Thorium monoxide explained

Thorium monoxide (thorium(II) oxide), is the binary oxide of thorium having chemical formula ThO. The covalent bond in this diatomic molecule is highly polar. The effective electric field between the two atoms has been calculated to be about 80 gigavolts per centimeter, one of the largest known internal effective electric fields.^{[1] [2] [3] [4]}

Simple combustion of thorium in air produces thorium dioxide. However, laser ablation of thorium in the presence of oxygen gives the monoxide.^[5] Additionally, exposure of a thin film of thorium to low-pressure oxygen at medium temperature forms a rapidly growing layer of thorium monoxide under a more-stable surface coating of the dioxide.^[6]

At extremely high temperatures, thorium dioxide can convert to the monoxide either by a comproportionation reaction (equilibrium with liquid thorium metal) above 1850K or by simple dissociation (evolution of oxygen) above 2500K.^[7]

Notes and References

1. Skripnikov. L. V.. 2016-12-07. Combined 4-component and relativistic pseudopotential study of ThO for the electron electric dipole moment search. The Journal of Chemical Physics. en. 145. 21. 214301. 10.1063/1.4968229. 28799403. 0021-9606. 1610.00994. 2016JChPh.145u4301S . 42337394.
2. Denis. Malika. Fleig. Timo. 2016-12-07. In search of discrete symmetry violations beyond the standard model: Thorium monoxide reloaded. The Journal of Chemical Physics. en. 145. 21. 214307. 10.1063/1.4968597. 28799357. 2016JChPh.145u4307D . 0021-9606.
3. Skripnikov. L. V.. Petrov. A. N.. Titov. A. V.. 2013-12-14. Communication: Theoretical study of ThO for the electron electric dipole moment search. The Journal of Chemical Physics. en. 139. 22. 221103. 10.1063/1.4843955. 24329049. 0021-9606. 1308.0414. 2013JChPh.139v1103S . 42153944.
4. Web site: The ACME EDM Experiment. electronedm.org. 2018-08-16.
5. The pure rotational spectrum of the actinide-containing compound thorium monoxide . 10.1039/B709343H. 2007. Dewberry. Christopher T.. Etchison. Kerry C.. Cooke. Stephen A.. Physical Chemistry Chemical Physics. 9. 35. 4895–4897. 17912418 . 2007PCCP....9.4895D.
6. Heming . He . Jaroslaw . Majewski . David D. . Allred . Peng . Wang . Xiaodong . Wen . Kirk D. . Rector . 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 . 2017 . 288–296 . 10.1016/j.jnucmat.2016.12.046 . free . 2017JNuM..487..288H .
7. The Reaction Occurring on Thoriated Cathodes . Michael . Hoch . Herrick L. . Johnston . J. Am. Chem. Soc. . 1954 . 76 . 19 . 4833–4835 . 10.1021/ja01648a018 .