Cerium(III) sulfide, also known as cerium sesquisulfide, is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

with the formula Ce₂S₃. It is the

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...

salt of cerium(III) and exists as three polymorphs with different crystal structures. Its high melting point (comparable to

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

or alumina) and chemically inert nature have led to occasional examination of potential use as a

refractory In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...

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 A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...

(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 Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...

: : 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 °C: : 6 CeO₂ + 5 CS₂ → 3 Ce₂S₃ + 5 CO₂ + SO₂

Polymorphs

Ce₂S₃ exists in three polymorphic forms: α-Ce₂S₃ (

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

burgundy Burgundy (; french: link=no, Bourgogne ) is a historical territory and former administrative region and province of east-central France. The province was once home to the Dukes of Burgundy from the early 11th until the late 15th century. The c ...

colour), β-Ce₂S₃ ( tetragonal, red colour), γ-Ce₂S₃ (

cubic Cubic may refer to: Science and mathematics * Cube (algebra), "cubic" measurement * Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex ** Cubic crystal system, a crystal system w ...

, black 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 °C) and with longer reaction times. 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 Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

of β-Ce₂S₃ powder ''via''

hot pressing Hot pressing is a high-pressure, low-strain-rate powder metallurgy process for forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. This is achieved by the simultaneous application of heat a ...

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)BiS₂ family (for M=Ce). The reaction of Ce₂S₃ with Bi₂S₃ and Bi₂O₃ in a sealed tube at 950 °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 The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...

, where they were considered -but eventually not adopted- as advanced

materials. Their suggested application was as the material in crucibles for the casting of

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

and

plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...

metal. Although the sulfide's properties (high melting point and large, large negative Δ_f''G°'' and chemical inertness) are suitable and cerium is a relatively

common Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common, a central public park in Boston, Massachusetts * Cambridge Common, common land area in Cambridge, Massachusetts * Clapham Common, originally com ...

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. 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 generator A thermoelectric generator (TEG), also called a Seebeck generator, is a solid state device that converts heat flux (temperature differences) directly into electrical energy through a phenomenon called the ''Seebeck effect'' (a form of thermoele ...

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

References

{{Sulfides Sulfides Cerium(III) compounds Refractory materials Inorganic pigments