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Oxides and hydroxides

All of the lanthanides form sesquioxides, Ln O. The lighter/larger lanthanides adopt a hexagonal 7-coordinate structure while the heavier/smaller ones adopt a cubic 6-coordinate "C-M O " structure. All of the sesquioxides are basic, and absorb water and carbon dioxide from air to form carbonates, hydroxides and hydroxycarbonates. They dissolve in acids to form salts.

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

Europium and ytterbium form salt-like monoxides, EuO and YbO, which have a rock salt structure. EuO is ferromagnetic at low temperatures, and is a semiconductor with possible applications in spintronics. A mixed Eu /Eu oxide Eu O can be produced by reducing Eu O in a stream of hydrogen. Neodymium and samarium also form monoxides, but these are shiny conducting solids, although the existence of samarium monoxide is considered dubious.

All of the lanthanides form hydroxides, Ln(OH). With the exception of lutetium hydroxide, which has a cubic structure, they have the hexagonal UCl structure. The hydroxides can be precipitated from solutions of Ln. They can also be formed by the reaction of the sesquioxide, Ln O, with water, but although this reaction is thermodynamically favorable it is kinetically slow for the heavier members of the series. Fajans' rules indicate that the smaller Ln ions will be more polarizing and their salts correspondingly less ionic. The hydroxides of the heavier lanthanides become less basic, for example Yb(OH) and Lu(OH) are still basic hydroxides but will dissolve in hot concentrated NaOH.