Formation of coordination compounds in gadolinium-loaded liquid organic scintillators (GdLS): Use of mixed-ligand gadolinium complexes in GdLS preparation

Article abstract of DOI:10.1134/S0036023609070158

The formation of coordination compounds between gadolinium 2-methylvalerate (Gd(2MVA)₃) and trioctylphosphine oxide (TOPO) in a gadolinium-loaded liquid scintillator (GdLS) solution was studied by MALDI-TOF MS. The most abundant gadolinium-cont

Full text of DOI:10.1134/S0036023609070158

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 7, pp. 1082–1089. © Pleiades Publishing, Inc., 2009.
Original Russian Text © G.Ya. Novikova, I.R. Barabanov, L.B. Bezrukov, L.I. Belous, N.A. Danilov, A.A. Ivanov, R.H. Ziganshin, E.A. Yanovich, 2009, published in Zhurnal
Neorganicheskoi Khimii, 2009, Vol. 54, No. 7, pp. 1143–1150.
COORDINATION
COMPOUNDS
Formation of Coordination Compounds in Gadolinium-Loaded
Liquid Organic Scintillators (GdLS): Use of Mixed-Ligand
Gadolinium Complexes in GdLS Preparation
a
a
a
a
b
c
G. Ya. Novikova , I. R. Barabanov , L. B. Bezrukov , L. I. Belous , N. A. Danilov , A. A. Ivanov ,
d
a
R. H. Ziganshin , and E. A. Yanovich
a
Institute for Nuclear Research, Russian Academy of Sciences, pr. Shestidesyatiletiya Oktyabrya 7a, Moscow, 117312 Russia
b
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
c
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
d
Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences,
ul. Miklukho-Maklaya 16/10, Moscow, 117997 Russia
Received March 24, 2008
Abstract—The formation of coordination compounds between gadolinium 2-methylvalerate (Gd(2MVA)3)
and trioctylphosphine oxide (TOPO) in a gadolinium-loaded liquid scintillator (GdLS) solution was studied by
MALDI-TOF MS. The most abundant gadolinium-containing compound in the GdLS solution in the presence
of TOPO is the monomer complex Gd(2MVA) · (TOPO) . In the absence of TOPO, the solution contains only
3
2
Gd (2MVA) (n = 2–5) association species. Therefore, TOPO prevents carboxylate polymerization in the solu-
n
3n
tion and can be used as a stabilizer of the gadolinium compounds in GdLS. The existence of the Gd(2MVA)3 ·
TOPO complex is confirmed by IR spectroscopy. The possibility of coordination between Gd(2MVA) and
3
2
,5-diphenyloxazole (PPO), the most common scintillation admixture, has been investigated. No complexes
between Gd(2MVA) and PPO have been detected by MALDI-TOF MS in the scintillator solution. IR spectro-
3
scopic data also do not provide any unambiguous indication of coordination between Gd(2MVA) and PPO.
3
DOI: 10.1134/S0036023609070158
Gadolinium-loaded liquid organic scintillators factors in a decline in the light yield at a constant gad-
GdLS) are widely used in many physical measure- olinium concentration in the solution. The first is a
(
ments involved in neutron detection. Particularly great decrease in the scintillation efficiency of the molecules
interest in gadolinium-containing scintillators has been containing aromatic rings, as well as the formation of
expressed in recent years because of the start of large- new, “quenching” molecules capable of decreasing the
scale experiments on the detection of neutrino flows efficiency of excitation transfer between solvent mole-
from various sources, such as nuclear reactors and cules. The second factor that can reduce the transpar-
ency of the solution in the emission spectral range is the
possible formation of new compounds absorbing light
in this range. In addition, gadolinium complexes can
precipitate from the solution because of the decrease in
their solubility caused by their hydrolysis or polymer-
ization.
supernova outbursts. The reaction between a neutrino
and a hydrogen atom of the organic solvent yields a
neutron, which is then absorbed by a gadolinium
nucleus. Since the efficiency of neutron absorption by
the gadolinium nucleus is very high, the gadolinium
concentration in the organic scintillator is usually
~
0.1%. The problem of carrying out neutrino experi-
The purpose of this work was to evaluate only the
effect of the processes involving gadolinium com-
pounds and to study the complexes that these com-
pounds can form in solutions of the organic scintillator
upon prolonged use of the latter.
ments includes the need for large amounts (tens of tons)
of scintillator and taking measurements over a long
time of several years. These two circumstances impose
certain stability requirements on the scintillation prop-
erties of GdLS.
