Cerium(III) sulfide, also known as cerium sesquisulfide, is an inorganic compound with the formula Ce2S3. It is the sulfide salt of cerium(III) and exists as three polymorphs with different crystal structures.[1][2][3]
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.[2]
The distinctive red colour of two of the polymorphs (α- and β-Ce2S3) and aforementioned chemical stability up to high temperatures have led to some limited commercial use as a red pigment (known as cerium sulfide red).[3]
The oldest syntheses reported for 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:[1][4]
Newer synthetic procedures utilise less toxic carbon disulfide gas for sulfurisation, starting from cerium dioxide which is reduced by the CS2 gas at temperatures of 800–1000 °C:[2]
Ce2S3 exists in three polymorphic forms: α-Ce2S3 (orthorhombic, burgundy colour), β-Ce2S3 (tetragonal, red colour), γ-Ce2S3 (cubic, black colour).[1][2][3] They are analogous to the crystal structures of the likewise trimorphic Pr2S3 and Nd2S3.[2]
Following the synthetic procedures given above will yield mostly the α- and β- polymorphs, with the proportion of α-Ce2S3 increasing at lower temperatures (~700–900 °C) and with longer reaction times.[2][3] The α- form can be irreversibly transformed into β-Ce2S3 by vacuum heating at 1200 °C for 7 hours. Then γ-Ce2S3 is obtained from sintering of β-Ce2S3 powder via hot pressing at an even higher temperature (1700 °C).[2]
The α polymorph of cerium(III) sulfide has the same structure as α-Gd2S3. It contains both 7-coordinate and 8-coordinate cerium ions, Ce3+, with monocapped and bicapped trigonal prismatic coordination geometry, respectively. The sulfide ions, S2−, are 5-coordinate.[5] Two thirds of them adopt a square pyramidal geometry and one third adopt a trigonal bipyramidal geometry.[6]
The γ polymorph of cerium(III) sulfide adopts a cation-deficient form of the Th3P4 structure. 8 out the 9 metal positions in the Th3P4 structure are occupied by cerium in γ-Ce2S3, with the remainder as vacancies. This composition can be represented by the formula Ce2.667□0.333S4. The cerium ions are 8-coordinate while the sulfide ions are 6-coordinate (distorted octahedral).[5][6]
Some reported reactions of cerium(III) sulfide are with bismuth compounds in order to form superconducting crystalline materials of the M(O,F)BiS2 family (for M=Ce).[7]
The reaction of Ce2S3 with Bi2S3 and Bi2O3 in a sealed tube at 950 °C gives the parent compound CeOBiS2:
This material is superconducting on its own, but the properties can be enhanced if it is doped with fluoride by including BiF3 in the reaction mixture.[7]
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.[2] Their suggested application was as the material in crucibles for the casting of uranium and plutonium metal.[2][4]
Although the sulfide's properties (high melting point and large, large negative ΔfG° and chemical inertness) are suitable and cerium is a relatively common element (66 ppm, about as much as copper), the danger of the traditional H2S-involving production route and the difficulty in controlling the formation of the resulting Ce2S3/CeS solid mixture meant that the compound was ultimately not developed further for such applications.[2]
The main non-research use of cerium(III) sulfide is as a specialty inorganic pigment.[3] The strong red hues of α- and β-Ce2S3, 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-Ce2S3 polymorph has a band gap of 2.06 eV and high Seebeck coefficient, thus it has been proposed as a high-temperature semiconductor for thermoelectric generators.[2] A practical implementation thereof has not been demonstrated so far.
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