If sufficient quantities of the metastable tetragonal phase is present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion. In some cases, the tetragonal phase can be metastable. By adding small percentages of yttria, these phase changes are eliminated, and the resulting material has superior thermal, mechanical, and electrical properties.

Upon heating, zirconia undergoes disruptive phase changes. Zirconia is often more useful in its phase 'stabilized' state. Effective dopants include magnesium oxide (MgO), yttrium oxide ( Y 2O 3, yttria), calcium oxide ( CaO), and cerium(III) oxide ( Ce 2O 3). When the zirconia is blended with some other oxides, the tetragonal and/or cubic phases are stabilized. The change of volume caused by the structure transitions from tetragonal to monoclinic to cubic induces large stresses, causing it to crack upon cooling from high temperatures. ZrO 2 adopts a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures.

Zirconium dioxide is one of the most studied ceramic materials. This conversion is the basis for the purification of zirconium metal and is analogous to the Kroll process. When heated with carbon in the presence of chlorine, it converts to zirconium(IV) chloride. When heated with carbon, it converts to zirconium carbide. It is slowly attacked by concentrated hydrofluoric acid and sulfuric acid. This difference is attributed to the larger size of the zirconium atom relative to the titanium atom. Unlike TiO 2, which features six-coordinated titanium in all phases, monoclinic zirconia consists of seven-coordinated zirconium centres. The very rare mineral tazheranite, (Zr,Ti,Ca)O 2, is cubic. A small percentage of the oxides of calcium or yttrium stabilize in the cubic phase. The trend is for higher symmetry at higher temperatures, as is usually the case. Three phases are known: monoclinic below 1170 ☌, tetragonal between 1170 ☌ and 2370 ☌, and cubic above 2370 ☌. Zirconia is produced by calcining zirconium compounds, exploiting its high thermostability. Production, chemical properties, occurrence A dopant stabilized cubic structured zirconia, cubic zirconia, is synthesized in various colours for use as a gemstone and a diamond simulant. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. 2) (sometimes known as zirconia, not to be confused with zircon) is a white crystalline oxide of zirconium.

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