Crystal structures and pressure-induced redox reaction of Cs2PdI4·I2 to Cs2PdI6

Article abstract of DOI:10.1021/ic990670+

Two iodopalladates of the same empirical formula with palladium in different oxidation states were synthesized from aqueous HI solution. Their crystal structures were characterized by single-crystal X-ray analysis, and the effect of hydrostatic pressure on the structural properties has been investigated. Dicesium hexaiodopalladate(IV), Cs₂PdI₆, crystallizes in a cubic system, space group Fm3m, with a = 11.332(1) A and Z = 4, and is isotypic to K₂PtCl₆. The second compound, dicesium tetraiodopalladate(II) diiodine, Cs₂PdI₄·I₂, shows tetragonal symmetry with space group I4/mmm, a = 8.987(1) A, c = 9.240(1) A, and Z = 2. The crystal structure can be described in resemblance to the Cs₂Au(I)Au(III)Cl₆ type. Structural relationships and chemical and structural transformation between both compounds will be discussed. DTA measurements at ambient pressure showed liberation of I₂ and decomposition of the compounds. Cs₂PdI₄·I₂ represents an excellent example for studying a solid-state electron-transfer reaction. The redox reaction to Cs₂PdI₆ can be demonstrated by performing pressure-dependent X-ray studies.

Full text of DOI:10.1021/ic990670+

732
Inorg. Chem. 2000, 39, 732-735
Crystal Structures and Pressure-Induced Redox Reaction of Cs2PdI4‚I2 to Cs2PdI6
B. Schu1pp,† P. Heines,† A. Savin,‡ and H.-L. Keller*,†
Anorganische Chemie, Universita¨t Dortmund, D-44221 Dortmund, Germany, and Laboratoire de
Chimique The´orique, Universite´ Pierre et Marie Curie, F-75252 Paris Cedex 05, France
ReceiVed June 10, 1999
Two iodopalladates of the same empirical formula with palladium in different oxidation states were synthesized
from aqueous HI solution. Their crystal structures were characterized by single-crystal X-ray analysis, and the
effect of hydrostatic pressure on the structural properties has been investigated. Dicesium hexaiodopalladate(IV),
Cs2PdI6, crystallizes in a cubic system, space group Fm3hm, with a ) 11.332(1) Å and Z ) 4, and is isotypic to
K2PtCl6. The second compound, dicesium tetraiodopalladate(II) diiodine, Cs2PdI4‚I2, shows tetragonal symmetry
with space group I4/mmm, a ) 8.987(1) Å, c ) 9.240(1) Å, and Z ) 2. The crystal structure can be described
in resemblance to the Cs2AuIAuIIICl6 type. Structural relationships and chemical and structural transformation
between both compounds will be discussed. DTA measurements at ambient pressure showed liberation of I2 and
decomposition of the compounds. Cs2PdI4‚I2 represents an excellent example for studying a solid-state electron-
transfer reaction. The redox reaction to Cs2PdI6 can be demonstrated by performing pressure-dependent X-ray
studies.
Introduction
characterized by Krebs et al.,2 who studied hexaiodometalates
by X-ray powder diffraction (a ) 11.311 Å, isotypic to K2-
PtCl611).
Ternary and quaternary halides with palladium in the oxida-
tion states +2 and +4 are known. Most of these halogenopal-
ladates (X ) Cl, Br, I) contain palladium(II), which exhibits a
square-planar coordination, whereas in compounds of the general
type M2PdX6 (M ) alkaline metal, X ) Cl, Br, I)1,2 palladium-
(IV) shows an octahedral coordination geometry. Compounds
containing large organic cations3-6 form Pd2X6 units with edge-
linked PdX4 groups. In pure inorganic structures condensation
of PdX4 groups was found in Tl8Pd7Br22,7 where a pentameric
motif is created by connecting simple PdX4 units via edges and
vertexes.
Here we report our efforts in preparation, single-crystal X-ray
structure determination, DTA analysis, and pressure-dependent
X-ray studies of the two compounds Cs2PdI6 (1) and Cs2PdI4‚
I2 (2). Both are of identical chemical composition but occur in
different oxidation states of Pd. These compounds, which were
prepared in aqueous HI solution, serve as a very convenient
model for electron-transfer reactions in the solid state. Upon
application of pressure to 2 in a direct solid-state reaction,
compound 1 can be afforded with oxidation from Pd(II) to Pd-
(IV) and cleavage of the inserted iodine dumbbell to iodide.
