Thorium dioxide

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Thorium dioxide
IUPAC names
Thorium dioxide
Thorium(IV) oxide
Other names
Thorium anhydride
3D model (JSmol)
ECHA InfoCard 100.013.842
Molar mass 264.04 g/mol
Appearance white solid
Odor odorless
Density 10.00 g/cm3
Melting point 3,390 °C (6,130 °F; 3,660 K)
Boiling point 4,400 °C (7,950 °F; 4,670 K)
Solubility insoluble in alkali
slightly soluble in acid
−16.0·10−6 cm3/mol
2.200 (thorianite)
Fluorite (cubic), cF12
Fm3m, No. 225
a = 559.74(6) pm[1]
Tetrahedral (O2−); cubic (ThIV)
65.2(2) J K−1 mol−1
−1226(4) kJ/mol
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
400 mg/kg
Related compounds
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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Infobox references

Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in color. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production.[1] 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.[2] All thorium compounds are radioactive because there are no stable isotopes of thorium.

Structure and reactions[edit]

Thoria exists as two polymorphs. One has a fluorite crystal structure. This is uncommon among binary dioxides (others 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).[3] 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 an 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).[4]


Nuclear fuels[edit]

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 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.[5][6]


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.[7]

Radiocontrast agents[edit]

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

Lamp mantles[edit]

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.[9] Some mantles still use thorium, but yttrium oxide (or sometimes zirconium oxide) is used increasingly as a replacement.

Glass manufacture[edit]

Thorium dioxide was formerly added to glasses during manufacture to increase their refractive index, producing thoriated glass with up to 40% ThO2 content. These glasses were used in the construction of high-quality photographic lenses. However, the radioactivity of the thorium caused both a safety and pollution hazard and self-degradation of the glass (turning it yellow or brown over time). Lanthanum oxide has replaced thorium dioxide in almost all modern high-index glasses.


  1. ^ 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. doi:10.1016/S0022-3115(96)00750-7.
  2. ^ Emsley, John (2001). Nature's Building Blocks (Hardcover, First ed.). Oxford University Press. p. 441. ISBN 0-19-850340-7.
  3. ^ 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. doi:10.1016/j.jnucmat.2016.12.046.
  4. ^ Hoch, Michael; Johnston, Herrick L. (1954). "The Reaction Occurring on Thoriated Cathodes". J. Am. Chem. Soc. 76 (19): 4833–4835. doi:10.1021/ja01648a018.
  5. ^ Mitchell, Brian S (2004). An Introduction to Materials Engineering. and Science for Chemical and Materials. p. 473. ISBN 978-0-471-43623-2.
  6. ^ Robertson, Wayne M. (1979). "Measurement and evaluation of hydrogen trapping in thoria dispersed nickel". Metallurgical and Materials Transactions A. 10 (4): 489–501. doi:10.1007/BF02697077.
  7. ^ Wolfgang Stoll "Thorium and Thorium Compounds" Ullmann's Encyclopedia of Industrial Chemistry 2012 Wiley-VCH, Weinheim. doi:10.1002/14356007.a27_001
  8. ^
  9. ^ Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 1425, 1456. ISBN 0-08-022057-6.