Cerium(III) sulfide

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.

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-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).

Synthesis

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 Ã‚°C in an atmosphere of hydrogen sulfide:

Ce₂O₃ + 3 H₂S → Ce₂S₃ + 3 H₂O

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 Ã‚°C:

6 CeO₂ + 5 CS₂ → 3 Ce₂S₃ + 5 CO₂ + SO₂

Polymorphs

Ce₂S₃ exists in three polymorphic forms: α-Ce₂S₃ (orthorhombic, black colour), β-Ce₂S₃ (tetragonal, burgundy colour), γ-Ce₂S₃ (cubic, red colour). They are analogous to the crystal structures of the likewise trimorphic Pr₂S₃ and Nd₂S₃.

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 Ã‚°C) and with longer reaction times. The α- form can be irreversibly transformed into β-Ce₂S₃ by vacuum heating at 1200 Ã‚°C for 7 hours. Then γ-Ce₂S₃ is obtained from sintering of β-Ce₂S₃ powder via hot pressing at an even higher temperature (1700 Ã‚°C).

α polymorph

The α polymorph of cerium(III) sulfide has the same structure as α-. It contains both 7-coordinate and 8-coordinate cerium ions, , with monocapped and bicapped trigonal prismatic coordination geometry, respectively. The sulfide ions, , are 5-coordinate. Two thirds of them adopt a square pyramidal geometry and one third adopt a trigonal bipyramidal geometry.

γ polymorph

The γ polymorph of cerium(III) sulfide adopts a cation-deficient form of the structure. 8 out the 9 metal positions in the structure are occupied by cerium in γ-, with the remainder as vacancies. This composition can be represented by the formula . The cerium ions are 8-coordinate while the sulfide ions are 6-coordinate (distorted octahedral).

Reactions

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).

The reaction of Ce₂S₃ with Bi₂S₃ and Bi₂O₃ in a sealed tube at 950 Ã‚°C gives the parent compound CeOBiS₂:

3 Ce₂S₃ + Bi₂S₃ + 2 Bi₂O₃ → 6 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.

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_fG° 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.

Pigment and other uses

The main non-research use of cerium(III) sulfide is as a specialty inorganic pigment. γ-Ce₂S₃ is usually the main component of these pigments due to its bright red color. They are less toxic than heavy metal pigments such as cadmium sulfide, however, they are still hazardous due to the release of environmentally toxic cerium ions.

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. A practical implementation thereof has not been demonstrated so far.

References