In this paper from a collaboration of researchers of CNR-ISM, CNR-ISSMC and the Physics Department of UFSCar (Sao Carlos, Brazil), and published in Physical Review B, new microscopic information is provided on the behaviour and effects of the O vacancies (VO) in BaTiO₃ and similar ferroelectric titanate perovskites.
This type of materials is widely used in multilayer capacitors and other devices, and VO play a significant role in their degradation. They can be introduced during the deposition of the metallic electrodes in reducing atmosphere or even created under working conditions under intense electric fields, then accumulate near the electrodes and finally cause the device failure.
In spite of their importance, VO are difficult to be probed, due to the lack of electric dipole and small atomic number of O, but they have an elastic dipole, which can cause anelastic relaxation. Measuring the complex elastic modulus as a function of temperature and frequency, one finds peaks in the elastic energy absorption at the temperatures where the mean hopping rates of the VO are close to the measuring frequency, so finding that, in addition to the jumps of isolated VO, there are slower reorientations of VO pairs. Each VO dopes two mobile electrons as itinerant Ti³⁺, which increase the conductivity and lower the Curie temperature, but it has been independently observed by XPS, that the linear clustering of VO partially transforms Ti³⁺ into Ti²⁺, which are two electrons trapped at Ti⁴⁺ within the VO-Ti-VO pairs. Therefore, the formation of pairs of VO halves the doping of mobile electrons, and VO clustering after cooling from high temperature is reflected in an increase of Tc with aging in the ferroelectric phase. On the other hand, on a longer time scale a decrease of Tc is observed. This is interpreted in terms of migration of the VO, which have lower energy isolated at the ferroelectric domain walls, than paired within the domains.