# Cerium(III) sulfide

Cerium(III) sulfide, also known as cerium sesquisulfide, is an inorganic compound with the formula Ce$_2$S$_3$. 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-Ce$_2$S$_3$) 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 °C in an atmosphere of hydrogen sulfide:
    Ce$_2$O$_3$ + 3 H$_2$S → Ce$_2$S$_3$ + 3 H$_2$O
Newer synthetic procedures utilise less toxic carbon disulfide gas for sulfurisation, starting from cerium dioxide which is reduced by the CS$_2$ gas at temperatures of 800–1000 °C:
    6 CeO$_2$ + 5 CS$_2$ → 3 Ce$_2$S$_3$ + 5 CO$_2$ + SO$_2$

## Polymorphs

Polymorphs of cerium(III) sulfide :
| Polymorph | T of formation | Colour | Crystal system | Space group | Lattice constants |
| α-Ce$_2$S$_3$ | <900 °C | burgundy | Othorhombic | Pnma (No. 62) | a=7.63 Å, b=4.12 Å, c=15.71 Å |
| β-Ce$_2$S$_3$ | 900–1200 °C | red | Tetragonal | I4$_1$/acd (No. 142) | a=15.37 Å, c=20.35 Å |
| γ-Ce$_2$S$_3$ | >1200 °C | black | Cubic | I43d (No. 220) | a=8.63 Å |
Ce$_2$S$_3$ exists in three polymorphic forms: α-Ce$_2$S$_3$ (orthorhombic, burgundy colour), β-Ce$_2$S$_3$ (tetragonal, red colour), γ-Ce$_2$S$_3$ (cubic, black colour). They are analogous to the crystal structures of the likewise trimorphic Pr$_2$S$_3$ and Nd$_2$S$_3$.
Following the synthetic procedures given above will yield mostly the α- and β- polymorphs, with the proportion of α-Ce$_2$S$_3$ increasing at lower temperatures (~700–900 °C) and with longer reaction times. The α- form can be irreversibly transformed into β-Ce$_2$S$_3$ by vacuum heating at 1200 °C for 7 hours. Then γ-Ce$_2$S$_3$ is obtained from sintering of β-Ce$_2$S$_3$ powder via hot pressing at an even higher temperature (1700 °C).

### α polymorph

The α polymorph of cerium(III) sulfide has the same structure as α-Gd$_2$S$_3$. It contains both 7-coordinate and 8-coordinate cerium ions, Ce$^{3+}$, with monocapped and bicapped trigonal prismatic coordination geometry, respectively. The sulfide ions, S$^{2−}$, are 5-coordinate. Two thirds of them adopt a square pyramidal geometry and one third adopt a trigonal bipyramidal geometry.
Coordination in α-Ce$_2$S$_3$ :
| Cerium Ce1 coordination | Cerium Ce2 coordination | Sulfur S1 coordination | Sulfur S2 coordination | Sulfur S3 coordination |

### γ polymorph

The γ polymorph of cerium(III) sulfide adopts a cation-deficient form of the Th$_3$P$_4$ structure. 8 out the 9 metal positions in the Th$_3$P$_4$ structure are occupied by cerium in γ-Ce$_2$S$_3$, with the remainder as vacancies. This composition can be represented by the formula Ce$_{2.667}$□$_{0.333}$S$_4$. 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)BiS$_2$ family (for M=Ce).
The reaction of Ce$_2$S$_3$ with Bi$_2$S$_3$ and Bi$_2$O$_3$ in a sealed tube at 950 °C gives the parent compound CeOBiS$_2$:
    3 Ce$_2$S$_3$ + Bi$_2$S$_3$ + 2 Bi$_2$O$_3$ → 6 CeOBiS$_2$
This material is superconducting on its own, but the properties can be enhanced if it is doped with fluoride by including BiF$_3$ 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$_f$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 H$_2$S-involving production route and the difficulty in controlling the formation of the resulting Ce$_2$S$_3$/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-Ce$_2$S$_3$, 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-Ce$_2$S$_3$ 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.