Thermally Stimulated Depolarization Studies of Ionic Solids

Abstract

Relevant physical properties of crystalline solids (for instance electric resistivity, color, mechanical strength, etc.) and applications (integrated electronics, laser etc.) are determined by the presence of lattice defects (vacancies, interstitials, impurities and complexes built by them). According to the specific nature of defects, suitable techniques have been developed. In ionic solids, defects often bear an electric charge (cation and anion vacancies, interstitial ions, aliovalent impurities). As a consequence of the Coulomb interaction between defects of opposite charge, complexes may be formed, which exhibit an electric dipole moment. Other defects, even in non ionic solids, may he endowed by their own dipole moment. Hence electrical methods such as dielectric losses (tgδ), isothermal depolarisation currents and more recently ionic thermocurrents (ITC) are suitable to monitor such defects. This last method, introduced by Bucci and Fieschi in 1964, deals with the detection and analysis of thermostimulated depolarization currents arising from ion redisplacements in solids whose electronic conductivity is neglegible (1,2). In a sense ITC enters in the wide class of methods based on the thermal release of stored energy, as for instance thermoluminescence (TL), thermally stimulated currents (TSC) and thermally stimu lated depolarization currents (TSDC). ITC deals with TSDC as well, but is restricted to polarization mechanisms, in which only ion redisplacements are involved ruling out a wide class of phenomena which occur in electrets such as carrier injection, electron and/or hole trapping. In this way ITC is more descriptive and specific term than the more general TSDC, which is used by some authors. Due to this restriction and its successful application to simple model systems, such as ionic crystals, it has opened the way for a quantitative study of dipolar processes also in more complex systems of technological interest.