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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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1st wave (V) 2nd wave (V)
Sm — 1.13 (rev) - 1.7
Eu - 0.03 - 1.79
Y — 0.71 (rev) - 1.71
The reduction potentials of the rare earths are found to be less negative in acetone than in water.
Magnetic Behaviour
33
The reduction of Eu3+ to Eu2+ possibly involves an electronically excited state [201]. The electron is first added to the unoccupied outer orbital and then drops down to the inner 4/ orbital. This situation may arise when an energy barrier prevents the ion from reaching the most stable electronic configuration. Eq. (12) represents the probable mechanism of the reduction process.
Eu3+ - [Eu2+]* + Eu2+ (12)
(4/6 5s2 бp*) (4/e 5s2 5рв &Z1) (4f 6s2 5p*)
The energy barrier occuring in this case is due to the added electron having to go through the cloud of the outer electrons.
Magnetic Behaviour
Metallic europium shows unusual magnetic behaviour. Bozorth and Van Vleck [202] suggested that the metal should be considered as consisting of Eu2+ ions embedded in a sea of electrons. This model accounts approximately the magnetic susceptibility above the Neel temperature. However, for the magnetic behaviour of solid Eu at high temperatures the non-interacting Eu2+ model is not a correct approximation. The values obtained for ju,ett and 0 by different workers [160, 202—204] employing various temperature ranges are summarized in Table 8.
Table 8. Values of цец and © for metallic europium
.. , Temperature ue tt ©
Investigator (Abs.) (Bohr mag.) (Abs.)
Elemm and Bommer [160] 167 — 293° 8.3 15°
H. LaBlanchetaist and Trombe [204] 140 — 300° 7.12 108°
Bozorth and Van Vleck [202] 100 — 300° 8.3 0
Colvin, Arajs and Peck [203] 300 — 400° 9.17 —3.3°
Colvin, Arajs and Peck [203] working at high temperatures (800°— 1099°) found that the magnetic susceptibility values are quite close to those resulting from non-interacting Eu+ ions. The melting process produces a slight anomaly in the 1jx — T plot near the melting point region. Liquid europium follows approximately the Curie-Weiss law. The magnetic moment in this state is very close to that expected from an interacting Eu2+ system.
The Curie temperatures, 0, for Eu—Lu are plotted against atomic number in Fig. 9. Klemm and Bommer [160] found that the curve (Fig. 10) obtained in a — T plot in the paramagnetic region gave a low
3 Sinha, Europium
34
Preparation and Properties of Europium
At No. —►
Fig. 9. A plot of Curie temperature, 0, against atomic number of Eu — Gd
value for the Curie temperature (0 = 15°) and also showed a deviation from the straight line at 150° K. They determined fieu for metallic Eu as 8.3 and commented on the deviation from the value expected for M3+ (3.4), and even from the M2+ species (7.9). This tends to support Colvin and Arajs theory of the non-interacting Eu+ion. Europium shows a
T°abs.
Fig. 10. A *lx — T plot for europium.
The Magnetic Susceptibility of Trivalent Europium 35
very weak ferromagnetic behaviour compared to ferromagnetic gadolinium. In the case of Gd the experimental fi value (7.8) agrees well with the calculated ju value for the M3+ ion (7.9). If in the case of europium the species happens to be M2+ the ferromagnetic behaviour would be expected to be the same as for Gd3+ (Eu2+ and Gd3+ have the same electronic configuration). Keemm and Bommer [160] put forward the following two reasons for the interesting behaviour of Eu.
(a) The numbers of neighbouring atoms in the case of Eu is not 12 but 8.
(b) The interatomic distance is appreciably larger.
Their measured values [160] for £atom are presented in Table 9.
Table 9. Magnetic susceptibility Xatom of europiumcUdifferent temperatures [160]
Temperature %atom * 106 Mag. field
(Abs.)
90 385000 540
307000 1060
245000 2050
150000 00
167 56300 indepenc
195 46000 99
293 30400 99
The Magnetic Susceptibility of Trivalent Europium
In 1925 Hund [205] calculated the magnetic moments of the trivalent rare earth ions. His theoretical values were in good agreement with the experimental results for all rare earth ions except Eu3+ and Sm3+, where the theoretical values were much too low (Table 10). Hund [205] and Laporte [206] suspected that the multiplet intervals in Eu3+ and Sm3+ are not really large compared to kT. In fact the interval [207] between the two lowest components 7Fo and 7Fi of the ground 7F term of Eu3+ is 1/21 of the overall width of the 7F term and that for Sm3+ {6Hs/2 and 6i?7/2) is 7/55 of the overall width of the 6H term, although the overall multiplet width in both cases is considerably larger than kT. Hence, the expression for magnetic susceptibility X instead of taking the form
„ Ng*fP J{J+1) . „
z =--aup-+ Na (13)
as in the case where the multiplet intervals are large compared to kT, takes [207] the complicated form given in eq. (14).
3*
36 Preparation and Properties of Europium
Table 10. Theoretical fien values of trivalent rare earths
M3+ Ground state Hund [205] Van Vleck [207] Observed [208] for M2(S04)3 • 8H20
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