Abstract

A microscopic interstitialcy-type diffusion mechanism for sodium beta-alumina involving three crystallographically different types of cation sites but only two equilibrium configurations of interstitialcy pairs is proposed. The resulting cation-site occupation probabilities are shown to agree with the experimental neutron-diffraction data of Roth et al. For various sizes of the associated regions around the charge-compensating oxygen interstitials (in which Na + ions are not freely mobile), the ratio of the radiotracer diffusivity with respect to the charge diffusivity (the so-called Haven ratio, H R )is determined. In agreement with the measurements of Kim et al., H R is found to increase with increasing temperature as a result of the decrease in the average size of the associated regions. It is concluded that, while at lower temperatures all excess Na + ions are found in the associated regions around the oxygen interstitials, with increasing temperature more and more of them are freely mobile in the unassociated regions of the conduction plane. The formation enthalpy associated with the thermally activated creation of mobile “interstitials” is determined to be about 0.05 eV. The slope of the slightly curved Arrhenius plot associated with the proposed mechanism therefore consists of the sum of the energy of formation of freely mobile interstitials and the energies of migration of two types of interstitialcy pairs. The Raman and IR-conductivity results are found to be compatible with this emerging picture for the microscopic diffusion process in sodium β-alumina. Spectroscopic diffusion techniques (NMR, internal friction, dielectric loss) show more than one apparent activation energy E A while, in the same temperature region, the d.c. ionic conductivity yields a unique value of E A ; reasons for this discrepancy are discussed.

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