Editing Lanthanide (section)

====Oxides and hydroxides====
All of the lanthanides form sesquioxides, Ln<sub>2</sub>O<sub>3</sub>. The lighter/larger lanthanides adopt a hexagonal 7-coordinate structure while the heavier/smaller ones adopt a cubic 6-coordinate "C-M<sub>2</sub>O<sub>3</sub>" structure.<ref name="Wells"/> All of the sesquioxides are basic, and absorb water and carbon dioxide from air to form carbonates, hydroxides and hydroxycarbonates.<ref name = "Adachi">Adachi, G.; Imanaka, Nobuhito and Kang, Zhen Chuan (eds.) (2006) ''Binary Rare Earth Oxides''. Springer. {{ISBN|1-4020-2568-8}}</ref> They dissolve in acids to form salts.<ref name=CottonSA2006/>

Cerium forms a stoichiometric dioxide, CeO<sub>2</sub>, where cerium has an oxidation state of +4.  CeO<sub>2</sub> is basic and dissolves with difficulty in acid to form Ce<sup>4+</sup> solutions, from which Ce<sup>IV</sup> salts can be isolated, for example the hydrated nitrate Ce(NO<sub>3</sub>)<sub>4</sub>.5H<sub>2</sub>O.  CeO<sub>2</sub> is used as an oxidation catalyst in catalytic converters.<ref name=CottonSA2006/> Praseodymium and terbium form non-stoichiometric oxides containing Ln<sup>IV</sup>,<ref name=CottonSA2006/> although more extreme reaction conditions can produce stoichiometric (or near stoichiometric) PrO<sub>2</sub> and TbO<sub>2</sub>.<ref name = "Greenwood&Earnshaw"/>

Europium and ytterbium form salt-like monoxides, EuO and YbO, which have a rock salt structure.<ref name=CottonSA2006/> EuO is ferromagnetic at low temperatures,<ref name = "Greenwood&Earnshaw"/> and is a semiconductor with possible applications in [[spintronics]].<ref name = "Nasirpouri">Nasirpouri, Farzad and Nogaret, Alain (eds.) (2011) ''Nanomagnetism and Spintronics: Fabrication, Materials, Characterization and Applications''. World Scientific. {{ISBN|9789814273053}}</ref> A mixed Eu<sup>II</sup>/Eu<sup>III</sup> oxide Eu<sub>3</sub>O<sub>4</sub> can be produced by reducing Eu<sub>2</sub>O<sub>3</sub> in a stream of hydrogen.<ref name = "Adachi"/> Neodymium and samarium also form monoxides, but these are shiny conducting solids,<ref name = "Greenwood&Earnshaw"/> although the existence of samarium monoxide is considered dubious.<ref name = "Adachi"/>

All of the lanthanides form hydroxides, Ln(OH)<sub>3</sub>.  With the exception of lutetium hydroxide, which has a cubic structure, they have the hexagonal UCl<sub>3</sub> structure.<ref name = "Adachi"/> The hydroxides can be precipitated from solutions of Ln<sup>III</sup>.<ref name=CottonSA2006/> They can also be formed by the reaction of the sesquioxide, Ln<sub>2</sub>O<sub>3</sub>, with water, but although this reaction is thermodynamically favorable it is kinetically slow for the heavier members of the series.<ref name = "Adachi"/> [[Fajans' rules]] indicate that the smaller Ln<sup>3+</sup> ions will be more polarizing and their salts correspondingly less ionic.  The hydroxides of the heavier lanthanides become less basic, for example Yb(OH)<sub>3</sub> and Lu(OH)<sub>3</sub> are still basic hydroxides but will dissolve in hot concentrated [[sodium hydroxide|NaOH]].<ref name = "Greenwood&Earnshaw"/>