Scintillators based on gadolinium carboxylates or
The stability of the scintillation properties of a scin- β-diketonates are being actively developed now for
tillator means the constancy of the total amount of light neutrino experiments [1–3]. There are only six gadolin-
collected by the photodetector. There are two possible ium–ligand bonds in these compounds, although lan-
1
082

FORMATION OF COORDINATION COMPOUNDS IN GADOLINIUM-LOADED
1083
thanides, including gadolinium, typically have a larger with acetone to remove water. The resulting salt was
coordination number. Therefore, the gadolinium atom dried at 110°ë for 2 h.
can form extra coordination bonds with electron donors
present in the liquid organic scintillator. These donors
The C, H, and Gd contents of the resulting Gd(2MVA)3
were determined by elemental microanalysis.
can be oxazole-based scintillation admixtures contain-
For Gd(2MVA) anal. calcd. (%): C, 42.96; H, 6.56;
ing the electron-donating elements oxygen and nitro-
gen, such as 2,5-diphenyloxazole (PPO). Furthermore,
gadolinium carboxylates and β-diketonates can dimerize,
and the formation of longer chains is not impossible.
3
Gd, 31.28. Found (%): C, 42.64; H, 6.59; Gd, 30.74.
Gd(2MVA) · 2PPO synthesis. PPO (2.83 g,
3
0
.0128 mol) was added to absolute benzene (5 mL) to
In this study, the gadolinium compound was gado- obtain a homogeneous solution. Next, dry gadolinium
linium 2-methylvalerate (Gd(2MVA) ), which is the most 2-methylvalerate (3.22 g, 0.0064 mol) was added in
3
small portions under magnetic stirring. This yielded a
viscous gel, which was stirred magnetically for 2 h. The
product was poured into a Petri dish and was placed
into a vacuum desiccator containing phosphorus(V)
oxide. Benzene was collected in a liquid-nitrogen trap
using a fore pump. The resulting complex was dried in
the desiccator for 3 days.
appropriate compound for GdLS preparation by extraction
[
1]. Solid Gd(2MVA) was first synthesized. Its solubility
3
and stability in the solution were enhanced by adding trio-
ctylphosphine oxide (TOPO); Gd : TOPO = 1 : 3 mol/mol.
Here, we report coordination between Gd(2MVA)3
and PPO and between Gd(2MVA) and TOPO and the
3
formation of dimer and polymer Gd(2MVA) associa-
3
Gd(2MVA) · TOPO synthesis. TOPO (1.2477 g)
3
tion species in an organic scintillator solution based on
a mixture of pseudocumene (20%) and dodecane.
was added to hexane (10 mL), and Gd(2MVA) (1.6 g)
3
was then added under stirring. The resulting complex
with a small amount of hexane was placed into a vac-
uum desiccator and was dried over ê é for 1 day. The
The formation of Gd(2MVA) association species
3
and Gd(2MVA) complexes with TOPO and PPO was
3
2
5
investigated by matrix-assisted laser desorption ioniza-
tion coupled with time-of-flight mass spectrometry
complex remained liquid, viscous, and transparent like
glass.
(
MALDI-TOF MS). The formation of coordination
Subsequent studies demonstrated that this complex
can be obtained in a simpler way by combining appro-
bonds between Gd(2MVA) and PPO and between
3
Gd(2MVA) and TOPO was also studied by IR spec-
3
priate amounts of Gd(2MVA) and TOPO powders.
3
troscopy.
IR spectra were recorded as KBr pellets on a
Specord UR 75 spectrophotometer.
