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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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Table 20. The lattice constants of various forms of rare earth oxides
M208 A-type a c a B-type b c P C-type a Reference
La2Os 3.93 6.12 [299]
Pr208 3.85 6.00 t— [299]
Nd2Oa 3.82 5.98 14.35 3.666 8.99 100.34° 11.080 [299]
3.841 6.002 [307]
Sm203 14.177 3.633 8.847 99.96° [308]
14.16 3.621 8.84 100.05° 10.934 r299]
Eu203 13.944 3.581 8.676 98.5° [305]
14.06 3.601 8.80 100.15° 10.860 [299]
Gd2Os 14.061 3.566 8.760 100.10° [304]
14.06 3.572 8.75 100.10° 10.8122 [299]
Tb2Os 10.729 [299]
By2o3 10.667 10.6647 [309] [299]
Ho208 10.6065 [299]
Er208 10.550 10.5473 [309] [299]
Tma03 10.4866 [299]
Yb208 10.435 10.4334 [309] [299]
Lu203 10.1178 [299]
y2o3 10.605 [309]
10.6021 [299]
naÉtiÉÉlflrfÉftÉÉftÜitifcfc^l II Hit III
Inorganic Coordination Compounds
59
The infrared spectra of the rare earth oxides can be used to distinguish the three polymorphic types. Baun and McDevitt [310] and the present writer [311] have studied the M—O stretching frequencies of the rare earth oxides, and the values are summarized in Table 21. A typical
Table 21. Characteristic infrared frequencies (in cmr1) of the rare earth oxides
Oxide Batjn and McDevitt [310] Sinha [311]
L&2O3 644 645
Pr6On 655 -
Nd3Os 655 675
SrtLjOj 640 530
Eu203 630 535
Gd203 535 535
Dy2Os 550 555
Ho203 559 560, 575 (sh)
Er203 563 565, 585 (sh)
Tm2Os 565 -
Yb203 569 -
Lu2Q3 570 578, 600 (sh)
spectrum of each type (A—B—EU2O3 and C—LU2O3) as recorded by the author (previously unpublished) is shown in Fig. 12. Using his results (Table 21) the author has plotted the M—0 stretching frequencies against the lattice parameter a, and confirmed the Roth-Schneider [299] classification.
Fig. 12. Infrared spectra of A—La203(—), B—Eu203(----) and C-Lu203(....)
showing M—O stretching frequency.
60
Compounds of Europium
Several mixed oxi 1о systems of the type A2O3 — M2O3, AO — M2O3, АгО*— MjOs and AOs — M2O3 have been investigated. The coordination tendencies of the metal ions with oxygen in solid mixed oxides can be rationalized just I'ke those in solution. The oxide ions are usually larger and tend to assume the closest packed arrangement forming tetrahedral or octahedral cavities into which smaller cations may fit. The large cations, however, may replace an oxide ion and form a twelve-fold coordination system with oxide ions. Thus, the ionic size and charge of the cations formmg mixed oxide systems become important factors.
In an ideal perovskite structure for an ABO3 compound, the larger, A, ions are surrounded by twelve oxygens and the smaller, B, ions by six oxygens. Eq. (31) shows the ionic radii relationship for a close-packed arrangement.
га H~ -®o = 1/2 (гв H~ J ’о) (31)
АВОз compounds containing lanthanum are closer to the ideal perovskite than those containing smaller rare earth ions. Compounds of the smaller rare ear+h ioos appear to have a distorted perovskite structure of lower symmetry. When, however, the relationship between the radii is very far from being ideal (eq. 31), strong distortion may result giving an entirely different structure. The Goldschmidt tolerance factor, t, for the perovskite structure is related to the ionic radii by
Га Ч- -Ко
1 = У 2 (rB + До) ( )
The factor t varies from 0.8 to 1. For an ideal cubic structure t should be > 0.89 (based on the Goldschmidt radii).
It was rather surprising when Hum> and Durrwachter [312] found that La^Os ^s mi^cible with ТОг to a great extent (52 mole per cent) whilst still preserving the cub с fluorite structure. The lattice constant of th'? mixed oxide has an a value 5.645 A compared to 5.592 A for ТОг. The lattice constants of some orthorhombic perovskite and cubic garnet-type europium compounds are listed in Table 22.
Table 22. Latf ire со nsiants of orthorhombic pcrovekUe- and cubic garnet- type europium compounds
Compound a (A) b (A) с (A) Reference
Orthorhombic perovskite structure
EuCrOj 5.33 7.61 5.50 [312]
EuFeOj 5.371 6.611 7.686 [314]
EuScOs 5.61 7.94 6.76 [313]
Cul c garnet structure
EuaGas012 12.403 [313]
Eu,Fe601# 12.518 [315]
Inorganic Coordination Compounds 61
Roth and his coworkers [313, 316, 317] have studied several n aed oxide systems involving europium and other rare earths. Figs. 13 and 14 show the EU2O3 — A2O3 (A = Al, Ga, Cr, Fe, Sc and In) phase diagram. The phase diagrams of EU2O3 with other rare earth oxides (La2(>3, Nd2C>3, Sm2Os, Gd203, Dy20a, Ho^Os, E^Os, Tm203, Yb203, LU2O3) are presented in Figs. 15 and 16. The individual oxides and mixed oxides of europium are d:scussed below.
EuzO^. — The normal oxide of europium is EU2O3. The lattice constant and the B C transformation temperature are given on p. 58. Recently Barret and Barry [318] have studied the interaction of Nd20s and EU2O3 w ;h hydrogen and oxygen. In the case of Eu^Os reduction w ith hydrogen at 650° C (1 ram H2,2 hrs.) gave EuOi.4923. The c< »nductivity of
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