Positron defect studies in oxides

Transition metal oxides show a variety of interesting properties and attract wide attention because of a broad application potential. Oxides often exhibit a complex defect structure and understanding such defect structures is of primary importance for planned applications. Defects in oxides involve vacancy-like defects and positron annihilation can be effectively used to investigate them. In this lecture, we summarize results of our recent theoretical and experimental investigations carried out for zinc oxide (ZnO) and yttria stabilized zirconia (YSZ), the latter being a solid solution of an appropriate amount of yttria (Y2O3) in zirconia (ZrO2). Hydrogen-related defects in these two oxides are also considered as hydrogen is existing in a non-negligible amount in both these oxides [1,2]. The structure of defects expected to be present in the studied materials is obtained using an ab initio computational technique. Since there is a large charge transfer between oxygen and transition metal sublattices in oxides, self-consistent calculations including positron induced forces are necessary to determine reliably positron characteristics.

First, we report on positron lifetime measurements performed with a pulsed low energy positron beam system and a conventional setup on as-grown and hydrogen plasma treated single crystal ZnO samples produced under hydrothermal conditions. The results of lifetime measurements are compared with lifetimes calculated for various configurations of vacancy-like defects taking into account also defects’ formation energies. We conclude that all calculated data support the idea that in as-grown ZnO samples positrons far from the surface annihilate in Zn vacancy-hydrogen complexes. In hydrogen plasma treated samples the situation is similar, but a component originating from delocalized positrons appears, which is attributed to a reduced concentration of vacancy-like defects capable of positron trapping because of the hydrogen plasma treatment. Vacancy agglomerates are also found in the subsurface region of both types of samples.

As for 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 obtained for YSZ single crystals, which suggests that the Zr vacancy and its complexes with hydrogen could be responsible for positron trapping in YSZ.

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