EXPERIMENTAL
MALDI-TOF MS experiments were carried out
on an Ultraflex TOF/TOF mass spectrometer (Bruker)
The starting chemicals were Gd O (99.9%) (REO),
2
3
2
-methylvaleric acid (98%, Aldrich), PPO (chemically using an N laser (wavelength of 337 nm). Mass spectra
2
pure grade), TOPO (99%, Aldrich), hydrochloric acid
highpurity grade), aqueous ammonia (25%, highpurity
grade), acetone (highpurity grade), pseudocumene
scintillation grade, P-357, Fischer Scientific Com-
were recorded in the positive ion mode at an accelerat-
ing voltage of 25 kV. The matrix was anthracene.
(
X-ray diffraction studies demonstrated that all of
(
the synthesized compounds are X-ray-amorphous.
pany), dodecane (highpurity grade), and benzene (high-
purity grade).
RESULTS AND DISCUSSION
2
-Methylvaleric acid was purified by vacuum distil-
lation; pseudocumene and dodecane, by passing them
through alumina-packed columns.
Lanthanide carboxylates and β-diketonates are
prone to polymerization, which can occur both in solu-
Gd(2MVA) synthesis. Gd(2MVA) was synthe- tion and in the solid phase, as is indicated by the
3
3
decrease in their solubility in the course of time [5–7].
As was demonstrated for anhydrous lanthanide acety-
lacetonates [5] and nickel acetylacetonates [8], their
polymerization is due to the sharing of oxygen atoms of
sized using a modified lanthanide carboxylate synthesis
procedure [4]. A dilute gadolinium chloride solution
was added in drops to a vigorously stirred (900 rpm) solu-
tion of 2-methylvaleric acid neutralized with concentrated
aqueous ammonia (gadolinium : acid = 1 : 3 mol/mol).
(
the β-diketone. The polymerization of the carboxylates
–
The GdCl solution was prepared by dissolving gado- is due to the COO groups functioning as bridging
3
ligands [9].)
linium oxide in concentrated hydrochloric acid and fil-
tering out excess Gd O . The resulting precipitate was
2
3
This polymerization can be hampered by using gad-
washed with deionized water to remove NH Cl, with olinium compounds with bulkier ligands (e.g., gadolin-
4
ethanol to remove excess 2-methylvaleric acid, and ium dipivaloylmethanate or pivalate) in the scintillator
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 7 2009

1
084
NOVIKOVA et al.
100
9
5
9
0
8
8
7
5
0
5
7
0
6
5
6
0
5
5
5
0
4
5
4
0
3
5
3
2
2
0
5
0
1
5
4
000
3500
3000
2500
2000
ν, cm
1500
1000
500
–
1
Fig. 1. IR spectrum of PPO.
cocktail. However, even in this case, polymerization attention has been focused on the synthetic conditions,
cannot be suppressed completely [10, 11].
structure, and properties of lanthanide β-diketonate
adducts. The Lewis bases that have been used in the
adducts are alkyl phosphates (tributyl phosphate, TBP)
In addition, because of the large coordination num-
bers typical of the gadolinium atoms, the synthesis of
gadolinium compounds in aqueous solutions yields
complexes containing water molecules in their inner
coordination sphere [11, 12]. When drying these com-
plexes, it is practically impossible to prevent hydrolysis
completely.
[
14, 15], phosphine oxides (trioctylphosphine oxide,
TOPO [11, 14]; triphenylphosphine oxide, TPPO
14, 16]), 1,1'-dipyridyl (Dipy) [14, 17, 18], 1,10-phenan-
[
throline (Phen) [14, 17], and dimethyl sulfoxide [19].
There have been similar studies aimed at the synthesis
and characterization of lanthanide carboxylate adducts.
The compounds that have been used as extra ligands are
pivalic acid [9], 1,10-Phen [9], TPPO [20, 21], and TOPO
None of the drying methods tested by Pope et al.
13]—room-temperature drying over MgClO , P O , or
[
4 2 5
CaCl and isothermal dehydration—afforded an anhy-
2
[21].
drous acetylacetonate. Hydroxo complexes were
present in all cases. Furthermore, it is likely that the for-
mation of hydroxo complexes always takes place when
hydrated β-diketonates are dissolved in organic sol-
vents (benzene, ethanol, methanol, etc.) [13]. Particu-
larly rapid hydrolysis is observed as the solution is
heated. The hydroxo complexes forming in the solution
polymerize, yielding hydroxo polymers.
