Exploring the low-temperature electrical relaxation of crystalline oxygen-ion and protonic conductors

In order to better understand the dielectric behavior known as NCL ('nearly constant loss') which occurs for complex materials, e.g. glasses and highl y doped crystals, at low temperatures and/or high frequencies, we examine v arious dilute crystalline materials in the same temperature/frequency range . These include Gd3+ and Y3+-doped CeO2 and Al3+ doped CaTiO3, both of whic h are oxygen-ion conductors, and Nd3+ doped BaCeO3, which is a proton condu ctor. In all cases, one or more discrete dielectric loss peaks are observed , all with low activation energies (similar to 0.2 eV). For each of the oxy gen-ion conductors, we find peak broadening with increasing concentration, eventually smearing out into true NCL behavior. In the case of the BaCeO3 t reated in H2O vapor, a peak appears which shows non-Arrhenius behavior, str ongly suggestive of proton tunneling. These various relaxation peaks are du e to collective motions of relatively large ionic configurations that have gone off-symmetry, involving small displacements and low activation barrier s. The fact that such relaxations are so prevalent suggests the need for a wider use of low-temperature dielectric spectroscopy. (C) 2000 Elsevier Sci ence B.V. All rights reserved.