Self-Diffusion and Na-K Exchange in βand β”-Na-Gallate Fast Ion Conductors

Abstract

The self-diffusion coefficients for Na+ migration in the two-dimensional, ionically conducting sodium gallates were determined as 2.0 x 10-5 and 7.6 x 10-5 cm2/sec for the β (hexagonal) and β” (rhombohedral) phases, respectively, at 350°C. Part of the sodium of these compounds is immobile. It is proposed that this portion substitutes for Ga+++ in the spinel block and compensates the charge of additional sodium in the conduction plane. The correlation factor suggests a noncolinear interstitialcy diffusion model for migration of the mobile Na+ in the β phase. The correlation factor for the β phase is unusually large and not explained by any present model. Activity coefficients for the Na- and K-gallates were determined in solid solutions. Na+and K+ show essentially ideal behavior in theβ structure and very nonideal behavior suggestive of ordering, in the β structure, in agreement with findings for βAl2O3. © 1978, The Electrochemical Society, Inc. All rights reserved.