Interstitial defects in monoclinic zirconia have been modeled in detail by Foster et al. . Using plane wave density functional theory, the tetragonal bonding and tripleplanar bonding geometries of lattice oxygen ions were determined. In addition, it was determined that interstitial defects can form stable defect pairs with either type of lattice oxygen ions (i. e., tetragonal or triply bonded). The analysis looked at defect pairs formed by interstitial oxygen ions with three possible charge states: 0, -1, and -2, bonded to triple-planar lattice oxygen ions. An analysis of oxygen vacancies both in the triple-planar and tetragonal geometries was also undertaken.
A neutral oxygen interstitial forming a defect pair with a triple-bonded oxygen is illustrated in Fig. 8 . Using the oxygen atomic energy as a reference, a single neutral oxygen can be incorporated in the lattice as an interstitial with an energy gain of -1.6 eV, if next to a triple-bonded lattice oxygen, and -0.8 eV, if next to a tetragonally bonded lattice oxygen. Figure 8 illustrates the fully relaxed charge density and positions of ions, showing that the interstitial and lattice oxygen form a strong covalent bond. The labels A and B associated with the lattice ions represent two different crystal planes within the structure. The lattice oxygen (OA), forming the defect pair with the interstitial oxygen, relaxes by up to 0.05 nm to accommodate the interstitial, distorting the triply-bonded oxygen with respect to the three zirconium ions bonded to it. The O-3Zr group has a slight pyramidal shape with its apex pointing away from the interstitial. The rest of the crystal remains more or less undisturbed, with the nearest zirconium (ZrA) only relaxing by about 0.005 nm. The case of a singly-charged oxygen interstitial forming a defect pair with a triply-bonded oxygen results in weakening of the covalent bond between the defect pair significantly. The extreme is the case of a doubly-charged oxygen interstitial in which the interstitial forms elongated bonds with the zirconium ions and occupies a new triple site, which is bonding with the ZrA, ZrB, and a new zirconium ion.