Rare Earth/Aluminum Bromide Vapor Complexes
UV-vis spectrometry,11-14 mass spectrometry,15 radiochem-
istry,16,17 quenching,18-22 entrainment,18 and chemical vapor
transport.23-28 Standard molar enthalpies and standard molar
entropies of the reactions have been derived from the
measurements for the chloride vapor complexes LnAlnCl3n+3
of the 16 rare earth elements Ln ) Sc,15,19 Y,19,24 La,21,22,27
Ce,21,22 Pr,21 Nd,11,21,26 Sm,12,21 Eu,21,23 Gd,16,18,21 Tb,21 Dy,20,21
Ho,13,21,22 Er,21 Tm,16,21 Yb,17,21 and Lu21 and interpolated for
that of the radioelement Ln ) Pm.21 However, the standard
thermodynamic property values are available only for the
bromide vapor complexes LnAlnBr3n+3 of Ln ) Y25 and La28
and for the iodide vapor complexes LnAlnI3n+3 of Ln ) Nd.14
These results have recently been discussed in the excellent
reviews of Boghosian and Papatheodorou,1 Adachi and co-
workers,2 and Oppermann and Schmidt.3 However, the
chemical vapor transport data have not been collected in ref
2 and have only been denoted as estimated values in ref 1
probably due to the relatively large experimental uncertain-
ties.
We19-22 have improved the phase equilibrium-quenching
technique and applied it to determine the stoichiometry and
thermodynamic properties of the reactions LnCl3(s) + (n/
2)Al2Cl6(g) ) LnAlnCl3n+3(g) for the 16 rare earth elements
Ln ) Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,
Tm, Yb, and Lu, and the results agree well with most of the
literature data. In this study, we extend the phase equilibrium-
quenching investigations to the reactions LnBr3(s) + (n/2)Al2-
Br6(g) ) LnAlnBr3n+3(g) for the 16 rare earth elements Ln
) Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb, and Lu. We are interested in whether the systematics
and anomalies in the LnAlnBr3n+3 vapor complexes are the
same as those in the LnAlnCl3n+3 vapor complexes.
Figure 1. The ampule.
for AlBr3, and more than 99.9% purity for CeO2, Pr6O11, Tb4O7,
and Ln* O3 (where Ln* ) Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er,
2
Tm, Yb, and Lu).
The anhydrous rare earth element bromides were prepared by
the direct reactions of their corresponding oxides with a large excess
of AlBr3 at an atomic ratio Ln:Al )1:4. The main reactions may
be expressed as (1/2)Ln* O3(s) + (1/2)Al2Br6(g) ) Ln*Br3(s) +
2
(1/2)Al2O3(s), CeO2(s) + (1/2)Al2Br6(g) ) CeBr3(s) + (1/2)Al2O3-
(s) + (1/4)O2(g), (1/6)Pr6O11(s) + (1/2)Al2Br6(g) ) PrBr3(s) + (1/
2)Al2O3(s) + (1/6)O2(g), and (1/4)Tb4O7(s) + (1/2)Al2Br6(g) )
TbBr3(s) + (1/2)Al2O3(s) + (1/8)O2(g), respectively, where the latter
three reactions produced oxygen, which may completely be removed
by adding a small amount of Al powder. In a typical reaction, after
placement of either 0.2 g of Ln* O3 and 1.2 g of AlBr3 or 0.2 g of
2
CeO2, Pr6O11, or Tb4O7, 1.2 g of AlBr3, and 0.015 g of Al powder
into an one-end sealed quartz tube, 20 mm in inner diameter and
300 mm in length, under Ar atmosphere, and the sealing of its other
end under vacuum, the reaction mixture was heated at 600 K for 1
h. The resultant LnBr3 was separated from the crude product by
the chemical vapor transport method. For doing so, the evacuated
and sealed quartz tube was placed in a tubular furnace with a
reduced temperature gradient from 750 to 460 K and the solid crude
product was placed at the hot end of the quartz tube. Each of the
chemical vapor transport reactions was carried out for 6 h. During
the reactions, the vapor complexes LnAlnBr3n+3 were produced by
the reactions of LnBr3 with the residual AlBr3 at the hot end of the
quartz tube, chemically transported from the hot end to the cold
end, and then decomposed into LnBr3(s) and Al2Br6(g) at the cold
end. After the reaction, the quartz tube was removed out quickly
from the furnace and its hot end was quenched with water, where
AlBr3 was quickly condensed. By using this method, LnBr3 may
easily be separated from the residual reactors and other resultants.
