Rare 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... 

PrS, NdS, GdS, and Pr2S3 were obtained for the first time. The ionic component of the chemical bonds in the mono- and sesquisulfides decreased with increasing atomic number of the rare-earth element. This was in agreement with the observation that the difference between the electronegativities of the rare-earth elements and sulfur decreased along the lanthanide series. 

The relationship between the joule and the calorie was assumed to be 4.184. The enthalpies of formation of rare-earth mono- and sesquisulfides... 

Table 1 lists the experimental values of the enthalpies of reactions (5), (2), and (3), which take account of the experimental errors and the reproducibility of the measurements. These values of AHg, AHj, and AH3 were used to calculate the enthalpies of formation of the rare-earth mono- and sesquisulfides in accordance with Eqs. (8) and (9). The results are given in Table 2. This table includes also the experimental values of the enthalpies of formation calculated per 1 g-atom of sulfur. The published values included in this table were taken from original papers or from textbooks [21], where these values were reviewed critically.

TABLE 2. Enthalpies of Formation of Rare-Earth Mono-and Sesquisulfides... 

Rare earth elements and sulfur combine to form a wide range of compounds as sulfides or oxysulfides Among them, sesquisulfides appeared to have the best potential as far as color is concerned (Table 4—1). We gave most of our attention to cerium sulfide, which was the most promising in terms of color intensity and purity. 

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. 

The author considered it best not to include in the reference book the properties of certain little-studied compounds rarely used in practice. Thus, in the presentation of the information on carbides, borides, nitrides, and other classes of metal-like compounds, no data are given on the refractory compounds of metals of the platinum group for the sulfides, data are given only for the class of sulfides of the rare-earth metals and actinides, in most of which the properties of refractory compounds in the wide sense are most clearly expressed, the proportion of ionic bond, in particular, being small. It was, however, found e qpedient to consider also the properties of oxysulfides of the rare-earth metals and actinides, which are very similar to the properties of sulfides and are obtained simply by replacement of two atoms of sulfur in a sesquisulfide by two atoms of oxygen. This is one of the few exceptions where the tables of the reference book give the properties of ternary and not binary compounds. 

Rare-earth sesquisulhdes have generally been prepared by the reaction of hydrogen sulfide with rare-earth oxides. However, such a procedure frequently gives oxysuliides or nonstoichiometric sulfides. Direct reaction of the elements readily gives pure stoichiometric sesquisulfides, and this procedure is easily modified to give crystals. ... 

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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