CONSTANT-LOSS RELAXATION RESPONSE IN CRYSTALS AND GLASSES

Nowick and his associates have stated that many ionic crystals and gla sses exhibit a loss per cycle which is independent of frequency over a n appreciable range and have suggested that such behavior constitutes a ''new universality''. Furthermore, much such data seem to approach a n asymptotic, nearly temperature-independent ac loss at sufficiently l ow temperatures. In order to further evaluate these conclusions, small -signal ac relaxation data for a CaTiO3:30% Al3+ ceramic material are analyzed in detail and the results compared to those published by Nowi ck and associates for the same material. It is found that a plausible conducting-system dispersion model based on the effective-medium appro ximation for hopping charges yields results globally similar to, but s omewhat different in detail from, those of Nowick et al. But a respons e model which includes both such conducting-system response and dielec tric-system dispersion well fits the data over a wide temperature rang e. To do so, it requires the presence of a non-zero high-frequency-lim iting resistivity probably arising from localized charge motion. No co nstant-loss individual dispersions appear in the model, but it neverth eless yields approximately constant loss over a limited frequency rang e at low temperatures. It suggests that asymptotic behavior is associa ted with the nearly temperature-independent dielectric-dispersion cont ribution to the response at low temperatures, and it does not verify t he Nowick conclusion that the slope of the ac conductivity approaches a constant value near 0.6 at high temperatures.