IMPEDANCE SPECTROSCOPY ON NA+ HO3+-BETA''-AL2O3 CRYSTALS/

Crystals of Na+-beta''-Al2O3 and Na+/Ho3+-beta''-Al2O3, with varying d egrees of exchange, have been investigated by impedance spectroscopy u sing sputtered chromium films as electrodes. For this work a sample ho lder was developed to enable studies of the in-plane conductivity of e ven very thin beta''-alumina crystals. The ionic conductivities of the investigated Na+-beta''-Al2O3 crystals were found to be one order of magnitude lower than those reported in the literature, a discrepancy c aused by a lower charge carrier concentration. On the other hand, the activation energy and the diffusion coefficients of the sodium ions, a s deduced from the Nernst-Einstein equation, agree well with previous studies. In the case of the Na+/Ho3+-beta''-Al2O3 crystals, a strong d ecrease of the ionic conductivity has been observed with increasing de gree of exchange. To determine the bulk conductivities an equivalent c ircuit had to be applied which differed from the one used in case of N a+-beta''-Al2O3. With an additional RC-element in parallel these spect ra could be fitted with satisfying results. This RC-element may be reg arded as a second relaxation process of the Na+ ions.