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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Positron lifetimes and high momentum profiles both for the perfect lattice and selected defects are calculated in three (cubic, tetragonal and monoclinic) zirconia polymorphs using the atomic superposition method. Theoretical data are compared with the measured positron lifetime for cubic and tetragonal monocrystals of yttria-stabilized zirconia (YSZ) and coincidence Doppler broadening measurements on tetragonal monocrystals of YSZ. Positron lifetime spectra of YSZ monocrystals exhibit a single component spectrum with lifetimes 178 ps and 174 ps for cubic and tetragonal phases, respectively. Possible interpretations of measured lifetime and Doppler data are discussed.1 Introduction Zirconia (ZrO 2 ) based materials are promising for many practical applications, including heat-resistant structural and functional ceramics, solid oxide fuel cells and oxygen sensors, as well as applications in nuclear fuel and waste confinement. Positron annihilation may bring important information on defects in this class of materials, which is a substantial prerequisite for understanding zirconia properties in general.At room temperature pure ZrO 2 exhibits the monoclinic baddelyite structure (m-ZrO 2 , space group c P 1 2 [1]) with the Zr 4+ ion in a distorted seven-fold coordination. On increasing the temperature, the structure transforms into a tetragonally distorted fluorite structure (t-ZrO 2 , nmc P 2 4[1]) at T ~ 1370 K, with Zr 4+ surrounded by eight anions, but with two slightly different Zr 4+ -O 2-distances. Perfect eightfold coordination is achieved at T ~ 2643 K with a transformation to a cubic fluorite structured phase (cZrO 2 , m Fm3 [1]), followed by melting at T ~ 2988 K. High-temperature c-ZrO 2 and t-ZrO 2 structures can be stabilized at room temperature by an addition of yttrium oxide (Y 2 O 3 ). These so-called "yttria- Obviously, positron annihilation spectroscopy (PAS) has a high potential for the investigation of defects in YSZ. However, the assignment of experimental results to corresponding defect configurations is still on the level of hypotheses without direct and convincing confirmation. Dominant positron trapping sites in YSZ are supposed to be associated with complexes (