Editing Zirconium dioxide (section)

==Production, chemical properties, occurrence==
Zirconia is produced by [[calcination|calcining]] zirconium compounds, exploiting its high [[thermostability]].<ref name="Ullmann">Ralph Nielsen "Zirconium and Zirconium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a28_543}}</ref>

===Structure===
Three phases are known: monoclinic below 1170&nbsp;°C, tetragonal between 1170&nbsp;°C and 2370&nbsp;°C, and cubic above 2370&nbsp;°C.<ref>R. Stevens, 1986. Introduction to Zirconia. Magnesium Elektron Publication No 113</ref>  The trend is for higher symmetry at higher temperatures, as is usually the case. A small percentage of the oxides of calcium or yttrium stabilize in the cubic phase.<ref name="Ullmann"/>  The very rare mineral [[tazheranite]], {{chem2|(Zr,Ti,Ca)O2}}, is [[cubic crystal system|cubic]]. Unlike {{chem2|TiO2}}, which features six-coordinated titanium in all phases, monoclinic zirconia consists of seven-coordinated zirconium centres. This difference is attributed to the larger size of the zirconium atom relative to the titanium atom.<ref>Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. {{ISBN|0-7506-3365-4}}</ref>

===Chemical reactions===
Zirconia is chemically unreactive.  It is slowly attacked by concentrated [[hydrofluoric acid]] and [[sulfuric acid]].  When heated with carbon, it converts to [[zirconium carbide]].  When heated with carbon in the presence of chlorine, it converts to [[zirconium(IV) chloride]].  This conversion is the basis for the purification of zirconium metal and is analogous to the [[Kroll process]].