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Europium - Sinha S.P.

Sinha S.P. Europium - Springer-Verlag, 1967. - 88 p.
Download (direct link): europium1967.djvu
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3Ca + 2MFS = 3CaF2 + 2M (7)
Fluorides are preferred to chlorides because of the hygroscopic nature of the latter. Daane and Spedding showed that all the rare earth metals except Sm, Eu and Yb can be conveniently prepared by the reduction of their fluorides with calcium. In the case of Sm, Eu and Yb, the trihalides were reduced to dihalides only, no further reduction being acheived.
Using lanthanum turnings as reducing agent Daane et al. [163] successfully prepared samarium and ytterbium from their respective oxides. In this process a 10% excess of lanthanum turnings is mixed with Sni203 or Yb2C>3 and the mixture is heated to 1450° C under vacuum to effect the reduction. Later Spedding, Hanak and Daane [164] extended the same reaction to reduce europium oxide to europium metal. This process makes use of the high vapour pressures of Sm, Eu and Yb and the low vapour pressure of La.
2La + M203 = La2Os + 2M (M = Sm, Eu and Yb) (8)
Spedding-Hanak-Daane’s Method
25
Spedding-Hanak-Daane’s Method [164] for the Preparation of Metallic
Europium
A schematic diagram of the reduction apparatus used by Spedding et al. [164] for the production of europium metal is shown in Fig. 6. The reduction chamber was made from two concentric tantalum cylinders welded together. A charge consisting of 86.5g of EU2O3 and 73g of lanthanum turnings was placed in the annular space of the chamber and gradually heated to 1000° C. The pressure was maintained below
1 x 10_4mm. Distillation of the metal started at 1100° C and as the temperature increased to 1200° C droplets of europium began to reflux. The metal vapour condensed in the lower part of the inner cylinder, and at intervals the metal was melted down into the tantalum crucible below by adjusting the height of the induction coil. The tantalum crucible was provided with a thermocouple for thermal analysis, and the temperature was controlled by the resistance furnace around the crucible. A water cooled silica tube served as the outer jacket for the apparatus. A yield of approximately 95% (70.4g) of highly pure europium was obtained. The total time for degassing and reduction was ~8 hours. The progress of the reduction was followed by taking cobalt-60 radiograms of the apparatus.
Fig. 6. A schematic diagram of the reduction apparatus for the production
of metallic europium
26
Preparation and Properties of Europium
It has been found that in the preparation of pure europium the starting materials need not be extremely pure. The common impurity viz. samarium is completely eliminated in the above process because samarium is less volatile than europium, and the reduction of Sm203 to the metal requires a higher temperature than the EU2O3 reduction. Commençai lanthanum turnings can also be used for the reduction in place of more expensive very pure lanthanum metal. Extreme care should be taken to ensure that the reactants contain no calcium as it appears as an impurity in the final product if present in the charge.
Properties of Europ im
Europium is a soft, malleable, grey metal. It is as soft as lead. It is rapidly attacked by moisture at ordinary temperatures and reacts with cold water as fast as does calcium. A pressure of 18,000 psi is required to extrude it into a wire of 1/s inch diameter at room temperature. It can be cut with a knife. It has a low tensile strength. It’s coefficient of thermal expansion (a)
Vt2 = Vtl (1 + 3 a At) (9)
%
was determined as (26 ± 4) X 10~6 /° C by measuring the volume at 20 and 780° C. On fusion a change of + 4.8 ± 0.8% in volume was observed. It’s melting point [164] is 826 ± 10° C. The calculated [164] boiling point for this metal is 1489° C. Daane [165] gives a boiling point for europium as 1712° C, which is more reasonable, with a heat of vaporization of 42 kcal/mole. The crystal structure of metallic europium was first determined by Klemm and Bommer [160] who found that it has a body centered cubic (bcc) structure (tungsten-type) with a lattice constant a = 4.582 ± 0.002 A. Spedding et al. [166] found a to be 4.606 ± 0.001 A. In a later study [164] the value of 4.582 ± 0.0004 A (25° C) was confirmed. A temperature dependent study by Barrett [167] showed that the bcc structure persists through a wide range of temperature. The following results were obtained.
T(°K) a (A)
5 4.551 ± 0.003
78 4.551 ± 0.004
300 4.577 ± 0.001
Properties of Europium
27
Europium is the only metal that has a bcc structure at ordinary temperatures. Usually in the case of rare earths the bcc structure is observed at higher temperatures only (Table 5).
Table 5. Crystallographic data of rare earth metals [160, 166—174]
Rare Earth Temp. (° C) Struc- ture Lattice constants (Â) a c
La -271° —310° hex 3.770 ± 0.0002 12.159 ± 0.0008
310° — 868° fee 5.304 ± 0.0003 —
868° — m.p. bee 4.26 —
Ce -150°--10° hex 3.68 11.92
-10° — 730° fee 5.1612 ± 0.0005 —
730° — m.p. bee 4.11 —
Pr up to 798° hex 3.6725 ± 0.0007 11.8354 ± 0.0012
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