Editing Lanthanide (section)

====Chalcogenides (S, Se, Te)====
All of the lanthanides form Ln<sub>2</sub>Q<sub>3</sub> (Q= S, Se, Te).<ref name=CottonSA2006/> The sesquisulfides can be produced by reaction of the elements or (with the exception of Eu<sub>2</sub>S<sub>3</sub>) sulfidizing the oxide (Ln<sub>2</sub>O<sub>3</sub>) with H<sub>2</sub>S.<ref name=CottonSA2006/> The sesquisulfides, Ln<sub>2</sub>S<sub>3</sub> generally lose sulfur when heated and can form a range of compositions between Ln<sub>2</sub>S<sub>3</sub> and Ln<sub>3</sub>S<sub>4</sub>. The sesquisulfides are insulators but some of the Ln<sub>3</sub>S<sub>4</sub> are metallic conductors (e.g. Ce<sub>3</sub>S<sub>4</sub>) formulated (Ln<sup>3+</sup>)<sub>3</sub> (S<sup>2−</sup>)<sub>4</sub> (e<sup>−</sup>), while others (e.g. Eu<sub>3</sub>S<sub>4</sub> and Sm<sub>3</sub>S<sub>4</sub>) are semiconductors.<ref name=CottonSA2006/> Structurally the sesquisulfides adopt structures that vary according to the size of the Ln metal. The lighter and larger lanthanides favoring 7-coordinate metal atoms, the heaviest and smallest lanthanides (Yb and Lu) favoring 6 coordination and the rest structures with a mixture of 6 and 7 coordination.<ref name=CottonSA2006/> 

Polymorphism is common amongst the sesquisulfides.<ref name="Flahaut handbookvol4">{{cite book |last=Flahaut |first=Jean |editor-first=K. A. Jr. |editor-last=Gschneider |title=Handbook on the Physics and Chemistry of Rare Earths vol 4|pages=100–110 |year=1979|chapter=Chapter 31:Sulfides, Selenides and Tellurides|publisher=North Holland Publishing Company|isbn=978-0-444-85216-8}}</ref> The colors of the sesquisulfides vary metal to metal and depend on the polymorphic form. The colors of the γ-sesquisulfides are La<sub>2</sub>S<sub>3</sub>, white/yellow; Ce<sub>2</sub>S<sub>3</sub>, dark red; Pr<sub>2</sub>S<sub>3</sub>, green; Nd<sub>2</sub>S<sub>3</sub>, light green; Gd<sub>2</sub>S<sub>3</sub>, sand; Tb<sub>2</sub>S<sub>3</sub>, light yellow and Dy<sub>2</sub>S<sub>3</sub>, orange.<ref name="HighPerfPigs">{{cite book |last=Berte |first=Jean-Noel |editor-first=Hugh M. |editor-last=Smith |title=High Performance Pigments|year=2009|chapter= Cerium pigments|publisher= Wiley-VCH|isbn=978-3-527-30204-8}}</ref> The shade of γ-Ce<sub>2</sub>S<sub>3</sub> can be varied by doping with Na or Ca with hues ranging from dark red to yellow,<ref name = "Atwood"/><ref name="HighPerfPigs"/> and Ce<sub>2</sub>S<sub>3</sub> based pigments are used commercially and are seen as low toxicity substitutes for cadmium based pigments.<ref name="HighPerfPigs"/>

All of the lanthanides form monochalcogenides, LnQ, (Q= S, Se, Te).<ref name=CottonSA2006/> The majority of the monochalcogenides are conducting, indicating a formulation Ln<sup>III</sup>Q<sup>2−</sup>(e-) where the electron is in conduction bands. The exceptions are SmQ, EuQ and YbQ which are semiconductors or insulators but exhibit a pressure induced transition to a conducting state.<ref name="Flahaut handbookvol4"/>
Compounds LnQ<sub>2</sub> are known but these do not contain Ln<sup>IV</sup> but are Ln<sup>III</sup> compounds containing polychalcogenide anions.<ref name="HOWI2">[[#Holleman|Holleman]], p.&nbsp;1944.</ref>

Oxysulfides Ln<sub>2</sub>O<sub>2</sub>S are well known, they all have the same structure with 7-coordinate Ln atoms, and 3 sulfur and 4 oxygen atoms as near neighbours.<ref>Liu, Guokui and Jacquier, Bernard (eds) (2006)  ''Spectroscopic Properties of Rare Earths in Optical Materials'', Springer</ref>
Doping these with other lanthanide elements produces phosphors. As an example, [[gadolinium oxysulfide]], Gd<sub>2</sub>O<sub>2</sub>S doped with Tb<sup>3+</sup> produces visible photons when irradiated with high energy X-rays and is used as a [[scintillator]] in flat panel detectors.<ref name="Intmicro">{{cite book |last=Sisniga |first=Alejandro |editor-first=Krzysztof |editor-last= Iniewski|title=Integrated Microsystems: Electronics, Photonics, and Biotechnology|year=2012|chapter= Chapter 15|publisher=CRC Press|isbn=978-3-527-31405-8}}</ref>
When [[mischmetal]], an alloy of lanthanide metals, is added to molten steel to remove oxygen and sulfur, stable oxysulfides are produced that form an immiscible solid.<ref name=CottonSA2006/>