Rare earth sulfides

Rare earth sulfides, selenides, and tellurides show semiconducting properties and have potential for application in thermoelectric generation. Thin film chalcogenides of various rare earths have been prepared by multisource evaporator systems [233]. 

Rare Earth Sulfides. In Group Ilia of the periodic classification, there are scandium, yttrium, and the rare earths. We have not yet studied the scandium sulfides, but we have studied the yttrium sulfides and the rare earth sulfides for the last six years (4,9, 10,20). We now turn to the research on these last sulfides done in our laboratory (6,7, 8). 

From a structural standpoint, the rare earth sulfides have several polymorphic forms (20), whose stability regions are represented in Figure 3. The high temperature form (y) exists from lanthanum to dysprosium. It is cubic and is of the Th3P4 type, with a defect structure. In each unit cell, there are 102/3 metal atoms which are distributed at random among the 12 sites of the metal lattice. The structures of the low temperature a and f forms are not yet known. The structure of the 8 form, which is peculiar to dysprosium, yttrium, and erbium, is monoclinic (20). The three last forms have low crystal symmetry, and certainly have no vacant lattices. 

Figure 3. Polymorphic varieties of rare earth sulfides, L2Ss...

In some cases, chiefly in the systems containing MgS and MnS, the ML2S4 compounds do not exist. However, we always observed solid solutions having the Th3P4-like structure very close to the rare earth sulfides (L2S3). 

To observe a solid solution from the L2S3 sulfides it is necessary to prepare the products at high enough temperatures (about 1300° C. for lanthanum and cerium products) and to quench them quickly. At the lower temperatures, the solid solution does not begin at but further on, because the rare earth sulfides have other polymorphic forms in the equilibrium conditions, and because these other forms, a and / , do not dissolve the MS sulfides. 

In our research, appropriate mixtures of a rare earth sulfide and an MS sulfide were prepared by grinding in an agate mortar. The mixtures were compressed in little tablets, which were placed in a sealed quartz tube, and heated at a constant temperature—for instance, 2 days at 1000° C., 3 hours at 1200° C., or 1 hour at 1300° C. Then the tubes were quenched in water. 

The results of our experiments are given in Table VII. Extended solid solutions are observed only with the last rare earth sulfides—Dy2S3, Y2S3, Er2S3, and Yb2S3—when added to MgS, MnS, or CaS. Extremely limited solid solutions are observed when one of these rare earth sulfides is added to SrS. We never observed solid solutions of the NaCl type with other rare earth sulfides, nor with BaS. 

Exclusion of water and oxygen is the primary criterion in rare earth oxysulfide synthesis procedures. This is generally analogous to the case of rare earth sulfides. The industry synthesis technology depends on sulfurizing rare earth oxide powders via solid-state reactions. For instance, the classical sulfide fusion method follows the schematic reaction ... 

E24.14 Transition metal and rare earth sulfides ate typically non-glass-forming sulfides, many of which have crystalline and layered phases. Any metal sulfide glasses would involve metalloid and non-metal sulfides. 

When used in glass compositions (at a low weight percentages) along with comparable amounts of titanium oxide, cerium oxide produces a deep yellow coloration [26]. Rare-earth sulfides, among them also cerium are used in glass and ceramics as colorants to replace toxic CdS [27]. 

MAJOR USES Used in the manufacture of rayon, carbon tetrachloride, floating agents, soil disinfectants, electronic vacuum tubes, optical glass, paint removers, varnishes, rubber cement used as a solvent in phosphorous, sulfur, selenium, bromine, iodine, fats, resins, rubbers, waxes, lacquers used as a chemical intermediate in cellophane, rubber compounds, fumigants, rare earth sulfides, xanthates. 

This method is suitable for all the rare earth sulfides, including those of Sc and Y. 

ENTHALPIES OF FORMATION OF SOME RARE-EARTH SULFIDES ... 

The present paper describes a determination of the enthalpies of formation of some rare-earth sulfides with the formulas LnS and Ln2S3 (Ln = La, Ce, Pr, Nd, and Gd). The samples of LnS and Ln2S3 were fused polycrystalline aggregates. A phase x-ray structure analysis, carried... 

The high solubility of rare-earth sulfides in cold aqueous solutions of inorganic acids [8, 15—19] made it possible to investigate synthesized substances and to determine their compositions by chemical and x-ray analyses. 

This increase in the covalence of the bonds should reduce the enthalpy of formation of rare-earth sulfides. 

Our experiments (Table 2) show that the enthalpy of formation does indeed decrease with ascending atomic number of the rare-earth element. The variation of along the rare-earth sulfide series is correlated with the values of the melting points of Ln8 and Ln2 and of the activation energy of carriers in the intrinsic conduction region of 1 1283. 

A few fluoride minerals of rare earths are known, but none for chlorides, bromides and iodides. Simple oxides of the rare earths are very rare in nature and no rare earth sulfide mineral is known. 

The crystal chemistry of the rare earth sulfides was described by Guittard et al. (1966), Flahaut eFal,(1965a), White et al. (1965a), Flahaut+ "( [968) Sleight et al. (1968). Greek letters were assigned by Flahaut et al. to each of the five structures of the R2S3 compounds, because of the many polymorphs in which they occur. For example. 

They are formed from the binary compounds by substitution of S or Se by a second non-metal, frequently arsenic, silicon, germanium, or tellurium. Such a choice of elements is evidence of the covalent character of the bonding in the rare earth sulfides and selenides. 

Rare Earth Sulfides and the Origins of their Color. 

Rare earth sulfide materials have received considerable attention over past years, mainly because of their interesting optical and magneto-optical properties. 

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