  Zirconia (ZrO2) is an important material with technological applications
which are affected by point defect physics. Ab-initio calculations are
performed to understand the structural and electronic properties of oxygen
vacancies and interstitials in different charge states in cubic zirconia. We
find oxygen interstitials in cubic ZrO2 can have five different configurations
- <110> dumbbell, <100> dumbbell, <100> crowd-ion, octahedral, and <111>
distorted dumbbell. For a neutral and singly charged oxygen interstitial, the
lowest energy configuration is the <110> dumbbell, while for a doubly charged
oxygen interstitial the octahedral site is energetically the most favorable.
Both the oxygen interstitial and the oxygen vacancy are negative-U, so that the
singly charged defects are unstable at any Fermi level. The thermodynamic
stability of these defects are studied in terms of Fermi level, oxygen partial
pressure and temperature. A method to determine the chemical potential of the
system as a function of temperature and pressure is proposed.
