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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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4*
52
Compounds of Europium
Europous chloride, ЕиСІг, can be prepared [269] by the reduction of ЕиСІз with a 1: 1 mixture of HC1 and hydrogen at an elevated temperature. It is a white solid with a bluish cast having a density of 4.89 at 25° C. The crystals were shown [277] to be orthorhombic (PbCl2 structure) with a = 4.493, b = 7.499 and c = 8.914 A. It was found to be isostructural with SmCb (a = 4.497, b = 7.532 and c = 8.973 A).
Doll and Klemm [277] also reported the Debye pattern of EuBr2 and Eul2. ЕиВгй and SmBr2 are isostructural with SrBr2 (orthorhombic) with almost the same lattice constants, but no data have been reported [277]. EuI2, however, is not isostructural with Srl2.
Europic Halides
The hydrated trihalides of the rare earths are easily obtained by reacting the oxides with appropriate HX acid solution. Anhydrous halides are, however, difficult to prepare. Attempts to dehydrate the hydrated halides usually result in oxyhalides. In the case of the chlorides and bromides
MX3 • yHaO -*■ MOX (26)
the hydrolysis reaction (eq. 26) can be prevented by using an HC1 or HBr atmosphere during dehydration. However, the iodides cannot be dehydrated in this way. An excellent review on the preparation of anhydrous rare earth halides has appeared [278].
Fluoride. — The crystal structure of EuFe varies, and was found [279] to be orthorhombic when it was prepared by the dry method i.e., by heating Еи20з at 700° C in a dry stream of a mixture of hydrogen and hydrogen fluoride. A hexagonal form [279], however, was obtained by treating the chloride solution with concentrated hydrofluoric acid and drying the product at ~ 150° C. The lattice constants, unit cell volumes and the densities of the two crystalline forms are compared in Table 16.
Table 16. Comparison of crystal data of the two forms of EuFz
Lattice parameters (A) Unit cell Density
Form a b c vol. g/cc
Orthorhombic
(YF, type) 6.622 7.019 4.396 204.3 6.793
Hexagonal
(LaF, type) 6.916 — 7.091 293.7 7.088
Chloride. — All the rare earth trichlorides except La and Pr crystallize with six molecules of water of crystallization; La and Pr chlorides crystallize as heptahydrates. Thermal decomposition studies [280] on
Inorganic Coordination Compounds
53
EuCIa • 6H2O showed that the hexahydrate began to evolve water of hydration at 80° C, and heating up to 165° C gave a hemihydrate, EuCl3.0.5 H2O. In the temperature range 225—265° C a mixed metal oxychloride- chloride phase EuOCl • 2EuCl3 is observed. Above 265° C, further decomposition occurs giving EuOCl at 380° C. It is of interest to note that no anhydrous metal chloride phase exists. The presence of a mixed metal oxychloride-chloride has also been observed for heavier rare earth chlorides. An infrared investigation [281] of the hydrated chlorides indicated moderately strong hydrogen bonding in the solid state.
Various methods [282] have been used to prepare anhydrous chlorides of the rare earths. Taylor and Carter [283] describe a general method for the preparation of high purity anhydrous halides in good yield. This method involves heating in vacuo, a molecularly dispersed mixture of hydrated rare earth halide with proper ammonium halide until the water and ammonium halide are expelled. All the trihalides except the iodides of Sm and Eu can be obtained using this proceedure. In the case of Sm and Eu the divalent iodides, Sml2 and Eul2 are obtained.
Various dehydrating agents have been tried to produce anhydrous chlorides. Matignon and Bourion [186, 284, 285] prepared anhydrous EuCl3 and other anhydrous chlorides by drying the hydrated salt at 100° C, and dehydrating it further in a stream of S2CI2 and CI2 by raising the temperature slowly. In the case of EuCl3 impurities due to the reducing action of S2CI2 may have contaminated the final product [285a].
EuCl3 is found [286] to be a hexagonal UCI3 type with lattice parameters a = 7.369 ± 0.004 and c = 4.133 ± 0.002 A. A structural analysis of the hexahydrate, Eu(OH2)6 • Cl3, showed [286a] a highly distorted antiprism geometry, and the presence of [Eu(OH2)6Cl2]+ and Cl- ions in the crystals, The Eu—O and Eu—Cl distances are 2.44 and 2.47 A respectively.
Bromide. — Like the trichlorides the rare earth tribromides form hexahydrates for all rare earths except La and Pr. The hydrated tribromides can be prepared by dissolving the oxides in 48% HBr followed by evaporation to crystallization. They can be divided into two thermal decomposition reaction groups [287]. The bromides of Pr, Nd, Sm and Eu follow a reaction sequence of the type.
MBr3 ■ 6H20 —■ MBr3 • H20 + MBr3 MOBr3 + M2Os (27)
The second group comprises the bromides of Gd—Lu. This group is distinguished from the first one by not having the monohydrate phase. The thermal decomposition steps are as follows.
MBr3 • 6H20 ^ (MBr3 + MOBr) + MOBr -- M2Oa (28)
54
Compounds of Europium
The decomposition pattern of the bromides is very similar to that of the chlorides.
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