Anelastic spectroscopy in ceramic materials and mobility of oxygen defects

A survey is provided of the studies of formation and mobility of O vacancies (VO), interstitials and their complexes with other defects in ceramic materials by anelastic spectroscopy; H defects in proton conducting perovskites are also considered. The anelastic spectra (complex elastic compliance as a function of temperature at angular frequency !) of materials containing defects with hopping/reorientation frequencies ¿¡1 (T) present peaks in the elastic energy loss (or imaginary part) centered at the temperatures Tm such that !¿ (Tm) ' 1 for each type of jump, so making it possible to distinguish di®erent types of defect motions and to measure their rates. After a brief introduction to anelastic relaxation, focusing on aspects that are generally overlooked but relevant for the phenomena treated here, the results on various classes of ceramic materials are considered. In the high-Tc superconducting (HTS) cuprates there is a fraction of mobile O atoms whose stoichiometry and ordering may be varied over broad ranges and determine doping and therefore the conducting properties. In the YBa2Cu3O6+x family, doping is exclusively provided by those O atoms in CuOx planes that order into long enough -Cu-O- chains, with x varying from almost 0 to 1. The anelastic spectra reveal three di®erent types of O jumps: those involving Cu-O chains also require changes in doping and electronic energy, and occur over a barrier of » 1 eV; the jumps of isolated O atoms in the semiconducting state occur over a barrier of only 0.11 eV. In the isostructural RuSr2GdCu2O8¡±, the non-stoichiometry range is limited to few percent of O vacancies in CuO2¡± planes, and the anelastic spectra can be interpreted in terms of jumps of VO taking into account their long range elastic interactions in a Curie-Weiss-like approach. In Bi2Sr2CaCu2O8+± and La2CuO4+± excess O enters in interstitial positions, and its jumps produce intense anelastic relaxation processes; partic- ularly in La2CuO4+± and in nonsuperconducting La2NiO4+± it is possible to reveal the strong interaction of O interstitials with the unstable tilt modes of the surrounding CuO6 and NiO6 octahedra, and the formation of interstitial pairs and ordered phases. The ferroelectric perovskites and related materials may contain at most small concentrations of VO, which however may adversely a®ect the properties of in- terest in devices, e.g. causing fatigue or polarization imprint phenomena. Some of the complex types of interaction between VO and domain walls are discussed and the anelastic experiments on ferroelectric perovskites PZT, BaTiO3 and Bi4Ti3O12 are reviewed. Relaxation processes are always present with acti- vation energies around 1 eV and whose amplitude is a®ected by reducing or oxygenation treatments, but it is di±cult to distinguish between jumps of VO 1 producing a reorientation of the associated elastic quadrupole, or long range mi- gration in the electric ¯eld produced by piezoelectric e®ect within the domains, or motion of domain walls hindered by the interaction with VO. The situation should be much simpler in cubic SrTiO3, but the anelastic spectra of SrTiO3 reduced in H2 contain several peaks likely due to H and polarons; the peak attributable to VO hopping is weak and broadened, supporting recent proposals that VO in SrTiO3 are clustered or con¯ned to near-surface regions. Hydrogen and VO are introduced at high concentrations in the ionic conduc- tors, often perovskite cerates and niobates, where their di®usion gives rise to an electric conduction that is higher than the electronic one over limited tem- perature ranges. Such ionic motions also produce rich anelastic spectra whose detailed interpretation still requires re¯nements, but it is agreed that the jumps of VO o