User:A. T. Galenitis/sandbox/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.

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:


 * 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, burgundy colour), β-Ce2S3 (tetragonal, red colour), γ-Ce2S3 (cubic, 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 of β-Ce2S3 powder via hot pressing at an even higher temperature (1700 °C).

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, negative ΔfG° i.e. 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. 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 Seeback coefficient, thus it has been proposed as a high-temperature semiconductor for thermoelectric generators. A practical implementation has not been demonstrated so far.