Editing Thorium dioxide

{{short description|Chemical compound}}
{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 433232426
| Name = Thorium dioxide
| ImageFile = Fluorite-unit-cell-3D-ionic.png
| ImageSize =
| ImageName =
| IUPACName = Thorium dioxide<br>Thorium(IV) oxide
| OtherNames = Thoria<br>Thorium anhydride
|Section1={{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 1314-20-1
| ChEBI = 37339
| ChemSpiderID = 14124
| EC_number = 215-225-1
| Gmelin = 141638
| PubChem = 14808
| UNNumber = 2910 2909
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 9XA7X17UQC
| InChI = 1S/2O.Th
| StdInChIKey = ZCUFMDLYAMJYST-UHFFFAOYSA-N
| SMILES = O=[Th]=O
  }}
|Section2={{Chembox Properties
| Formula = ThO<sub>2</sub>
| MolarMass = 264.037&nbsp;g/mol<ref name=crc1/>
| Appearance = white solid<ref name=crc1/>
| Odor = odorless
| Density = 10.0&nbsp;g/cm<sup>3</sup><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 = 3350
| MeltingPt_ref=<ref name=crc1/>
| BoilingPtC = 4400
| BoilingPt_ref=<ref name=crc1/>
| RefractIndex = 2.200 (thorianite)<ref>Haynes, p. 4.144</ref>
| MagSus = &minus;16.0·10<sup>−6</sup>&nbsp;cm<sup>3</sup>/mol<ref>Haynes, p. 4.136</ref>
  }}
|Section3={{Chembox Structure
| CrystalStruct = [[Fluorite structure|Fluorite]] (cubic), [[Pearson symbol|''cF12'']]
| SpaceGroup = Fm<u style="text-decoration:overline">3</u>m, No. 225 
| Coordination = Tetrahedral (O<sup>2−</sup>); cubic (Th<sup>IV</sup>)
| LattConst_a = 559.74(6) pm<ref name=Yamashita>{{Cite journal | title = Thermal expansions of NpO<sub>2</sub> 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 | bibcode = 1997JNuM..245...72Y }}</ref>
  }}
|Section4={{Chembox Thermochemistry
| DeltaHf = &minus;1226(4) kJ/mol
| Entropy = 65.2(2)&nbsp;J&thinsp;K<sup>&minus;1</sup>&thinsp;mol<sup>&minus;1</sup>
  }}
|Section7={{Chembox Hazards
| GHS_ref=<ref>{{cite web |title=Thorium dioxide |url=https://pubchem.ncbi.nlm.nih.gov/compound/14808#section=Safety-and-Hazards |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref>
| GHSPictograms = {{GHS06}}{{GHS08}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|301|311|331|350|373}}
| PPhrases = {{P-phrases|203|260|261|264|270|271|280|301+316|302+352|304+340|316|318|319|321|330|361+364|403+233|405|501}}
| ExternalSDS = 
| NFPA-H = 2
| NFPA-F = 0
| NFPA-R = 0
| NFPA-S = RA
| FlashPt = Non-flammable
| LD50 = 400&nbsp;mg/kg
  }}
|Section8={{Chembox Related
| OtherAnions = [[Thorium(IV) sulfide]]
| OtherCations = [[Hafnium(IV) oxide]]<br/>[[Cerium(IV) oxide]]
| OtherFunction = 
| OtherFunction_label = 
| OtherCompounds = [[Protactinium(IV) oxide]]<br/>[[Uranium(IV) oxide]]
  }}

}}
'''Thorium dioxide''' (ThO<sub>2</sub>), also called '''thorium(IV) oxide''', is a crystalline solid, often white or yellow in colour. Also known as '''thoria''',  it is mainly 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 = 978-0-19-850340-8 | 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]].

==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>

==Applications==
===Nuclear fuels===
Thorium dioxide (thoria) can be used in nuclear reactors as ceramic fuel pellets, typically contained in nuclear fuel rods clad with zirconium alloys.  Thorium is not fissile (but is "fertile", breeding fissile [[uranium-233]] under neutron bombardment); hence, it must be used as a nuclear reactor fuel in conjunction with fissile isotopes of either uranium or plutonium.  This can be achieved by blending thorium with uranium or plutonium, or using it in its pure form in conjunction with separate fuel rods containing uranium or plutonium.  Thorium dioxide offers advantages over conventional uranium dioxide fuel pellets, because of its higher thermal conductivity (lower operating temperature), considerably higher melting point, and chemical stability (does not oxidize in the presence of water/oxygen, unlike uranium dioxide).

Thorium dioxide can be turned into a [[nuclear reaction|nuclear]] fuel by breeding it into uranium-233 (see below and refer to the article on [[thorium]] for more information on this). The high [[thermal stability]] of thorium dioxide allows applications in flame spraying and high-temperature ceramics.

===Alloys===
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 | bibcode = 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>

===Catalysis===
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 (2012) "Thorium and Thorium Compounds" in ''Ullmann's Encyclopedia of Industrial Chemistry''. Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a27_001}}</ref>

===Radiocontrast agents===
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 [[iodinated contrast|injectable iodine]] or ingestable [[barium sulfate suspension]] as standard [[X-ray]] contrast agents.

===Lamp mantles===
{{Main|Gas mantle}}
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% ThO<sub>2</sub> and 1% [[cerium(IV) oxide]].  Even as late as the 1980s it was estimated that about half of all ThO<sub>2</sub> 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.

===Glass manufacture===
[[File:Yellowing of thorium lenses.jpg|left|thumb|alt=Three lenses from yellowed to transparent left-to-right|Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)]]
When added to [[glass]], thorium dioxide helps increase its [[refractive index]] and decrease [[dispersion (optics)|dispersion]]. Such glass finds application in high-quality [[lens (optics)|lenses]] for cameras and scientific instruments.<ref name=CRC>{{cite book| last= Hammond| first= C. R.| title= The Elements, in Handbook of Chemistry and Physics| edition= 81st| publisher= [[CRC Press]]| isbn= 978-0-8493-0485-9| date= 2004| url-access= registration| url= https://archive.org/details/crchandbookofche81lide}}</ref> The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal.<ref>{{Cite web|author=Oak Ridge Associated Universities|year=1999|title=Thoriated Camera Lens (ca. 1970s)|url=https://orau.org/health-physics-museum/collection/consumer/products-containing-thorium/camera-lens.html|access-date=29 September 2017}}</ref> Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced by rare-earth oxides such as [[lanthanum oxide]] in almost all modern high-index glasses, as they provide similar effects and are not radioactive.<ref>{{cite book |first=W. |last=Stoll |chapter=Thorium and Thorium Compounds |doi=10.1002/14356007.a27_001 |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH |year=2005 |isbn=978-3-527-31097-5 |page=32}}</ref>
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==References==
{{reflist}}

==Cited sources==
*{{cite book | editor= Haynes, William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = [[CRC Press]] | isbn = 978-1439855119| title-link = CRC Handbook of Chemistry and Physics }}

{{Thorium compounds}}
{{Oxides}}

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[[Category:Hepatotoxins]]
[[Category:Oxides]]
[[Category:Thorium(IV) compounds]]
[[Category:Refractory materials]]
[[Category:Fluorite crystal structure]]