LOW-FREQUENCY AND HIGH-FREQUENCY CONDUCTIVITIES OF RBAG4I5 AND AG-BETA''-ALUMINA AT LOW-TEMPERATURES

The specific ionic conductivities, sigma, of polycrystalline RbAg4I5 a nd Ag beta''-alumina superionic conductors were determined by two- and four-probe impedance measurements at low frequencies (10(-3) Hz less- than-or-equal-to f less-than-or-equal-to 10(2) Hz) and waveguide measu rements at high frequencies (18 x 10(9) Hz less-than-or-equal-to f les s-than-or-equal-to 26.5 x 10(9) Hz =: K-band) in the temperature range 15 K less-than-or-equal-to T less-than-or-equal-to 298 K. Above a tra nsition temperature T(t) = (90 +/- 20) K, the conductivity data repres ent clearly a silver ion transport process with thermal activation. Th e activation energies of the alpha, beta, and gamma phases of RbAg4I5 were determined for the low- and high-frequency ranges. Below T(t) a n early temperature-independent conductivity was observed in accordance with previous results. Around T(t) a long-time relaxation effect is ob served in low-frequency EIS measurements, changing sigma and the diele ctric constant epsilon. The low-temperature conductivity data can be e xplained neither in terms of the jump-relaxation nor in terms of the t wo-level tunneling system (TLS) model. A different mechanism which tak es into account the influence of inhomogeneities on the ionic conducti vity is proposed. This mechanism is obviously valid in a restricted te mperature range located between the ion TLS-tunneling and the jump-rel axation regimes.