Recently, we succeeded in preparing several ternary halo-
genopalladates in the system CsCl-PdCl2 utilizing hydrothermal
synthesis. Cs12Pd9Cl30‚2H2O, Cs6Pd5Cl16‚2H2O, and CsPdCl3
contain [Pd2Cl6]2- units. In CsPd2Cl5 these units are trans-
connected by vertexes to form one-dimensional endless chains.8
In addition we were able to isolate a new tetrameric [(PdCl2-
Cl2/2)4]4- group in thallium(I) chloropalladate(II) Tl4Pd3Cl10.9
Experimental Section
Synthesis and Characterization. Since PdI2 decomposes between
365 und 651 °C,12 the iodopalladates were not prepared by melting,
quenching, and annealing mixtures of CsI and PdI2. Preparation under
hydrothermal conditions was not successful, due to the limited solubility
product of PdI2 in H2O (KL ) 3.73 × 10-16 mol3/L3 at 25 °C,
potentiometric measurement).13 Thus, we made use of the complex
[PdI4]2-, which is formed from PdI2 in aqueous HI (57%): 0.3 g of a
mixture of CsI and PdI2 (CsI, Fluka; PdI2, own preparation) with the
molar ratio 10:1 was added to a quartz glass ampule. After addition of
1 mL of hydroiodic acid (57%, Chempur) the solvent was frozen, the
ampule (L 6 mm, length 6 cm) was evacuated, sealed, then heated for
48 h at 150 °C, and slowly cooled to room temperature. The product
contains three compounds: Cs2PdI4‚I2 (black needle-shaped crystals),
Cs2PdI6 (black cubes), and CsI3 (red plates) were found, which were
isolated after filtration and washing with water and diethyl ether.
Having refined the experimental conditions, it was possible to yield
the pure compounds Cs2PdI6 (1) and Cs2PdI4‚I2 (2). 1 is obtained by
rapidly cooling the hydrothermal HI solution. Addition of I2 (sublimed
two times, Merck) on the other hand and slow cooling gives a mixture
of 2 and CsI3, the latter of which is removed with ethanol.
It is surprising to note the small number of well-characterized
10
iodopalladates. Apart from K2PdI4 only Cs2PdI6 and its
decomposition product Cs2PdI4 are reported.2 Cs2PdI6 was
* To whom correspondence should be addressed. E-mail: keller@pop.uni-
dortmund.de. Phone: +49 231 755 3804. Fax: +49 231 755 5048.
† Universita¨t Dortmund.
‡ Universite´ Pierre et Marie Curie.
(1) Shape, A. G. J. Chem. Soc. 1953, 4177.
(2) Sinram, D.; Brendel, C.; Krebs, B. Inorg. Chim. Acta 1974, 64, L131.
(3) Gerisch, M.; Heinemann, F. W.; Markgraf, U.; Steinborn, D. Z. Anorg.
Allg. Chem. 1997, 623, 1651.
(4) Maassarani, F.; Pfeffer, M.; Le Borgne, G. Organometallics 1987, 6,
2029.
(5) Harris, C. M.; Livingstone, S. E.; Stephenson, N. C. J. Chem. Soc.
1958, 3697.
(6) Harries, D. C.; Gray, H. B. Inorg. Chem. 1974, 13, 2250.
(7) Duchaˆteau, M. Diplomarbeit, University of Kiel, 1989.
(8) Schu¨pp, B.; Keller, H.-L. To be published.
(9) Schu¨pp, B.; Keller, H.-L. Z. Anorg. Allg. Chem. 1999, 625, 241.
(10) Yamada, S.; Tsuchida, R. Bull. Chem. Soc. Jpn. 1953, 26, 480.
(11) Williams, R. J.; Dillin, D. R.; Milligan, W. O. Acta Crystallogr. 1973,
B29, 1369.
(12) Dupuis, T.; Duval, C. Anal. Chim. Acta 1950, 4, 615.
(13) Geissler, M. Z. Chem. 1982, 22, 341.
