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 highly doped crystals, at low temperatures and/or high frequencies, we examine various dilute crystalline materials in the same temperature/frequency range. These include Gd3+ and Y3+-doped CeO2 and Al3+ doped CaTiO3, both of which are oxygen-ion conductors, and Nd3+ doped BaCeO3, which is a proton conductor. In all cases, one or more discrete dielectric loss peaks are observed, all with low activation energies (∼0.2 eV). For each of the oxygen-ion conductors, we find peak broadening with increasing concentration, eventually smearing out into true NCL behavior. In the case of the BaCeO3 treated in H2O vapor, a peak appears which shows non-Arrhenius behavior, strongly suggestive of proton tunneling. These various relaxation peaks are due to collective motions of relatively large ionic configurations that have gone off-symmetry, involving small displacements and low activation barriers. The fact that such relaxations are so prevalent suggests the need for a wider use of low-temperature dielectric spectroscopy.