Thorium dioxide
IUPAC names
Thorium dioxide
Thorium(IV) oxide
Other names
Thorium anhydride
3D model (JSmol)
ECHA InfoCard 100.013.842 Edit this at Wikidata
EC Number
  • 215-225-1
UN number 2910 2909
  • InChI=1S/2O.Th
  • O=[Th]=O
Molar mass 264.037 g/mol[1]
Appearance white solid[1]
Odor odorless
Density 10.0 g/cm3[1]
Melting point 3,350 °C (6,060 °F; 3,620 K)[1]
Boiling point 4,400 °C (7,950 °F; 4,670 K)[1]
Solubility insoluble in alkali
slightly soluble in acid[1]
−16.0·10−6 cm3/mol[2]
2.200 (thorianite)[3]
Fluorite (cubic), cF12
Fm3m, No. 225
a = 559.74(6) pm[4]
Tetrahedral (O2−); cubic (ThIV)
65.2(2) J K−1 mol−1
−1226(4) kJ/mol
GHS labelling:[5]
GHS06: ToxicGHS08: Health hazard
H301, H311, H331, H350, H373
P203, P260, P261, P264, P270, P271, P280, P301+P316, P302+P352, P304+P340, P316, P318, P319, P321, P330, P361+P364, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazard RA: Radioactive. E.g. plutonium
Special hazard RA: Radioactive. E.g. plutonium
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
400 mg/kg
Related compounds
Other anions
Thorium(IV) sulfide
Other cations
Hafnium(IV) oxide
Cerium(IV) oxide
Related compounds
Protactinium(IV) oxide
Uranium(IV) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Thorium dioxide (ThO2), 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.[4] 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 °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.[6] 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 dioxides. (Other binary oxides with fluorite structure include cerium dioxide, uranium dioxide and plutonium dioxide.)[clarification needed] The band gap of thoria is about 6 eV. A tetragonal form of thoria is also known.

Thorium dioxide is more stable than thorium monoxide (ThO).[7] 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 1,850 K (1,580 °C; 2,870 °F) or by simple dissociation (evolution of oxygen) above 2,500 K (2,230 °C; 4,040 °F).[8]


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 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.


Thorium dioxide is used as a stabilizer in tungsten electrodes in 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 electrons (thermions). 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.[9][10][11][12][13]


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 petroleum cracking, conversion of ammonia to nitric acid and preparation of sulfuric acid.[14]

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.[15] This use was replaced with injectable iodine or ingestable barium sulfate suspension as standard X-ray contrast agents.

Lamp mantles

Main article: 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% 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.[16] Some mantles still use thorium, but yttrium oxide (or sometimes zirconium oxide) is used increasingly as a replacement.

Glass manufacture

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. Such glass finds application in high-quality lenses for cameras and scientific instruments.[17] 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.[18] 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.[19]


  1. ^ a b c d e f g Haynes, p. 4.95
  2. ^ Haynes, p. 4.136
  3. ^ Haynes, p. 4.144
  4. ^ a b Yamashita, Toshiyuki; Nitani, Noriko; Tsuji, Toshihide; Inagaki, Hironitsu (1997). "Thermal expansions of NpO2 and some other actinide dioxides". J. Nucl. Mater. 245 (1): 72–78. Bibcode:1997JNuM..245...72Y. doi:10.1016/S0022-3115(96)00750-7.
  5. ^ "Thorium dioxide".
  6. ^ Emsley, John (2001). Nature's Building Blocks (Hardcover, First ed.). Oxford University Press. pp. 441. ISBN 978-0-19-850340-8.
  7. ^ He, Heming; Majewski, Jaroslaw; Allred, David D.; Wang, Peng; Wen, Xiaodong; Rector, Kirk D. (2017). "Formation of solid thorium monoxide at near-ambient conditions as observed by neutron reflectometry and interpreted by screened hybrid functional calculations". Journal of Nuclear Materials. 487: 288–296. Bibcode:2017JNuM..487..288H. doi:10.1016/j.jnucmat.2016.12.046.
  8. ^ Hoch, Michael; Johnston, Herrick L. (1954). "The Reaction Occurring on Thoriated Cathodes". J. Am. Chem. Soc. 76 (19): 4833–4835. doi:10.1021/ja01648a018.
  9. ^ Mitchell, Brian S (2004). An Introduction to Materials Engineering. and Science for Chemical and Materials. John Wiley & Sons. p. 473. ISBN 978-0-471-43623-2.
  10. ^ Robertson, Wayne M. (1979). "Measurement and evaluation of hydrogen trapping in thoria dispersed nickel". Metallurgical and Materials Transactions A. 10 (4): 489–501. Bibcode:1979MTA....10..489R. doi:10.1007/BF02697077. S2CID 137105492.
  11. ^ Kumar, Arun; Nasrallah, M.; Douglass, D. L. (1974). "The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys". Oxidation of Metals. 8 (4): 227–263. doi:10.1007/BF00604042. hdl:2060/19740015001. ISSN 0030-770X. S2CID 95399863.
  12. ^ Stringer, J.; Wilcox, B. A.; Jaffee, R. I. (1972). "The high-temperature oxidation of nickel-20 wt.% chromium alloys containing dispersed oxide phases". Oxidation of Metals. 5 (1): 11–47. doi:10.1007/BF00614617. ISSN 0030-770X. S2CID 92103123.
  13. ^ Murr, L. E. (1974). "Interfacial energetics in the TD-nickel and TD-nichrome systems". Journal of Materials Science. 9 (8): 1309–1319. Bibcode:1974JMatS...9.1309M. doi:10.1007/BF00551849. ISSN 0022-2461. S2CID 96573790.
  14. ^ Stoll, Wolfgang (2012) "Thorium and Thorium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. doi:10.1002/14356007.a27_001
  15. ^ Thorotrast.
  16. ^ Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 1425, 1456. ISBN 978-0-08-022057-4.
  17. ^ Hammond, C. R. (2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC Press. ISBN 978-0-8493-0485-9.
  18. ^ Oak Ridge Associated Universities (1999). "Thoriated Camera Lens (ca. 1970s)". Retrieved 29 September 2017.
  19. ^ Stoll, W. (2005). "Thorium and Thorium Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. p. 32. doi:10.1002/14356007.a27_001. ISBN 978-3-527-31097-5.

Cited sources