10.1021/ic990670+ CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/01/2000

Pressure-Induced Reaction of Cs2PdI4‚I2 to Cs2PdI6
Table 1. Crystallographic Data for Cs2PdI6 and Cs2PdI4‚I2
Inorganic Chemistry, Vol. 39, No. 4, 2000 733
Cs2PdI6
Cs2PdI4‚I2
space group
a, Å
Fm3hm (no. 225)
I4/mmm (no. 139)
8.987(1)
9.240(1)
746.3
5.045/5.105(8)
1133.66
2
11.332(1)
c, Å
V, Å3
1455.2
F
calcd/obsd, g‚cm-3 5.174/5.209(6)
M, g‚mol-1
Z
1133.66
4
T, K
293
293
18.41
0.71069
0.0637
µ, mm-1
λ(Mo KR), Å
wR2a
18.88
0.71069
0.0858
R1 (Fo > 4σ(Fo))b 0.0327 (98 reflections) 0.0248 (229 reflections)
R1 (all Fo)
0.0377 (113 reflections) 0.0351 (248 reflections)
a wR2 ) {∑(w(Fo - Fc2)2/∑(w(Fo )2)}0,5
.
b R1 ) ∑||Fo| - |Fc||/
2
2
∑|Fo|.
Table 2. Fractional Atomic Coordinates and Displacement
Parameters Ueq(Å2) (Ueq ) 1/3∑i∑jUijai*aj*aiaj) of Cs2PdI6 and
Cs2PdI4‚I2
atom
site
x
y
z
Ueq
Figure 1. Crystal structure of Cs2PdI6 and Cs2PdI4‚I2, and transforma-
tion of Cs2PdI6 into a “tetragonal setting” (calculated parameters for a
tetragonal setting of Cs2PdI6: I4/mmm, a′ ) a/x2 ) 8.013(1) Å, c′ )
a ) 11.332(1) Å, V ) 727.6 Å3). Thermal ellipsoids are at the 50%
probability level.
Cs2PdI6
1
1
1
Cs
Pd
I
(8c)
(4a)
(24e)
/
/
/
4
0.0440(6)
0.0282(7)
0.0405(5)
4
4
0
0
0
0
0
0.23799(8)
Cs2PdI4‚I2
1
1
Cs
Pd
I(1)
I(2)
(4d)
(2a)
(8h)
(4e)
0
0
/
/
0.0525(4)
0.0312(4)
0.0413(4)
0.0453(4)
2
4
Table 3. Selected Interatomic Distances (Å) and Angles (deg) for
Cs2PdI6 and Cs2PdI4‚I2
0
x
0
0
0
0.79379(5)
0
Cs2PdI6
0.6501(1)
Pd-I(1)
Cs-I(1)
2.697(1)
4.009(1)
6×
X-ray Studies. X-ray diffraction experiments with different crystals
on a κ-CCD single-crystal diffractometer (Nonius) showed that, apart
from red crystals of CsI3,14 two other compounds were isolated. Two
black lustrous crystals, needle-shaped Cs2PdI4‚I2 as well as cubic Cs2-
PdI6, were examined with monochromatic Mo KR radiation (λ )
0.71069 Å). æ-Scans were performed with a rotation angle of 1°, and
360 frames have been taken, allowing a measuring time of 15 s/frame.
Detector distances of 25 mm (1) and 26 mm (2) were chosen. Lattice
parameters were determined and refined using the Denzo+Scalepack15
program package. We regret the lack of the possibility of perfoming
absorption correction for the measurements at that time. We suppose
no serious inaccuracy is contained. For 1, a total of 2884 reflections
with -12 e h e 12, -14 e k e 14, and -14 e l e 14 were collected.
The number of independent reflections was 113 (Rint ) 0.0687). For 2,
2801 collected and 248 independent reflections were observed with
-11 e h,k,l e 11 (Rint ) 0.0818). Starting models for structure
refinement were found using direct methods (SHELXS 8616), and the
structural data were refined versus F2 by full-matrix least-squares
methods (SHELXL 9317).
