Mobility and clustering of O vacancies in the paraelectric/ferroelectric phases of BaTiO3 and (Ba/Ca)(Ti/Zr)O3

Oxygen vacancies are considered as the most important point defects responsible for degradation, aging and pinning of domain walls in oxide ferroelectrics, but are also very elusive. Controlled amounts of O vacancies up to 1.5% have been introduced in ceramic BT and BCTZ and their anelastic and dielectric spectra have been studied. Oxygen vacancies strongly depress the Curie temperature, may induce a reversed thermal hysteresis and dependence on the thermal history. It is argued that the major effect on Tc is from electron doping, in turn dependent on the aggregation state of the vacancies. The latter can be probed through elastic energy loss peaks in the paraelectric phase, which allow the barriers for hopping of isolated and reorientation of paired O vacancies to be measured. These barriers are found to decrease with increasing concentration of O vacancies. To explain the reversed thermal hysteresis it is assumed that O vacancies dope electrons more effectively when isolated than aggregated, and in the ferroelectric phase they disperse to decorate the domain walls as isolated point defects. In this manner, after aging in the ferroelectric phase, the pairs formed in the paraelectric phase dissolve, electron doping increases and Tc is reduced.