There is some probability that 2,5-diphenyloxazole,
which is commonly used as a scintillation admixture in
GdLS, cal also form coordination bonds with gadolin-
ium carboxylates through the oxygen and nitrogen het-
eroatoms of its oxazole ring. However, IR spectro-
scopic data (Figs. 1–3) provide no unambiguous indica-
tion that these coordination bonds exist in the GdR ·
3
In our opinion, in order to avoid both the polymer- 2PPO adduct, because the characteristic bands of the
ization and hydrolysis of the gadolinium complexes, it phenyl radicals of PPO (Fig. 1; ν(C–C(arom)) =
–1
is necessary to introduce, into these complexes, a com- 1609.07, 1587.48, 1543.40, 1480.95, and 1445.47 cm )
pound that can replace water molecules and completely are overlapped with the characteristic bands of the car-
saturate the coordination sphere without being favor- boxylate groups of Gd(2MVA) (Fig. 2). (The IR spec-
3
able for polymerization.
trum of the starting metal salt (Fig. 2) shows carboxyl
–1
–1
bands at 1533.42 cm (ν (COO)) and 1423.52 cm
It is well known that the coordinatively unsaturated
β-diketonates and carboxylates of paramagnetic lan- (ν
as
(ëéé)), indicating that the carboxyl group is coordi-
s
thanide ions can add Lewis base molecules. Particular nated in a bidentate bridging mode. In the IR spectrum
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 7 2009

FORMATION OF COORDINATION COMPOUNDS IN GADOLINIUM-LOADED
1085
9
9
8
8
7
7
6
6
5
5
4
4
3
3
5
0
5
0
5
0
5
0
5
0
5
0
5
0
4
000
3500
3000
2500
2000
ν, cm–1
1500
1000
500
Fig. 2. IR spectrum of Gd(2MVA)3.
of the adduct, the vibration frequencies of the carboxyl isotopes, so the mass spectrum of any gadolinium-con-
–
1
taining complex has a distinctive profile. This makes it
possible to identify the complex in the total mass spec-
trum of GdLS. An example of an isotopic multiplet is
shown in Fig. 5.)
groups are the same (1533.25 and 1422.15 cm ) and
–
1
the bands at 1585.24 and 1443.59 cm are likely due to
the phenyl radicals of PPO.) The characteristic bands
were identified using reference data [22, 23].
The spectrum of the initial salt (Gd(2MVA) ) has a
3
–1
broad band at 3417 cm (ν(éç)), suggesting that the
initial carboxylate contains water molecules. It follows
from the elemental analysis data that the water content
of the initial salt does not exceed 0.2 wt %. In the IR
Table 1. Assignment of signals from Gd-containing complexes
in the MALDI-TOF mass spectrum of Gd(2MVA) · 2PPO
3
in pseudocumene (anthracene matrix)
spectrum of the GdR · 2PPO adduct, the OH stretching
3
band is missing. However, this is not an unambiguous
indication that PPO molecules coordinate to the initial
salt, replacing the water molecules. It is possible that
m/z
Assignment
(strongest peak)
Relative
intensity, %
the residual water leave the salt during GdR · 2PPO
3
observed calculated
synthesis upon the evaporation of the absolute benzene
used as the solvent.
+
Our MALDI-TOF MS data do not indicate the exist- [Gd (2MVA) ]
891.3546 891.2288
1394.3662 1393.3810
100
2
5
ence of stable complexes between Gd(2MVA) and
3
PPO, but they demonstrate the presence of carboxylate
oligomers in the solution of (Gd(2MVA) ) · 2PPO in
+
3
[Gd (2MVA) ]
64.5
27.2
13.0
3
8
pseudocumene. (The applicability of the MALDI-TOF
MS method to the identification of lanthanide carboxy-
late association species in solutions was demonstrated
in [20, 21].)
+
[
Gd (2MVA) ] 1896.4895 1895.5328
4 11
Figure 4 shows the mass spectrum of a GdR · 2PPO
3
+
solution in pseudocumene (ÒGd = 1 g/L) with an [Gd (2MVA) ] 2398.6130 2397.6846
5
14
anthracene matrix. (Note that gadolinium has six stable
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 7 2009

1
086
NOVIKOVA et al.
