Crystalline and Noncrystalline Structures
The complex crystallography of zirconia plays an important role in the challenges to develop commercially viable applications of this material. At room temperature, bonding in this material is a combination of ionic and covalent and results in a structure in which zirconium is seven-coordinated, which is rather unusual and is a product of the large difference in ionic sizes between zirconium and oxygen. The formation of this material from pure a-Zr and oxygen starts at around 23 at.% O, corresponding to a composition of ZrO03 . The perfect stoichiometry for this material in which there is one zirconium and two oxygen ions for each formula unit is not used in industrial applications. Doping of the structure produces oxygen vacancies resulting
in chemical formulas of the type ZrCaxO2-x, for the case of CaO doping, to maintain charge balance in the structure.