High-quality single crystals of ZnO in the as-grown and N + ion-implanted states have been investigated using a combination of three experimental techniques-namely, positron lifetime/slow positron implantation spectroscopy accompanied by theoretical calculations of the positron lifetime for selected defects, temperaturedependent Hall ͑TDH͒ measurements, and deep level transient spectroscopy ͑DLTS͒. The positron lifetime in bulk ZnO is measured to be ͑151± 2͒ ps and that for positrons trapped in defects ͑257± 2͒ ps. On the basis of theoretical calculations the latter is attributed to Zn+ O divacancies, existing in the sample in neutral charge state, and not to the Zn vacancy proposed in previous experimental work. Their concentration is estimated to be 3.7ϫ 10 17 cm −3 . From TDH measurements the existence of negatively charged intrinsic defects acting as compensating acceptors is concluded which are invisible to positrons-maybe interstitial oxygen. This view is supported from TDH results in combination with DLTS which revealed the creation of the defect E1, and an increase in concentration of the defect E3 after N + ion implantation, and peculiarities in the observation of the defect E4.

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Combining photoluminescence and positron annihilation studies of hydrothermally grown ZnO crystals with stoichiometry varied by controlled annealing enabled us to clarify the origin of green luminescence. It was found that green luminescence in ZnO has multiple origins and consists of a band at 2.3(1) eV due to recombination of electrons of the conduction band by zinc vacancy acceptors coupled with hydrogen and a band at 2.47(2) eV related to oxygen vacancies. The as-grown ZnO crystals contain zinc vacancies associated with hydrogen and exhibit a green luminescence at 2.3(1) eV. Annealing in Zn vapor removed zinc vacancies and introduced oxygen vacancies. This led to disappearance of the green luminescence band at 2.3(1) eV and appearance of a green emission at higher energy of 2.47(2) eV. Moreover, the color of the crystal was changed from colorless to dark red. In contrast, annealing of the as-grown crystal in Cd vapor did not remove zinc vacancies and did not cause any significant change of green luminescence nor change in coloration.

The main objective of the present paper is to communicate a study of defects behavior in zirconia-based nanomaterials-pressure-compacted yttria-stabilized zirconia ͑YSZ͒ nanopowders with different contents of Y 2 O 3 and ceramics obtained by sintering the YZS nanopowders. In addition, YZS single crystals were also investigated. Positron annihilation techniques including positron lifetime and coincidence Doppler broadening with a conventional positron source and Doppler broadening experiments on a monoenergetic positron beam were involved in this study as the principal tools. These techniques were supplemented with transmission electron microscopy and x-ray diffraction observations. In order to get better support of the experimental data interpretation, the state-of-art theoretical calculations of positron parameters were performed for the perfect ZrO 2 lattice and selected defect configurations in the YSZ. Theoretical calculations have indicated that neither the oxygen vacancies nor their neutral complexes with substitutional yttrium atoms are capable of positron trapping. On the other hand, the zirconium vacancies are deep positron traps and obviously are responsible for the saturated positron trapping observed in the YSZ single crystals. In the compacted YSZ nanopowders, a majority of positrons is trapped either in the vacancylike defects situated in the negative space-charge layers along grain boundaries ͑ 1 Ϸ 185 ps͒ or in vacancy clusters at intersections of grain boundaries ͑ 2 Ϸ 370 ps͒. The intensity ratio I 2 / I 1 was found to be correlated with the mean grain size d as I 2 / I 1 ϳ d −2. A small fraction of positrons ͑Ϸ10%͒ form positronium in large pores ͑ 3 Ϸ 2 ns, 4 Ϸ 30 ns͒. A significant grain growth during sintering of the YSZ nanopowders above 1000°C was observed.

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