Cd8As7Cl: A Novel Pnictidohalide with a New Structure Type

Article abstract of DOI:10.1006/jssc.1997.7555

A novel cadmium arsenide chloride, Cd₈As₇Cl (I), has been prepared by a two-step stoichiometric synthesis. It crystallizes in the cubic space groupP23 (No. 195) with a unit cell parametera=7.2660(10) A (Z=1). The crystal structure has been solved based on single-crystal X-ray diffraction data (ω-2θscans, least-squares refinement onF²) toR=0.0258 and wR²=0.0435. I possesses a new type of crystal structure that is described as a distorted (not dense) primitive cubic packing of cadmium atoms, the cubic voids of which are filled by either arsenic or chlorine atoms or As-As pairs. The structure of I exhibits a position disorder for one-half of the cadmium atoms. The As-As pairs are cubically surrounded by eight cadmium atoms, the As-As separation being 2.43 A, which is longer than in all known cadmium and mercury arsenide halides. Attempts to synthesize the corresponding bromide and iodide failed.

Full text of DOI:10.1006/jssc.1997.7555

JOURNAL OF SOLID STATE CHEMISTRY 134, 282—285 (1997)
ARTICLE NO. SC977555
Cd8As7Cl: A Novel Pnictidohalide with a New Structure Type
Andrei V. Shevelkovꢀ, Ludmila N. Reshetova, and Boris A. Popovkin
Inorganic Synthesis Laboratory, Department of Chemistry, Moscow State University, 119899 Moscow, Russia
Received April 2, 1997; in revised form July 15, 1997; accepted July 15, 1997
halides, we have unexpectedly prepared a novel cadmium
arsenide chloride, Cd As Cl, as a byproduct. In this article
A novel cadmium arsenide chloride, Cd8As7Cl (I), has been
prepared by a two-step stoichiometric synthesis. It crystallizes in we report the synthesis of Cd As Cl, the phase possessing
ꢈ
ꢉ
ꢈ
ꢉ
the cubic space group P23 (No. 195) with a unit cell parameter
a ؍ 7.2660(10) A (Z ؍ 1). The crystal structure has been solved
based on single-crystal X-ray diffraction data (x–2h scans, least-
squares refinement on F2) to R ؍ 0.0258 and wR2 ؍ 0.0435.
I possesses a new type of crystal structure that is described as
a distorted (not dense) primitive cubic packing of cadmium
atoms, the cubic voids of which are filled by either arsenic or
chlorine atoms or As–As pairs. The structure of I exhibits
a position disorder for one-half of the cadmium atoms. The
ꢂ\
a new type of crystal structure, in which As dumbbells are
ꢃ
surrounded cubically by eight cadmium atoms. Unsuccess-
ful attempts to synthesize the respective bromide and iodide
are also reported.
EXPERIMENTAL
Samples. Cadmium, arsenic, and cadmium diiodide of
As–As pairs are cubically surrounded by eight cadmium atoms, a high purity ('99.99%) were used as received. Cadmium
the As–As separation being 2.43 A, which is longer than in all
known cadmium and mercury arsenide halides. Attempts to
synthesize the corresponding bromide and iodide failed. ( 1997
Academic Press
dichloride was prepared by heating cadmium powder under
flowing HCl gas at 723 K. Cadmium dibromide was pre-
pared by heating cadmium powder under Ar/Br gas flow at
ꢃ
723 K. Cadmium dichloride and dibromide were handled
and manipulated in a glovebox.
For the preparation of Cd As Cl, a stoichiometric mix-
ꢈ
ꢉ
INTRODUCTION
ture of cadmium, arsenic, and cadmium dichloride was
sealed in a silica tube under vacuum and heated at 623 K for
3 days. The sample then was reground and heated in
a sealed silica tube for 3 days at 773 K. An X-ray analysis
Cadmium and mercury pnictidohalides exhibit a large
variety of compositions and structural types. Among them,
phases containing different types of the group 15 element
anions and polyanions can be found, such as Zꢁ\ anions
(Z"P, As), Zꢂ\ dumbbells (Z"P, As, Sb), and helical Z\
chains (P, As). A review (1) gives a survey of the pnictidohal-
ides, with special respect to their crystal structures; further
(Enraf—Nonius FR-552 chamber, CuKa radiation) of
ꢀ
a black air-stable product showed no traces of starting
materials. All reflections were indexed in the cubic system
with a unit cell parameter a"7.2671(5) A. The composition
Cd As Cl was further confirmed by a crystal structure re-
ꢃ
ꢈ
ꢉ
(recent) examples can be found in Refs. (2—7). Zꢂ\ dumb-
finement.
We also attempted to synthesize the corresponding bro-
ꢃ
bells are the most frequently observed anions. With few
exceptions, the dumbbells are surrounded octahedrally
by six metal atoms; exceptions are Hg Sb I (6) and
mide and iodide. For this purpose, the stoichiometric mix-
tures of cadmium, arsenic, and cadmium dibromide or
diiodide, respectively, were annealed under the same condi-
tions as those for Cd As Cl. X-ray analysis of the products
ꢄ
ꢃ ꢅ
Hg As Br (5), in which the dumbbells are surrounded
ꢀꢆ ꢀꢇ ꢀꢈ
by five metal atoms in such a way that octahedra with one
vertex missing can be easily imagined. The Z—Z distances in
dumbbells do not vary much for atoms of one sort—e.g., the
As—As separation falls in the short range of 2.38—2.41 A
(4, 7—10)—and these distances do not depend on the type of
arrangement of octahedra in crystal structures.
ꢈ
ꢉ
showed that they were mixtures of Cd As X (X"Br, I)
ꢃ ꢁ
and cadmium arsenides, CdAs and Cd As .
ꢂ
ꢃ
ꢁ
ꢃ
X-ray diffraction experiment. A suitable single crystal
was selected from the reaction product, glued with epoxy
cement on the tip of a Pyrex fiber, and mounted on
a goniometer head of a CAD-4 (Enraf—Nonius) diffrac-
tometer. Unit cell parameters were refined based on 24
well-centered reflections in the angular range of 14°(h(16°.
In the course of our work dealing with new synthetic
routes of single-crystal preparation of cadmium arsenide
ꢀTo whom correspondence should be addressed.
282
0022-4596/97 $25.00
Copyright ( 1997 by Academic Press
All rights of reproduction in any form reserved.

