Editing Samarium (section)

===Halides===

[[File:Samarium(III) chloride hexahydrate.jpg|thumb|right|Samarium trichloride]]

Samarium metal reacts with all the [[halogen]]s, forming trihalides:<ref name="g1236">[[#Greenwood|Greenwood]], pp. 1236, 1241</ref>
:2 Sm (s) + 3 X<sub>2</sub> (g) → 2 SmX<sub>3</sub> (s) (X = F, Cl, Br or I)

Their further reduction with samarium, lithium or sodium metals at elevated temperatures (about 700–900&nbsp;°C) yields the dihalides.<ref name="smcl2" /> The diiodide can also be prepared by heating SmI<sub>3</sub>, or by reacting the metal with [[1,2-Diiodoethane|1,2-diiodoethane]] in anhydrous [[tetrahydrofuran]] at room temperature:<ref name="g1240">[[#Greenwood|Greenwood]], p. 1240</ref>
:Sm (s) + ICH<sub>2</sub>-CH<sub>2</sub>I → SmI<sub>2</sub> + CH<sub>2</sub>=CH<sub>2</sub>.

In addition to dihalides, the reduction also produces many [[Non-stoichiometric compound|non-stoichiometric]] samarium halides with a well-defined crystal structure, such as Sm<sub>3</sub>F<sub>7</sub>, Sm<sub>14</sub>F<sub>33</sub>, Sm<sub>27</sub>F<sub>64</sub>,<ref name="smf2" /> Sm<sub>11</sub>Br<sub>24</sub>, Sm<sub>5</sub>Br<sub>11</sub> and Sm<sub>6</sub>Br<sub>13</sub>.<ref>{{cite journal |last1=Baernighausen |first1=H. |last2=Haschke |first2=John M. |title=Compositions and crystal structures of the intermediate phases in the samarium-bromine system |journal=Inorganic Chemistry |volume=17 |page=18 |date=1978 |doi=10.1021/ic50179a005}}</ref>

Samarium halides change their crystal structures when one type of halide anion is substituted for another, which is an uncommon behavior for most elements (e.g. actinides). Many halides have two major crystal phases for one composition, one being significantly more stable and another being metastable. The latter is formed upon compression or heating, followed by quenching to ambient conditions. For example, compressing the usual monoclinic samarium diiodide and releasing the pressure results in a PbCl<sub>2</sub>-type orthorhombic structure (density 5.90&nbsp;g/cm<sup>3</sup>),<ref>{{cite journal|last1=Beck|first1=H. P.|title=Hochdruckmodifikationen der Diiodide von Sr., Sm und Eu. Eine neue PbCl2-Variante?|journal=Zeitschrift für anorganische und allgemeine Chemie|volume=459|page=81|date=1979|doi=10.1002/zaac.19794590108}}</ref> and similar treatment results in a new phase of samarium triiodide (density 5.97&nbsp;g/cm<sup>3</sup>).<ref>{{cite journal|last1=Beck|first1=H. P.|last2=Gladrow|first2=E.|title=Zur Hochdruckpolymorphie der Seltenerd-Trihalogenide|journal=Zeitschrift für anorganische und allgemeine Chemie|volume=453|page=79|date=1979|doi=10.1002/zaac.19794530610}}</ref>