12×
Cs2PdI4‚I2
Pd-I(1)
2.6208(5)
3.233(1)
3.9676(5)
3.7064(8)
3.9676(5)
3.9832(6)
2.774(2)
4×
2×
8×
2×
4×
2×
1×
4×
I(1)-Pd-I(1)
I(2)-Pd-I(2)
I(1)-Pd-I(2)
90
180
90
4×
Pd-I(2)
Cs-I(1)
I(1)-I(1)
I(1)-Cs
I(1)-I(2)
I(2)-I(2)
I(2)-I(1)
3.9832(6)
AuIAuIIICl6
and Cs2HgPdCl6,21 though [AuCl2]-- and
19,20
[HgCl2]-dumbbells, respectively, are substituted by I2 molecules
here. Palladium(II) shows a square-planar stereochemistry with
palladium-iodine bond distances of 2.621(1) Å. The coordina-
tion sphere is supplemented by two more distant iodine atoms
belonging to embedded I2 molecules to form a strongly
elongated octahedron around palladium. The iodine-iodine
distance in the embedded iodine molecule is 2.774(2) Å, hereby
slightly stretched compared to the value found in elementary
See Tables 1-3 for crystallographic data, atomic coordinates and
displacement parameters, and bond distances and angles, respectively,
for Cs2PdI6 and Cs2PdI4‚I2.
iodine (dI-I ) 2.715(2) Å).22,23 In polyiodides similar distances
had been found. As examples we will consider the anion I12
2-
,
Results and Discussion
observed in [K(Crypt-2.2.2)]2I12,24 which can be described as
(I3-‚I2)‚I2‚(I3-‚I2) with I-I distances for the I2 units of 2.758-
(2) and 2.797(2) Å, and the compound [Co(NH3)6]I3I425 contain-
ing I42- units, which can be described by the species (I-‚I2‚I-)
showing a bond length of 2.791(2) Å for the I2 group. In
Crystal Structure of Cs2PdI6. Cs2PdI6 (Figure 1) crystallizes
isotypic to K2PtCl6 as well as the other dicesium hexahalo-
genopalladates(IV) Cs2PdX6.1,18
Crystal Structure of Cs2PdI4‚I2. The crystal structure of Cs2-
PdI4‚I2 (Figure 1) resembles the basic structure motif of Cs2-
(19) Elliott, N.; Pauling, L. J. Am. Chem. Soc. 1938, 60, 1846.
(20) Tindemans, J. C. M.; Verschoor, G. C. Mater. Res. Bull. 1974, 9,
1667.
(21) Schro¨der, L.; Keller, H.-L. Z. Anorg. Allg. Chem. 1991, 603, 69.
(22) Van Bolhuis, F.; Koster, P. B.; Migchelsen, T. Acta Crystallogr. 1967,
23, 90.
(14) Runsink, J.; Swen-Walstra, S.; Migchelsen, T. Acta Crystallogr. 1972,
B28, 1331.
(15) Otwinowsky, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(16) Sheldrick, G. M. SHELXS 86, Program for Crystal Structure Deter-
mination, University of Go¨ttingen, 1986.
(17) Sheldrick, G. M. SHELXL 93, Program for Crystal Structure Refine-
ment, University of Go¨ttingen, 1993.
(18) Hoppe, R.; Klemm, W. Z. Anorg. Allg. Chem. 1952, 268, 364.
(23) Ibberson, R. M.; Moze, O.; Petrillo, C. Mol. Phys. 1992, 76, 395.
(24) Tebbe, K.-F.; Kavoosian, A. Z. Naturforsch.1993, 48b, 438.
(25) Tebbe, K.-F. Acta Crystallogr. 1983, C39, 154.

734 Inorganic Chemistry, Vol. 39, No. 4, 2000
Schu¨pp et al.
Figure 2. Section of the crystal structure of Cs2PdI4‚I2 showing the
coordination sphere of the Cs site. Thermal ellipsoids are at the 50%
probability level.
compounds with a simple I2 molecule (e.g., C6H12N4‚3I2) an
iodine-iodine bond length of 2.746(1) Å has been reported.26
The centers of [PdI4]2- units and of I2 molecules are located
1
1
alternately on a 4-fold rotation axis, 0 0 z and
/
/ z,
2
2
1
respectively, with z ) 0 and z ) /2 belonging to the point
symmetry of 4hm2. Cesium is coordinated by four pairs of iodine
atoms; each atom has a distance of 3.9676(5) Å to the central
cesium atom. Each pair of these iodine atoms belongs to a
[PdI4]2- unit, so that there are different iodine-iodine distances
within the resulting CsI8 polyhedron. They range from 3.7064-
(8) Å for iodine atoms from the same [PdI4]2- unit, to 4.752(4)
Å for iodine atoms from different [PdI4]2- units in the direction
of the crystallographic c-axis, to 5.2806(9) Å in the directions
[100] and [010]. This results in a strong distortion of the iodine
polyhedron around cesium (Figure 2).