Cd As Cl: A NOVEL PNICTIDOHALIDE
283
ꢈ
ꢉ
Data were recorded at room temperature with data collec-
tion parameters listed in Table 1 and corrected for polariza-
tion and Lorentz effects. A semiempirical absorption
correction was applied to data based on azimuthal scans of
five reflections having their s angles close to 90°. Data
reduction was performed using SDP programs (11).
TABLE 2
Atomic Coordinates and Equivalent Isotropic Displacement
Parameters? for Cd8As7Cl
Atom
Position
x/a
y/b
z/c
º(eq)@
Cd(1)
Cd(2ꢀ)A
Cd(2ꢀꢀ)A
As(1)
As(2)
Cl
4e
4e
4e
1a
6h
1b
0.2170(2)
0.7158(6)
0.7637(8)
0
0.2170
0.7158
0.7637
0
0.2170
0.7158
0.7637
0
0.3328(2) 0.0013(1)
0.0020(2)
0.0023(1)
0.0023(1)
0.0023(1)
0.0014(1)
Crystal structure refinement. The crystal structure of
Cd As Cl was solved by direct methods (SHELXS-86 (12)),
ꢈ
ꢉ
which revealed positions of two cadmium and two arsenic
atoms. The position of the chlorine atoms was found from
a Fourier map. Isotropic refinement (SHELXL-93 (13))
showed that one of two independent cadmium atoms had an
enormously large thermal displacement parameter. Inspec-
tion of a difference Fourier map showed that this atom was
split into two positions lying approximately 0.6 A from each
other. These two peaks were introduced into the refinement
as half-occupied cadmium positions and refined isotropi-
cally to R"0.0376. Occupancy factors of the Cd(2ꢀ) and
Cd(2ꢀꢀ) atoms were then refined assuming that the sum of the
factors was equal to unity (to maintain the electroneutral-
ity). Final refinement with the anisotropic thermal displace-
ment parameters of all atoms except for Cd(2ꢀ) and Cd(2ꢀꢀ),
which were isotropic, led to R"0.0258 and the atomic
parameters listed in Table 2. Selected interatomic distances
and valence bond angles are listed in Table 3.
ꢀ
ꢃ
ꢀ
ꢃ
0
ꢀ
ꢀ
ꢃ
ꢃ
? º(eq) is defined as one-third of the trace of the orthogonalized º
GH
tensor.
@ Cd(2ꢀ) and Cd(2ꢀꢀ) atoms were refined isotropically.
A Occupancies: 0.535(3), Cd(2ꢀ); 0.465(3), Cd(2ꢀꢀ).
cadmium atom has a slightly distorted tetrahedral coord-
ination, 3As(2)#1As(1) in the cases of the Cd(1) and Cd(2ꢀꢀ)
atoms, and 3As(2)#1C1 in the case of the Cd(2ꢀ) atom. The
separation between the Cd(2ꢀ) and Cd(2ꢀꢀ) positions is only
0.60 A, which means that these atoms cannot occupy neigh-
