Cerium(III) sulfide
Cerium(III) sulfide, also known as cerium sesquisulfide, is an inorganic compound with the formula CeS. It is the sulfide salt of cerium(III) and exists as three polymorphs with different crystal structures. Its high melting point (comparable to silica or alumina) and chemically inert nature have led to occasional examination of potential use as a refractory material for crucibles, but it has never been widely adopted for this application. The distinctive red colour of two of the polymorphs (α- and β-CeS) and aforementioned chemical stability up to high temperatures have led to some limited commercial use as a  red pigment (known as cerium sulfide red). Synthesis. The oldest syntheses reported for the of cerium(III) sulfide follow a typical rare earth sesquisulfide formation route, which involves heating the corresponding cerium sesquioxide to 900–1100 °C in an atmosphere of hydrogen sulfide: Newer synthetic procedures utilise less toxic carbon disulfide gas for sulfurisation, starting from cerium dioxide which is reduced by the CS gas at temperatures of 800–1000 °C: Polymorphs. CeS exists in three polymorphic forms: α-CeS (orthorhombic, burgundy colour), β-CeS (tetragonal, red colour), γ-CeS (cubic, black colour). They are analogous to the crystal structures of  the likewise trimorphic PrS and NdS. Following the synthetic procedures given above will yield mostly the α- and β- polymorphs, with the proportion of α-CeS increasing at lower temperatures (~700–900 °C) and with longer reaction times. The α- form can be irreversibly transformed into β-CeS by vacuum heating at 1200 °C for 7 hours. Then γ-CeS is obtained from sintering of β-CeS powder "via" hot pressing at an even higher temperature (1700 °C). Reactions. Some reported reactions of cerium(III) sulifde are with bismuth compounds in order to form superconducting crystalline materials of the  M(O,F)BiS family (for M=Ce). The reaction of CeS with BiS and BiO in a sealed tube at 950 °C gives the parent compound CeOBiS: This material is superconducting on its own, but the properties can be enhanced if it is doped with fluoride by including BiF in the reaction mixture. Applications. Refractory material. Cerium(III) and cerium(IV) sulfides were first investigated in the 1940s as part of the Manhattan project, where they were considered -but eventually not adopted- as advanced refractory materials. Their suggested application was as the material in crucibles for the casting of uranium and  plutonium metal. Although the sulfide's properties (high melting point and large, large negative Δ"G°" and chemical inertness) are suitable and cerium is a relatively common element (66 ppm, about as much as copper), the danger of the traditional HS-involving production route and the difficulty in controlling the formation of the resulting CeS/CeS solid mixture meant that the compound was ultimately not developed further for such applications. Pigment and other uses. The main non-research use of cerium(III) sulfide is as a specialty inorganic pigment. The strong red hues of α- and β-CeS, non-prohibitive cost  of cerium, and chemically inert behaviour up to high temperature are the factors which make the compound desirable as a pigment. Regarding other applications, the γ-CeS polymorph has a band gap of 2.06 eV and high Seeback coefficient, thus it has been proposed as a high-temperature semiconductor for thermoelectric generators. A practical implementation thereof has not been demonstrated so far.