Editing Thorium dioxide (section)

==Structure and reactions==
Thoria exists as two polymorphs. One has a [[fluorite]] crystal structure. This is uncommon among [[binary compound|binary]] dioxides. (Other binary oxides with fluorite structure include [[cerium dioxide]], [[uranium dioxide]] and [[plutonium dioxide]].){{clarify|date=August 2018}}<!-- these other examples are fluorite structures also; need secondary ref to discuss why this is worth mentioning and [[WP:V]] that it is true --> The [[band gap]] of thoria is about 6&nbsp;[[Electronvolt|eV]]. A tetragonal form of thoria is also known.

Thorium dioxide is more stable than [[thorium monoxide]] (ThO).<ref>{{cite journal |first1= Heming |last1= He |first2= Jaroslaw |last2= Majewski |first3= David D. |last3= Allred |first4= Peng |last4= Wang |first5= Xiaodong |last5= Wen |first6= Kirk D. |last6= Rector |title= Formation of solid thorium monoxide at near-ambient conditions as observed by neutron reflectometry and interpreted by screened hybrid functional calculations |journal= Journal of Nuclear Materials |volume= 487 |year= 2017 |pages= 288–296 |doi= 10.1016/j.jnucmat.2016.12.046 |bibcode= 2017JNuM..487..288H |doi-access= free }}</ref> Only with careful control of reaction conditions can oxidation of thorium metal give the monoxide rather than the dioxide. At extremely high temperatures, the dioxide can convert to the monoxide either by a [[disproportionation reaction]] (equilibrium with liquid thorium metal) above {{convert|1850|K|°C °F}} or by simple dissociation (evolution of oxygen) above {{convert|2500|K|°C °F}}.<ref>{{cite journal |title= The Reaction Occurring on Thoriated Cathodes |first1= Michael |last1= Hoch |first2= Herrick L. |last2= Johnston |journal= J. Am. Chem. Soc. |year= 1954 |volume= 76 |issue= 19 |pages= 4833–4835 |doi= 10.1021/ja01648a018 }}</ref>