In addition, the anhydrous rare earth element bromides can also be
prepared by dropping liquid Br2 directly into a solid mixture
consisting of the corresponding rare earth oxides and a large excess
of Al powder in one-end sealed quartz tubes under Ar atmosphere
and then chemically transported via their vapor complexes in the
evacuated and sealed quartz tubes. All anhydrous chemicals were
handled in a glovebox containing a dry argon atmosphere with a
water vapor level less than 20 ppm.
Experimental Section
The chemicals used in this study were of 99.999% purity for Al
powder, more than 99.5% purity for Br2, more than 99.99% purity
(11) Øye, H. A.; Gruen, M. J. Am. Chem. Soc. 1969, 91, 2229.
(12) Papatheodorou, G. N.; Kucera, G. H. Inorg. Chem. 1979, 18, 385.
(13) Hoekstra, H. R.; Hessler, J. P.; Williams, C. W.; Carnall, W. T. In
High Temperature Metal Halide Chemistry; Hildenbrand, D. L.,
Cubicciotti, D. D., Eds.; The Electrochenical Society: Pennington,
NJ, 1978; PV 78-1, p 123.
(14) Kulset, N. High Temperature Study of Noedymium Halide Gas
Complexes. Ph.D. Thesis, University of Trondheim, Trondheim,
Norway, 1986.
(15) Scha¨fer, H.; Flo¨rke, U. Z. Anorg. Allg. Chem. 1981, 479, 89.
(16) Steidl, G.; Ba¨chmann, K.; Dienstbach, F. J. Phys. Chem. 1983, 87,
5010.
(17) Steidl, G.; Ba¨chmann, K.; Dienstbach, F. Polyhedron 1983, 2, 727.
(18) Cosandey, M.; Emmenegger, F. P. J. Electrochem. Soc. 1979, 126,
1601.
(19) Wang, Z.-C.; Wang, L.-S.; Gao, R.-J.; Su, Y. J. Chem. Soc., Faraday
Trans. 1996, 92, 1887.
(20) Wang, L.-S.; Gao, R.-J.; Su, Y.; Wang, Z.-C. J. Chem. Thermodyn.
1996, 28, 1093.
(21) Wang, Z.-C.; Wang, L.-S. Inorg. Chem. 1997, 36, 1536.
(22) Wang, Z.-C.; Wang, L.-S. J. Alloys Compd. 1998, 265, 153.
(23) Lange, F. Th.; Ba¨rnighausen, H. Z. Anorg. Allg. Chem. 1993, 619,
1747.
(24) Oppermann, H.; Huong, D. Q. Z. Anorg. Allg. Chem. 1995, 621, 659.
(25) Oppermann, H.; Hennig, Z.; Dao Quoc, H. Z. Naturforch. 1998, 53b,
361.
The phase equilibrium-quenching experiments were carried out
in closed ampules made from Pyrex glass with a special shape as
shown in Figure 1. Less AlBr3 and an excess of LnBr3 were placed
in the deep ditch of the ampule (part A in Figure 1), and the ampule
was then sealed under vacuum. That may ensure AlBr3 to evaporate
completely and to react with part of the LnBr3(s) to reach an
equilibrium at high temperature among LnBr3(s), Al2Br6(g), and
LnAlnBr3n+3(g) in each ampule.
(26) Oppermann, H.; Zhang, M.; Hennig, Z. Z. Naturforch. 1998, 53b, 1343.
(27) Oppermann, H.; Dao Quoc, H.; Morgenstern, A. Z. Naturforch. 1999,
54b, 1410.
(28) Oppermann, H.; Dao Quoc, H.; Zhang-Presse, M. Z. Naturforch. 2001,
56b, 908.
Four ampules were placed in a graphite container and then placed
in a furnace, where the temperature was kept constant within (0.5
K measured with a Pt-PtRh10 thermocouple. Preliminary experi-
ments showed that the maximum temperature difference in the
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