Iodine of site I(1) is 5-fold coordinated by one neighboring
palladium atom and four cesium atoms, extending to a tetragonal
pyramidal structure. Each I(2) is coordinated linearly by another
I(2) atom, forming the I2 molecule. At a distance of 3.233(1) Å
Pd extends the coordination sphere of I(2).
Figure 3. Illustration of the ELF for Cs2PdI4‚I2 (a, left) and Cs2PdI6
(b, right) with an isovalue of 0.42.
(tight-binding linear muffin-tin orbital in the atomic sphere
approximation).30 In Figure 3 the electron localization function
is visualized by the program SciAn31 and displayed for an
isovalue of 0.42. In the case of 2 (Figure 3a) we can see that
this isosurface encloses a maximum of ELF (“localized elec-
trons”) between the iodine atoms of the embedded I2 dumbbell
which does not appear in the case of 1 (Figure 3b) for the same
isovalue. The relatively low ELF value between the I atoms is
related to the weakness of the bond in I2 (compair the long I-I
distance). Pd-I bonds can be made visible (not displayed) for
an isovalue of 0.57 in both compounds. This higher level shows
the stronger attraction for localized electrons within the Pd-I
bond.
The crystal structure of 2 can be described analogously to
Cs2HgPdCl6 as the “filled-up” K2PdCl4 type.21 The empty
channels in the K2PdCl4-type structure are filled by I2 and
[PdI4]2- along 1/2 1/2 z in an alternating sequence. This special
motif causes a change of the empirical formula A2BX4 in K2-
PdCl4 to A2BX4‚X2 and A2BX4‚HgX2, respectively.
DTA Measurements. The chemical relationship of the
cesium iodopalladates 1 and 2 is obvious in the alternating
sequence of [PdI4]2- and I2 units in the direction [001]. It should
be possible to cause a transition between the compounds in direct
solid-state redox reaction by application of pressure or temper-
ature. To verify our assumption, we first performed TG/DTA
experiments. We expected to gain additional information about
the decomposition reaction of Cs2PdI6 as reported by Krebs.2
Thermogravimetry showed for 1 a loss of mass in two steps,
which corresponds to liberation of iodine (I2). The products,
which were isolated from step 1 and step 2, show different X-ray
powder patterns. Several peaks in the powder pattern of the
decomposition product of step 1 can be assigned to CsI. The
product of the second decomposition step shows a melting point
at 430 ( 5 °C. Following our observations, we assume the
formation of at least one other compound; due to the lowered
CsI content it will contain linked PdI4 units. This would
correspond to the results of our experiments in cesium chloro-
palladates(II) which show this kind of linkage.8 Recently we
were able to isolate a pure sample of the first decomposition
step, and detailed structure investigations are to be published.
DTA and TG lines of Cs2PdI6 and Cs2PdI4‚I2 are the same.
Pressure-Dependent X-ray Diffraction Experiments. Fur-
thermore, application of pressure to 2 should induce a solid-
state reaction to dicesium hexaiodopalladate(IV) involving
cleavage of the I2 dumbbell and oxidation of Pd(II) to Pd(IV).
The first example of this structure type is Cs2AuIAuIIICl6 and
has already been reported by Elliott and Pauling19 in 1938.
Unlike the examples discussed so far, Cs2PdI4‚I2 reveals a
smaller c/a-ratio of 1.03 (Cs2AuAuCl6 1.45, Cs2AgAuCl619 1.49,
and Cs2HgPdCl6 1.46). The reason for this lies in the shorter
length of the dumbbell I2 in relation to AuCl2-, AgCl2-, and
HgCl2.
Substitution of triatomic dumbbells by I2 molecules implies
a change of the coordination number of Cs from CN ) 12 [Cs2-
AuAuCl6, d(Cs-Cl) ) 3.768 Å (4×) and 3.802 Å (8×); Cs2-
AgAuCl6, d(Cs-Cl) ) 3.710 Å (4×) and 3.806 Å (8×);
Cs2HgPdCl6, d(Cs-Cl) ) 3.739 Å (4×) and 3.796 Å (8×)] to
CN ) 8, or CN ) 8 + 4 with d(Cs-Cl) ) 3.9676(5) Å (8×)
and 4.5873(5) Å (4×), respectively.
