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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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The stability constants of the rare earth and actinide chloride complexes have been investigated by several authors [288—291]. It is highly interesting that the values of the stability constant for the chloride, nitrate and sulphate complexes of Eu3+ and Am3+ are of the same order of magnitude although the ionic radius of Ams+ (0.99 A) differs from that of Eu3+ (0.95 A). The stability constant data of Choppin and Unrein [289] on the halide complexes of Eu3 f are summarized in Table 17. Their data compare well with Bansal et al. [290]. The value of k\ for AmCl2+ reported by Bansal et al. as 0.9 ± 0.1 is of the same magnitude as for EuCl2+. Choppin and Unrein also gave the values of the thermodynamic functions for the EuCl2+ complexes. However, there was an error in the A$ value which was corrected in a later work [353]. The following values were reported [353].
EuCl2+(ju = 1.0 at 25° C): AF° = 0.07 ± 0.03 kcal/mole; AH° = -0.005 ±
0.03 kcal/mole; AS° = —0.4 ± 0.2 eu.
These very small values for the thermodynamic functions tend to suggest that chloride ions form very weak complexes indeed if not just forming an ion-pair. The first formation constant, ki, shows a decrease with increase in ionic radii of the halide ions (Table 17).
Table 17. Stability constants of the halide complexes [289] of Euz+ (fi = 1.0 at 25° C)
Complexing k1 k2
ion
Cl 0.80 ± 0.12 0.19 ± 0.10
Br 0.58 ± 0.09 0.35 ± 0.20
I 0.49 ± 0.06 —
Electrical conductance, cation transference number and activity coefficient of the halide systems are discussed on page 37.
LiEuFt. — The solid state reaction of LiF with the trifluorides of the heavy lanthanides (Eu—Lu and Y) results in the double fluoride LiMF4. These double fluorides have a scheelite type of structure. The lattice constants of LiEuF4 are found to be a = 5.288 and c = 11.03 A. Hydroxide. — Addition of alkali to a solution of a europic salt yields the hydroxide. Even certain amines and ammonia are basic enough to precipitate the hydroxides of most trivalent rare earths. The crystalline hydroxide of europium, Eu(0H)3, is prepared reacting europium metal with water at room temperature and evaporating the solution to dryness after the evolution of hydrogen has ceased. Eu(0H)a belongs [293] to the
Inorganic Coordination Compounds
55
hexagonal space group 063/m with lattice parameters a = 6.365 and c = 3.645 A (± 0.001 A). The unit cell contains two formula units. Thermal decomposition of the hydroxide has been carried out. Given below is a summary of the experimental data obtained by Rau and Glover [293]. The decomposition starts at 200—250° C and continues to 300° C. The first plateau extends over the temperature range 300 to 435°
2Eu(OH)s + 2HaO + 2EuOOH + Eu2Os + H20 (29)
and corresponds to a weight loss of 8.87 per cent (the theoretical weight loss for the conversion of Eu(OH)s to EuOOH is 8.87 per cent). Above 345° C the transformation of EuOOH to EU2O3 occurs and is essentially completed at 465° C. The crystal structure of EuOOH was thought by Rau and Glover to be orthorhombic, but a definite structural assignment has recently been made by Barnighausen [336] (p. 68).
Moeller et al. [294, 295] have thoroughly studied the pH values at which the rare earth hydroxides are precipitated from various salt solutions, and also the solubility and solubility product constants of the hydroxides. Their results are summarized in Table 18.
Nitrate. — Rare earth oxides dissolve in moderately concentrated nitric acid to give the nitrates. The salts crystallize [296] as hexahydrates, M(NOs)s * 6H2O. However, Wendlandt and Bear [297] obtained tetrahydrates M(NOs)s * 4H20 (M = Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu), by crystallization in a desiccator over concentrated H2SO4. According to their thermal decomposition patterns, the nitrates of Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu can be divided into three groups. The general sequence is as follows
Step I Step H M(NOs)s • 4HaO ---> MONOs ->M203 (30)
The nitrates of Eu, Gd and Dy belong to group A and for this group the minimum decomposition temperatures for Step I are 75—80° C whilst for Step II, they are 455—480° C. The minimum oxide level temperatures are in the range 730°—760° C. The composition, MONOs is found for Eu and Dy, but not for Gd. The composition of the pyrolysis product could perhaps be represented better by a mixture of MONO3 and M2O3 rather then a stoichiometric compound. For groups B (Ho, Er and Tm) and C (Yb and Lu), the thermal decomposition pattern consists essentially of the same sequence except for the abscence of the MONO3 + M2O3 mixed phase. The minimum decomposition temperatures for Step I are 55—120° C and those for Step II are 380 to 460° C. With the exception of Yb, the minimum oxide level temperatures are in the range 500—600° C The group B nitrates show a definite horizontal weight level corresponding to the composition MONO3 and are thus differentiated from group C.
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