The Gd-Se System

A compound Gd3Se4 does not occur. A eutectic appears near this composition at 57.0 at.% Se and 1550 C between GdSe and Gd2Se3 [1]. Other authors [7,8] give 1380 C as the eutectic temperature and remark that the eutectic makes a compound Gd3Se4 questionable [7,8]. A compound Gd3Se4 melting at 1500°C was, however, mentioned in [9,13], also see p. 290. 

The phase richest in Se is a polyselenide of variable composition between GdSe 775 and GdSei 862- Its existence was concluded from vapor pressure results in the range Se Gd =1.6 to 

Barrett [14]. The existence of a relatively unstable diselenide GdSe2 losing Se above 200°C is claimed in [7, 8, 10]. 

Data on the phase diagram are summarized by Elliott [11] and Shunk [12]. 

The Se vapor pressure at compositions between Gd Se = 1 1.58 and 1 2.0 has been measured by a radioactive dew point method. Isotherms at 943,1000 and 1075 K are given in Fig. 129. They indicate the existence of a polyselenide phase in the range Gd Se = 1 1.775 to 1 1.862. The vapor pressure between 926 and 1133 K in the two-phase region just below 1 1.775 can be represented by log p (in Torr) = 10.85-10750/T, Veale, Barrett [14]. 

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Fig. 130. Electrical resistivity Q (Fig. 130a) and Seebeck coefficient a (Fig. 130 b) in the Gd-Se system at -300 K versus composition.

Gas Chromatography. This can be used to detect these compounds using 3% SE-30 on 80-100 mesh Chromosorb G (System GD, p. 194). The compounds should be converted to their methyl derivatives by heating the Acid Fraction obtained from the solvent extraction of horse urine (see above) in a sealed tube at 100 for 30 minutes with 180 il of iodomethane and 50 mg of potassium carbonate. Trimethylanilinium hydroxide is not recommended for methylating these compounds. 

There has been no structural study made on the phases observed in these systems which often have complicated structures. For the trivalent state of titanium or vanadium some phases have been pointed out, but the description of the networks should be reexamined. LaTiXs where X = S, Se and LaVX3 where X = S, Se are probably monoclinic (Donohue, 1975). R6Ti4Si5 where R = La to Gd and R6V4S15 where R = La to Gd are probably also monoclinic (Patrie et al., 1969c). 

Only three ternary rare earth systems, namely, Yb-Sb-Te, Gd-Bi-Te, Yb-In-Se, have been completely investigated over the whole range of concentrations, among other ternary systems with the properties indicated above. Information about binary sections of ternary systems is more complete (in many cases these sections are quasi-binary). Some of these sections are given in this chapter as an illustration. The work has been mostly done by chemists from the Academy of Sciences of Azerbaidjan. 

In the systems with arsenic (R-As-Se), the compounds with the composition RAsSe (R = La, Gd) and Eu4As2Se were identified (Hulliger 1968, 1979). 

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