The relation to chloroperovskites can be rationalized by the
group-subgroup relationship of space groups as described for
the compound Cs2HgPdCl6.21
Relationship between 1 and 2. The structural relationship
between 1 and 2 can be more clearly understood by transforma-
tion of the original setting for Cs2PdI6 into a tetragonal setting,
which is illustrated in Figure 1 including the corresponding
transformation matrix.
The contrasting kind of chemical bonding in both compounds
can be indicated by ELF calculations (ELF ) electron localiza-
tion function).27-29 ELF describes chemical bonding by localized
pairs of electrons. On the basis of density functional theory,
calculations were performed with the program TB-LMTO-ASA
(28) Savin, A.; Becke, A. D.; Flad, J.; Nesper, R.; Preuss, H.; von Schnering,
H. G. Angew. Chem. 1991, 103, 421; Angew. Chem., Int. Ed. Engl.
1991, 30, 409.
(29) Savin, A.; Nesper, R.; Wengert, S.; Fa¨ssler, Th. F. Angew. Chem. 1997,
109, 1892; Angew. Chem., Int. Ed. Engl. 1997, 36, 1809.
(30) Krier, G.; Jepsen, O.; Burkhardt, A.; Andersen, O. K. TB-LMTO
program, Max-Planck-Institut fu¨r Festko¨rperforschung, Stuttgart,
Germany, 1994.
(26) Tebbe, K.-F.; Nagel, K. Acta Crystallogr. 1995, C51, 1388.
(27) Becke, A. D.; Edgecombe, K. E. J. Chem. Phys. 1990, 92, 5397.
(31) Pepke, E.; Murray, J.; Lyons, J.; Hwu, T.-Z., Florida State University,
Tallahassee, FL, 1993.

Pressure-Induced Reaction of Cs2PdI4‚I2 to Cs2PdI6
Inorganic Chemistry, Vol. 39, No. 4, 2000 735
change in the diffraction patterns as a result of the structural
transition from Cs2PdI4‚I2 to Cs2PdI6 was quite obvious. At
pressures p e 1.24 GPa we obtain a tetragonal indexing in
agreement with the crystal structure of Cs2PdI4‚I2 (Figure 4).
The length of the crystallographic a-axis decreases with increas-
ing pressures (p e 1.24 GPa), while the c-axis remains virtually
constant. At pressures p g 2.03 GPa the powder patterns can
be indexed face-centered cubic. Presumably, at higher pressures
another transition takes place. Further experiments are in
progress to obtain more detailed information.
Conclusions
Investigating the system CsI-PdI2, we were able to isolate
two compounds with the same empirical formula and related
crystal structures. The proposed transition from Cs2PdI4‚I2 to
Cs2PdI6 can be demonstrated by pressure-dependent X-ray
diffraction. The high-pressure experiments gave rise to the
existence of another unknown modification (p > 8 GPa). The
reverse reaction of 1 to 2 has not been observed yet, because
performing temperature-dependent measurements has only been
possible at ambient pressure so far.
DTA analysis demonstrated the liberation of iodine and
simultaneous partial decomposition of 1 and 2 to CsI and another
compound with a presumably higher palladium iodide content.
Acknowledgment. We are grateful to Dr. Hans-Josef Hesse
(group of Prof. Dr. Wortmann) and Dipl.-Phys. Ing. Werner
Sievers (group of Prof. Dr. Holzapfel), Universita¨t-Gesamthoch-
schule Paderborn, for their essential support concerning the
pressure measurements. For their kind support in translating this
manuscript, we thank Dipl.-Chem. Frank Hipler and Heather
N. Heines. We also thank Degussa for donating chemicals and
the Fonds der Chemischen Industrie for financial support.
Figure 4. Results of the pressure measurements for Cs2PdI4‚I2.
For this purpose energy-dispersive X-ray experiments with
synchrotron radiation (Hasylab, DESY, Hamburg) were per-
formed at different pressures. Pressures were generated by
diamond anvil cells (DAC); details concerning this method are
given by Huber, Syassen, and Holzapfel.32 The ruby lumines-
cence method was performed for pressure determination, as
pressure-transmitting media mineral oil and N2 were used. The
Supporting Information Available: X-ray crystallographic files,
in CIF format, and a table giving a comparison of relevant structure
types. This material is available free of charge via the Internet at
http://pubs.acs.org.
(32) Huber, G.; Syassen, K.; Holzapfel, W. B. Phys. ReV. 1977, B15, 5123.
IC990670+