9
9
9
9
8
8
8
8
8
7
7
7
7
7
6
6
6
6
6
4
2
0
8
6
4
2
0
8
6
4
2
0
8
6
4
2
3
500
3000
2500
2000
1500
1000
500
–
1
ν, cm
Fig. 3. IR spectrum of Gd(2MVA) · 2PPO.
3
Table 2 presents the assignment of the main peaks topic multiplets and were calculated using the Isotope
from gadolinium complexes in the pseudocumene solu- Pattern program.)
tion. (The masses refer to the strongest peaks of the iso-
It is clear from Table 1 that gadolinium 2-methylval-
erate in the solution exists mainly as dimers (100%) and
association species containing three gadolinium atoms
(64.5%), four ones (27.2%), or five ones (13.0%).
9
We suggested use of TOPO in order to prevent car-
I × 10 , rel. units
boxylate polymerization in the scintillator solution, and
3
55
we synthesized and characterized the Gd(2MVA) ·
3
1
0
0
0
0
.0
.8
.6
TOPO adduct.
The IR spectrum of the Gd(2MVA) · TOPO com-
3
plex (Fig. 6) indicates that the extra ligand is likely in
the inner coordination sphere. The characteristic
–1
absorption band ν(ê=é) of the ligand (1135 cm ) is
shifted to lower frequencies relative to the same band of
–1
891
free TOPO (1160 cm ) [14]. The spectrum of the com-
.4 222
plex does not show any ν(OH) absorption band in the
1
394
–1
3
000–3600 cm range, indicating that the complex is
.2
0
completely dehydrated. In addition, the spectrum of
1
869
Gd(2MVA) · TOPO exhibits a band due to the free car-
3
4
62
1683
2399
1
178
–
1
5
32
boxylic acid (1710.36 cm ), which is evidence that the
extra ligand TOPO partially replaces the acid anion in
the carboxylate molecule.
2
50 500 750 1000 1250 1500 1750 2000 2250 m/z
The complexes between Gd(2MVA) and TOPO in the
3
scintillator solution were also studied by MALDI-TOF MS.
Fig. 4. MALDI-TOF mass spectrum of a solution of
We chose to examine a scintillator containing gado-
linium 2-methylvalerate (1 g/L in terms of Gd), scintil-
Gd(2MVA) · 2PPO in pseudocumene (anthracene matrix).
3
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FORMATION OF COORDINATION COMPOUNDS IN GADOLINIUM-LOADED
1087
8
I × 10 , rel. units
1
161
5
1160
158
1
4
3
2
1
0
1
163
1162
1
157
1164
1
164
1
165 1166
1
156
1169
167
1
1
169
1
155
1
170
1
154
1156
1158
1160
1162
1164
1166
1168
1170 m/z
Fig. 5. MALDI-TOF mass spectrum of the gadolinium-containing scintillator. Scintillator composition: Gd(2MVA ) + PPO (3 g/L) + bis-
3
MSB (30 mg/L). Solvent: pseudocumene (20%) + dodecane. The scintillator storage time before recording the spectrum was 1 year.
The matrix is anthracene.
+
lation admixtures (PPO, 2 g/L; bis-MSB, 30 mg/L), and Table 2. Mass spectrum of [Gd(2MVA) (TOPO) ]
2
2
TOPO as the stabilizing agent (Gd : TOPO = 1 : 3 mol/mol).
The solvent was a mixture of pseudocumene (20%)
with dodecane, which has a high flash point and is com-
patible with acryl, a constructional material used in
large-volume scintillators. Before experiments, the
scintillator was stored for about 1 year. (Figure 5 shows
the MALDI-TOF mass spectrum of this scintillator
with an anthracene matrix.)
Relative
Relative
Observed
Calculated
intensity, %
intensity, %
1
156.4584
1157.4922
1158.4989
10.27
43.50
79.88
80.360
100
1156.8077
1157.8095
1158.8102
1159.8120
1160.8124
1161.8151
1162.8148
1163.8176
1164.8207
1165.8238
5.455
39.853
75.539
80.385
100
Figure 7 displays the mass spectrum of the
+
[
Gd(2MVA) (TOPO) ] complex as an example of gad-
2
2
olinium clusters. Table 2 lists the observed and calcu-
lated masses for the peaks from all isotopes of this com-
plex. Mass calculations were carried out using the Iso-
tope Pattern program.
