Characterization of point defects in yttria stabilized zirconia single crystals

Yttria stabilized zirconia (YSZ) has a big potential of use in a wide area of high-temperature applications. Although pure zirconia is monoclinic at room temperature and exhibits relatively poor mechanical properties, high temperature tetragonal or cubic zirconia phases with superior properties can be stabilized down to room temperature by the addition of an appropriate amount of yttria (Y2O3). The deviation from stoichiometry caused by the addition of trivalent yttrium ions leads to the formation of a high density of point defects in YSZ. These defects are believed to influence significantly not only phase stabilization itself but also material characteristics of YSZ’s important for their practical use. Hence, the detailed characterization of defects in YSZ is a very important task.
In this work we performed a thorough investigation of point defects in tetragonal and cubic YSZ single crystals. Experimental data were obtained by three complementary techniques of positron annihilation spectroscopy (PAS), namely slow positron implantation spectroscopy, positron lifetime measurements, and coincidence Doppler broadening. The interpretation of PAS data was performed with the help of state-of-the-art ab-inito theoretical calculations of positron parameters for various types of vacancy-like defects.
Present experimental data suggest that both tetragonal and cubic YSZ single crystals contain a high concentration of vacancy-like defects. Theoretical calculations showed that neither oxygen vacancies nor their neutral complexes with substitute yttrium atoms are capable of positron trapping. On the other hand, zirconium vacancies are deep positron traps and most probably responsible for positron trapping observed in YSZ single crystals. However, the calculated positron lifetime for a zirconium vacancy is apparently longer than the experimental value estimated for YSZ single crystals. We argue that this effect could be explained by hydrogen atoms trapped at zirconium vacancies. On the basis of structure relaxations, we found that zirconium vacancy – hydrogen complexes represent positron traps with the calculated lifetimes close to the experimental ones. In the vicinity of a zirconium vacancy the hydrogen atom forms an O-H bond with one of the nearest neighbour oxygen atoms. This result testifies that hydrogen is an important impurity in YSZ which strongly interacts with vacancies. Hence, the hydrogen content in YSZ materials should be considered as a very important parameter. In this work, therefore, we employed also nuclear reaction analysis for a determination of the hydrogen content in YSZ single crystals. It was found that the hydrogen content in our studied samples is in a range of 0.1-0.3 at.-%, which is sufficient for the formation of hydrogen complexes with zirconium vacancies capable of saturated positron trapping.