Editing Rare-earth element (section)

==Compounds==
Rare-earth elements occur in nature in combination with [[phosphate]] ([[monazite]]), [[carbonate]]-[[fluoride]] ([[bastnäsite]]), and oxygen anions.

In their oxides, most rare-earth elements only have a valence of 3 and form [[sesquioxide]]s (cerium forms {{chem2|CeO2}}). Five different crystal structures are known, depending on the element and the temperature. The X-phase and the H-phase are only stable above 2000&nbsp;K. At lower temperatures, there are the hexagonal A-phase, the monoclinic B-phase, and the cubic C-phase, which is the stable form at room temperature for most of the elements. The C-phase was once thought to be in [[space group]] ''I''2{{sub|1}}3 (no. 199),<ref>{{cite journal |last1=William Zachariasen |title=The Crystal Structure of the Modification C of the Sesquioxides of the Rare Earth Metals, and of Indium and Thallium |journal=[[Norsk Geologisk Tidsskrift]] |date=Jan 11, 1927 |volume=9 |pages=310–316 |url=https://njg.geologi.no/images/NJG_articles/NGT_09_3_4_310-316.pdf}}</ref> but is now known to be in space group ''Ia''{{overline|3}} (no. 206). 

The structure is similar to that of [[fluorite]] or [[cerium dioxide]] (in which the cations form a [[face-centred cubic]] lattice and the anions sit inside the tetrahedra of cations), except that one-quarter of the anions (oxygen) are missing. The [[unit cell]] of these sesquioxides corresponds to eight unit cells of fluorite or cerium dioxide, with 32 cations instead of 4. This is called the [[bixbyite]] structure, as it occurs in a mineral of that name ({{chem2|(Mn,Fe)2O3}}).<ref>{{cite journal |author=M. V. Abrashev |author2=N. D. Todorov |author3=J. Geshev |title=Raman spectra of R 2O3 (R—rare earth) sesquioxides with C-type bixbyite crystal structure: A comparative study |journal=Journal of Applied Physics |date=Sep 9, 2014 |volume=116 |issue=10 |page=103508 |doi=10.1063/1.4894775 |bibcode=2014JAP...116j3508A |hdl=10183/107858 |s2cid=55024339 }}</ref>