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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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Level Wavenumber in cm 1 LaCl3 Ethylsulphate
7F0 0 0
1F1 355.05 356
405.27 398
7F% 1022.54 1015
1027.52 1031
1084.33 1106
7FZ 1846.77 1856
1862.41 1903
1880.36 1913
1900.61 1929
1919.70 —
7 2751.48 2782
2833.30 2841
2866.87 2890
2867.38 2893
2903.14 3056
3041.15 —
7F g 3801.98 3858
3838.74 3880
3844.31 3955
3918.05 3937
3957.10 4021
3992.70 —
4010.01 -
7F6 4907.35 4915 ♦
4930.05 4937
4949.63 4958
4967.42 4976 4992 4996
* This line is considered [570] to be spurious.
The Absorption Spectra of Trivalent Rare Earths in Solution
In solution rare earths also show very sharp spectral transitions similar to those in the crystalline state. These transitions are again to be identified with the excitation of the electrons from the ground level to the excited J levels of the 4/n configuration.
In the presence of complexing agents, three types of changes in the spectrum profile of the rare earth are noticeable with respect to the free ion (as free ion data for most of the rare earths are not available, we shall choose aquo ions as a convenient standard).
Absorption Spectra of the Europium Ion and Its Complexes 121
(a) Shift of spectral lines
(b) Splitting of spectral lines
(c) Change in line intensity
(a) Ephraim and Bloch [574,575] during 1926—28 observed a red shift of the absorption bands of anhydrous rare earth salts compared with the aquo ions. Subsequently, such red shifts of bands with respect to the aquo ions were observed for rare earth ions with many complexing agents other than fluoride. Fluorides in fact show a small blue shift with respect to the aquo ion. This effect usually called the nephelauxetic effect is conveniently explained by Jctrgensen by assuming a decrease of the Fjc parameters in the complexes [576]. Thus the nephelauxetic parameter P can be expressed in terms of Ft s as
P = ntn (60)
where the superscripts c and / stand for the complexed species and for the free ion respectively. It has usually been assumed that p does not vary with k, and hence /9 defined in terms of the Racah parameters (Ek) will be the same as that for the i'Ys. It has been found in the case of Pr3+ complexes (fortunately the free ion parameters are known [577] for Pr3+) that ft varies considerably [578] with k when eq. (60) is used to define it.
As for most of the rare earth ions the free ion parameters are not known, and (3 can be approximated as
P = 1/» 2 vclv<a (61)
<
’T
where i ^ 0 but equal to n the number of bands taken into account for the calculation and v*e and v*a are the wavenumber of the intra /—/ transition bands for the complex and the corresponding aquo ion respectively. It is very important that in minimizing errors for obtaining an average p value n should be as large as possible. The reader will at once notice that in eq. (61) it is implied that the ratio rc/v« for all the transitions will be the same or very nearly the same. Putting this in other words the ratio, P, for the excited levels is the same as for the ground term multiplets. However, this is not the case. From our previous discussion we will recall that the excited terms of the fn configuration are mainly determined by the Fjc parameters and the contribution of £4/ is small, whereas the main contribution of £4/ is to the ground multiplets. From actual experiments it is seen that the variation of Fk’s from free ion to complex are not the same as for £4/, as for example [578] in the case of Pr34 where F2 decreases —5 per cent in going from the free ion to the chloro complex (Pr3+ in LaCU) whilst £4/ decreases only ~1.5 per cent. This is possibly
122
Spectroscopic Properties of Europium
the main reason why J0rgensen’s modified formula [579] (eq. (62)) for the Ufcphelauxetic effect, although predicting with reasonable accuracy the nephelauxetic ratio when only the visible and ultraviolet transitions are taken into account, fails to correlate the band shifts in the infrared region. Sinha [258] has examined a number of neodymium complexes
»'comp ^aqu == ^ ^aqu (^)
and pointed out that the shift of the bands in the infrared region is less than that in the visible or ultraviolet region. This essentially says that £4/ varies less than the Fu s in Nd3+ complexes. This effect should not be restricted only to Pr3+ and Nd3+ but should also be observed for all other rare earth ions. In fact we would predict to vary slightly more than Fjc,b in passing from the free ion to stronger complexes for the heavier rare earths where the ground state multiplets are widely spread (Fig 20). Sinha and Schmidtke [578] tried to give an explanation of the variations of the F* and £4/ parameters and found that /? of eq. 60 for Pr3* complexes can be expressed predominantly by the ratio of F%}F{. However, no simple expression for /9* (given by eq. 63) has
p* = tt,ia, (63)
been found, considering the transition in the infrared region.
(b) The ligand field splitting of the absoption bands will mainly depend of the effective point group symmetry around the ion in question. The splitting of the J levels has been treated previously, and Table 44 is a handy reference for predicting the number of components of the J levels of a known symmetry.
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