Table 3 presents the assignment of the main peaks
from the gadolinium-containing complexes in the scin-
tillator solution. (The masses refer to the strongest
peaks of the gadolinium clusters.) It is clear from Table 3
that, in the presence of TOPO, most of the gadolinium
1159.5018
1160.5089
1161.5121
53.735
70.085
43.441
22.166
3.383
52.480
70.266
39.475
13.024
3.050
1162.5151
1163.5171
is in the form of a carboxylate monomer containing two
+
TOPO molecules ([Gd(2MVA) (TOPO) ] ).
2
2
Thus, TOPO hampers carboxylate polymerization in the
scintillator solution, thereby stabilizing the scintillator.
In our opinion, it is appropriate to presynthesize a
gadolinium adduct with TOPO or another extra ligand
for saturating the coordination sphere of the gadolin-
1
164.4960
164.9504
1
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 7 2009

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088
NOVIKOVA et al.
1
1
1
1
1
40
30
20
10
00
9
8
7
6
5
4
3
0
0
0
0
0
0
0
3
500
3000
2500
2000
ν, cm
1500
1000
500
–
1
Fig. 6. IR spectrum of Gd(2MVA) · TOPO.
3
8
I × 10 , rel. units
1
161
5
4
3
2
1
1
664
1
277
7
74
1
090
1594
5
62
1
451
0
6
00
800
1000 1200 1400 1600 m/z
+
Fig. 7. MALDI-TOF mass spectrum of [Gd(2MVA) (TOPO) ] .
2
2
ium atom and to introduce this adduct into the scintilla-
tor solution.
The advantage offered by the mixed-ligand gadolin-
ium complexes in the preparation of liquid scintillators
is that these complexes are coordinatively saturated
compounds readily soluble in organic solvents and
The gadolinium adduct should be prepared immedi-
ately after the synthesis of a gadolinium carboxylate in resist hydrolysis and polymerization when stored as a
order to avoid hydrolysis and carboxylate polymerization. pure substance or in a scintillator solution.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 7 2009

FORMATION OF COORDINATION COMPOUNDS IN GADOLINIUM-LOADED
1089
Table 3. Assignment of signals from Gd-containing complexes in the MALDI-TOF mass spectrum of the GdLS solution
(
Gd(2MVA) + 3TOPO + 1.4PPO + 0.02bis-MSB; anthracene matrix; scintillator storage time of 1 year)
3
m/z
Assignment (strongest peak)
Relative intensity, %
observed
calculated
[
Gd(2MVA) TOPO]+
773.8613
1160.5089
1277.4779
1547.0785
1664.0220
774.4440
1160.8114
1277.5955
1547.1792
1663.9632
7.6
100
2
+
[Gd(2MVA) (TOPO) ]
2 2
+
[Gd (2MVA) (TOPO)]
11.7
3.7
2
5
+
[Gd(2MVA) (TOPO) ]
2 3
+
[Gd (2MVA) (TOPO) ]
18.0
2
5
2
ACKNOWLEDGMENTS
The authors are grateful to I.V. Fadeeva (Baikov
10. D. N. Suglobov, G. V. Sidorenko, and E. K. Legin, Vola-
tile Organic and Complex Compounds of f-Elements
(Energoatomizdat, Moscow, 1987) [in Russian].
Institute of Metallurgy and Materials Science, Russian
Academy of Sciences) for helpful discussions of IR
spectra.
1
1
1
1
1
1
1. C. S. Erasmus and J. C. A. Boeyns, J. Cryst. Mol. Struct.
, 83 (1971).
1
2. S. Onuma, H. Inoue, and S. Shibata, Bull. Chem. Soc.
Jpn. 49, 644 (1976).
This study was supported by the Russian Founda-
tion for Basic Research, grant no. 06-02-16751.
3. G. W. Pope, I. F. Steinbach, and W. F. Wagner, J. Inorg.
Nucl. Chem. 20, 304 (1961).
4. I. A. Murav’eva, L. I. Martynenko, and V. I. Spitsyn, Zh.
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