| MolarMass = 264.04037&nbsp;g/mol<ref name=crc1/>

| Appearance = white solid<ref name=crc1/>

| Density = 10.000&nbsp;g/cm³<ref name=crc1>Haynes, p. 4.95</ref>

| Solubility = insoluble<ref name=crc1/>

| SolubleOther = insoluble in [[alkali]]<br>slightly soluble in [[acid]]<ref name=crc1/>

| MeltingPtC = 33903350

| RefractIndex = 2.200 (thorianite)<ref>Haynes, p. 4.144</ref>

| MagSus = &minus;16.0·10⁻⁶&nbsp;cm³/mol<ref>Haynes, p. 4.136</ref>

| CrystalStruct = [[Fluorite structure|Fluorite]] (cubic), [[Pearson symbol|''cF12'']]

| LattConst_a = 559.74(6) pm<ref name=Yamashita>{{Cite journal | title = Thermal expansions of NpO₂ and some other actinide dioxides | journal = J. Nucl. Mater. | volume = 245 | issue = 1 | year = 1997 | pages = 72–78 |author1=Yamashita, Toshiyuki |author2=Nitani, Noriko |author3=Tsuji, Toshihide |author4=Inagaki, Hironitsu | doi = 10.1016/S0022-3115(96)00750-7 | postscriptbibcode = 1997JNuM..245...72Y }}</ref>

| OtherAnions = [[Thorium(IV) sulfide]]

}}

'''Thorium dioxide''' (ThO₂), also called '''thorium(IV) oxide''', is a crystalline solid, often white or yellow in colorcolour. Also known as '''thoria''', it is produced mainly as a by-product of [[lanthanide]] and [[uranium]] production.<ref name=Yamashita/> [[Thorianite]] is the name of the mineralogical form of [[thorium]] dioxide. It is moderately rare and crystallizes in an isometric system. The melting point of thorium oxide is 3300&nbsp;°C – the highest of all known oxides. Only a few elements (including [[tungsten]] and [[carbon]]) and a few compounds (including [[tantalum carbide]]) have higher melting points.<ref>{{cite book | last = Emsley | first = John | title = Nature's Building Blocks | edition = Hardcover, First | publisher = [[Oxford University Press]] | year = 2001 | pages = [https://archive.org/details/naturesbuildingb0000emsl/page/441 441] | isbn = 978-0-19-850340-78 | url = https://archive.org/details/naturesbuildingb0000emsl/page/441 }}</ref> All thorium compounds, including the dioxide, are radioactive because there are no stable [[isotopes of thorium]].

Thoria exists as two polymorphs. One has a [[fluorite]] crystal structure. This is uncommon among [[binary compound|binary]] dioxides. (othersOther 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 in by ana [[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>

Thorium dioxide is used as a stabilizer in [[tungsten]] electrodes in [[tungsten inert gas welding|TIG welding]], electron tubes, and aircraft gas turbine engines. As an alloy, thoriated tungsten metal is not easily deformed because the high-fusion material thoria augments the high-temperature mechanical properties, and thorium helps stimulate the emission of [[electron]]s ([[thermion]]s). It is the most popular oxide additive because of its low cost, but is being phased out in favor of non-radioactive elements such as [[cerium]], [[lanthanum]] and [[zirconium]].

Thoria -dispersed nickel finds its applications in various high -temperature operations like combustion engines because it is a good creep -resistant material. It can also be used for hydrogen trapping.<ref>{{cite book | url = https://books.google.com/books?id=iQQcERxsNywC&pg=PA473 | page = 473 | isbn = 978-0-471-43623-2 | title = An Introduction to Materials Engineering. and Science for Chemical and Materials. | author1 = Mitchell, Brian S | year = 2004 | publisher = John Wiley & Sons }}</ref><ref>{{cite journal | first = Wayne M. | last = Robertson | title = Measurement and evaluation of hydrogen trapping in thoria dispersed nickel | journal = Metallurgical and Materials Transactions A | volume = 10 | issue = 4 | year = 1979 |doi = 10.1007/BF02697077 | pages =489&ndash;501 | postscriptbibcode = 1979MTA....10..489R | s2cid = 137105492 }}</ref><!ref name="KumarNasrallah1974">{{cite journal|last1=Kumar|first1=Arun|last2=Nasrallah|first2=M.|last3=Douglass|first3=D. L.|title=The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys|journal=Oxidation of Metals|volume=8|issue=4|year=1974|pages=227–263|issn=0030-770X|doi=10.1007/BF00604042|hdl=2060/19740015001|s2cid=95399863|hdl-access=free}}</ref><ref name="StringerWilcox1972">{{cite journal|last1=Stringer|first1=J.|last2=Wilcox|first2=B. A.|last3=Jaffee|first3=R. I.|title=The high-temperature oxidation of nickel-20 wt.% chromium alloys containing dispersed oxide phases|journal=Oxidation of Metals|volume=5|issue=1|year=1972|pages=11–47|issn=0030-770X|doi=10.1007/BF00614617|s2cid=92103123}}</ref><ref name="Murr1974">{{cite journal|last1=Murr|first1=L. E.|title=Interfacial energetics in the TD-nickel and TD-nichrome systems|journal=Journal of Materials Science|volume=9|issue=8|year=1974|pages=1309–1319|issn=0022-2461|doi=10.1007/BF00551849--|bibcode=1974JMatS...9.1309M |s2cid=96573790}}</ref>

Thorium dioxide has almost no value as a commercial catalyst, but such applications have been well investigated. It is a catalyst in the [[Ruzicka large ring synthesis]]. Other applications that have been explored include [[Cracking (chemistry)|petroleum cracking]], conversion of [[ammonia]] to [[nitric acid]] and preparation of [[sulfuric acid]].<ref name=Ullmann>Stoll, Wolfgang Stoll(2012) "Thorium and Thorium Compounds" in ''Ullmann's Encyclopedia of Industrial Chemistry 2012''. Wiley-VCH, Weinheim. {{DOIdoi|10.1002/14356007.a27_001}}</ref>

Thorium dioxide was the primary ingredient in [[Thorotrast]], a once-common [[radiocontrast agent]] used for [[cerebral angiography]], however, it causes a rare form of cancer (hepatic [[angiosarcoma]]) many years after administration.<ref>[https://radiopaedia.org/articles/thorotrast Thorotrast]. radiopaedia.org</ref> This use was replaced with injectable [[iodinated contrast|injectable iodine]] or ingestable [[barium sulfate suspension]] as standard [[X-ray]] contrast agents.

Another major use in the past was in [[gas mantle]] of lanterns developed by [[Carl Auer von Welsbach]] in 1890, which are composed of 99 percent% ThO₂ and 1% [[cerium(IV) oxide]]. Even as late as the 1980s it was estimated that about half of all ThO₂ produced (several hundred tonnes per year) was used for this purpose.<ref>{{Greenwood&Earnshaw1st|pages=1425, 1456}}</ref> Some mantles still use thorium, but [[yttrium oxide]] (or sometimes [[zirconium oxide]]) is used increasingly as a replacement.

[[Category:Thorium(IV) compounds]]