procedure to an empirical tempera ture-dependence function with four adjustable parameters. In order to interpret these results and to understand the physical meaning of the fitted parameters, a microscopic model is proposed that allows us to d educe a theoretical function of temperature for the activation enthalp y similar to the empirical function. Then, from this function, we dete rmine the association (0.57 eV) and migration (0.73 eV) enthalpies for oxygen vacancies, and analyze the temperature variation of the free e nergy (DELTAG) and entropy (DELTAS), as well as the degree of dissocia tion of the vacancies in the conduction process for this material. A n oteworthy result is that, for the range of temperature studied here, t he extrinsic dissociated regime (where it is assumed that all oxygen v acancies are free) is never reached. Finally, taking into account the contribution of the jumps up to the second-next-nearest anionic neighb ors, we obtain the value of 1.31 X 10(13) Hz for the attempt frequency of the oxygen vacancies.