Rare earth sesquisulfides

Rare-earth sesquisulfides have generally been prepared by the reaction of hydrogen sulfide with rare-earth oxides. However, such... 

Crystals of rare-earth sesquisulfides have been prepared by this method except when Ln is La, Er, Tm, or Y. In these cases the LnSI compounds are stable even at 1250°C. Anal. Calcd. for Gd2S3 Gd, 76.58 S, 23.42. Found Gd, 76.4 S, 22.2. 

The rare-earth sesquisulfides are reasonably stable when exposed to air at room temperature, although a weak odor of hydrogen sulfide is frequently present. The orthorhombic A structure is found for La, Ce, Pr, Nd, Sm, Gd, Tb, and Dy. The monoclinic D structure is found for Dy, Ho, Er, and Tm (Dy2S3 is dimorphic), and the rhombohedral E structure is found for Yb and Lu. The B... 

Table 4-1 Color of some rare earth sesquisulfides in their y-form.

Rare earth sesquisulfides exist in different allotropic forms (Figure 4-1). Cerium sesquisulfide is known to exist in 3 different allotropic forms a, (3, and y, all stable at different temperatures and exhibiting different colors (Table 4-2). 

Figure 4-1 Different allotropic forms of the sesquisulfides of rare earth elements l Table 4-2 Color and properties of rare earth sesquisulfide allotropic forms.

The majority of processes aimed at obtaining rare earth sesquisulfides of high purity use solid/gas reactions. 

Rhodia has now developed and commercialized a special process to produce rare earth sesquisulfides, notably cerium sesquisulfides with the adequate purity and size necessary for pigment application. 

Pressure-temperature-induced transition to the cubic, deficient ThjP -type structure for rare earth sesquisulfides and sesquiselenides, determined on quenched samples by Eatough et al. (1969) and Eatough and Hall (1970). The selenide phases were prepared from stoichiometric mixtures of the components. 

Polycrystalline rare-earth sesquisulfides have been prepared by this method for La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y. Europium sesquisulfide does not exist. For the more reactive rare-earth metals (La to Sm), the silica ampule will be severely attacked unless protected. This may also be a problem for other rare earths if high temperatures and long heating times are employed. Carbon is the most suitable material for protecting the silica in these syntheses. A graphite crucible may be used, but it is generally satisfactory simply to coat the inside of the silica tube with carbon by the pyrolysis of benzene. Benzene is poured into the silica tube, which is closed at one end it is then poured back out with the residue left clinging to the tube. The tube is placed in a furnace at 800° for a few minutes. 

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