Complex defect structure of oxides studied by positrons

Transition metal oxides show a variety of interesting properties and attract wide attention because of a broad application potential. At the same time, oxides often exhibit a complex defect structure comprising vacancy-like defects and positron annihilation can be effectively used to investigate and, thereby, to understand such structures. In this contribution, we concentrate on two oxides, namely these are zinc oxide (ZnO) and yttria stabilized zirconia (YSZ), the latter being a solid solution of an appropriate amount of yttria (Y2O3) in zirconia (ZrO2). As hydrogen has been detected in both these oxides and its content is not negligible [1,2], we also deal with positron characteristics of H-related defects, in addition to ‘bare’ vacancies. The structure of studied defects is obtained using an ab initio computational technique. Since there is a large charge transfer between oxygen and transition metal atoms in oxides, self-consistent calculations including positron induced forces are necessary to determine reliably positron characteristics.
As for ZnO, we discuss briefly the structure of studied vacancy-like defects. Then, the results of positron lifetime calculations are shown and compared with experimental data. Next, we present calculated formation energies of the same defects. We conclude that all calculated data support idea that in hydrothermally grown ZnO positrons annihilate in Zn vacancy-hydrogen complexes. Concerning YSZ, due to yttria doping a significant amount of O vacancy-yttrium complexes is present. Despite of earlier assumptions our calculations indicate that such complexes do not constitute positron traps. We further concentrate on the Zr vacancy and its complexes with hydrogen. First results for positron calculations with positron induced forces are presented. Calculated positron lifetimes are compared with experimental data, which suggests that the Zr vacancy and its complexes with hydrogen could be responsible for positron trapping in YSZ. Finally, further prospects of positron studies of defects in oxides are outlined.

[1] G. Brauer et al., Phys. Rev. B 79, 115212 (2009)
[2] O. Melikhova et al., Mat. Res. Symp. Proc. 1216, W07-10 (2010)