Positron annihilation in three zirconia polymorphs

Abstract

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 (

Positron Lifetimes in Zirconia-Based Nanomaterials

“…On the other hand, the interpretation of lifetime τ 1 , observed for the monocrystalline specimens, as that of free positrons in the bulk is not straightforward. As shown in our recent study [10], slightly shorter bulk lifetimes are suggested by theoretical calculations for zirconia polymorphs. A more open-space structure of the lattice due to the non-stoichiometry-introduced oxygen vacancies in the YSZ system was ibidem tentatively considered as a possible cause for such a discrepancy.…”

Sintering of yttria‐stabilized zirconia nanopowders studied by positron annihilation spectroscopy

“…A remarkable decrease of τ av with sintering temperature evidences that a recovery of open-volume defects takes place during sintering. By similarity of present lifetimes with those observed for compacted ZrO 2 +3 mol.% Y 2 O 3 and other similar YSZ nanopowders at the room temperature [7,8] we attribute τ 2 -and τ 3 -components of Table 1 to the annihilation of positrons trapped in vacancy-like defects situated in the space charge layers along the GB's (τ 2 ≈ 0.180 ns) and triple points (τ 3 ≈ 0.375 ns). Based on simple spatial considerations, the intensity ratio I 3 /I 2 was found to correlate with average grain size d as…”

“…• C is expected to contain also a contribution from positron trapped in defects inside grains known from the investigations on YSZ single crystals [6,8].…”

“…A potential use of the short-lived LT components for disclosing further details of sintering process in the YSZ nanopowders was discussed in Ref. [8]. In the present Contribution, therefore, we focus on the microstructural changes induced in the tetragonal ZrO 2 +3 mol.% Y 2 O 3 during sintering nanopowders.…”

Defects in yttria‐stabilised zirconia: a positron annihilation study

“…These measurements are supplemented by a coincidence Doppler broadening (CDB) investigation using two HPGe detectors. The nature of defects observed in these materials is discussed on the basis of available experimental data and their comparison with the theoretical calculations of positron lifetimes and high-momentum parts of Doppler profiles, performed [5] within the framework of the atomic superposition method considering appropriate vacancy-type defect configurations. …”