boring positions, but alternate randomly within the whole
structure.
Since cadmium and chlorine atoms possess formal oxida-
tion states #2 and !1, respectively, the formal charge
distribution between the As(1) and As(2) atoms is evident:
!3 for the As(1) atoms and !2 for the As(2) atoms; thus,
the crystallochemical formula is written as Cd Asꢁ\
(As )ꢂ\Cl\. The As(2) atoms are joined into As dumb-
bells, the As—As distance being 2.43 A. Each As(2) atom is
further bound to four cadmium atoms, two Cd(1) atoms and
two of four randomly distributed Cd(2ꢀ) and Cd(2ꢀꢀ) atoms.
The Cd—As distances vary in the range of 2.62—2.97 A. These
values exceed the normal range of the Cd—As separation in
binary phases (2.53—2.58 A in Cd As (14) and 2.59 A in
RESULTS AND DISCUSSION
ꢃ>
ꢈ
The crystal structure of Cd As Cl is drawn in Fig. 1,
ꢈ
ꢉ
ꢂ\
ꢃ
ꢃ
where three independent cadmium atoms are labeled. Each
TABLE 1
Crystal Data and Structure Refinement for Cd8As7Cl
Unit cell parameter a (A)
Space group
7.2660(10)
P23 (No. 195)
ꢁ
ꢃ
CdAs (15)), as well as in cadmium arsenide halides that
contain As—As pairs. In arsenide halides, such pairs are
Volume (Aꢁ)
383.61(9)
ꢃ
Formula weight
1459.09
Z
1
F(0 00)
632
Wavelength (A)
0.71069 (MoKa)
6.316
0.06;0.06;0.06, black
293(2)
25.979
2.80—24.88
436
148, 0.0517
TABLE 3
Density (calculated) (Mg/mꢁ)
Crystal size (mmꢁ) and color
Temperature (K)
Absorption coefficient (mm\ꢀ)
h range for data collection (°)
Reflections collected
Independent reflections, R(int)
Data/parameters
Extinction coefficient (13)
Refinement method
Final R indices (I'2p(I))
R indices (all data)
Goodness-of-fit on Fꢃ
Largest diff peak and hole (e/Aꢁ)
Selected Interatomic Distances (A) and Angles (°) for Cd8As7Cl
Atom—Atom
Distance
Atom—Atom—Atom Bond angle (deg)
Cd(1)—As(2)
Cd(1)—As(1)
Cd(1)—Cl
Cd(2ꢀ)—As(2)
Cd(2ꢀ)—Cl
Cd(2ꢀ)—As(1)
Cd(2ꢀꢀ)—As(2)
Cd(2ꢀꢀ)—As(1)
Cd(2ꢀꢀ)—Cl
2.7245(9) (;3) As(2)—Cd(1)—As(2)
112.48(4) (;3)
106.26(5) (;3)
2.731(2)
3.575(2)
As(2)—Cd(1)—As(1)
148/16
0.0039(6)
2.6167(5) (;3) As(2)—Cd(2ꢀ)—As(2)
119.90(2) (;3)
91.8(2) (;3)
2.716(8)
3.589(4)
As(2)—Cd(2ꢀ)—Cl
Full-matrix least-squares on Fꢃ
R "0.0258, wR "0.0435?
2.667(2) (;3) As(2)—Cd(2ꢀꢀ)—As(2)
116.29(13) (;3)
101.2(2) (;3)
ꢀ
ꢃ
R "0.0292, wR "0.0445?
1.153
2.973(9)
3.284(5)
2.430(3)
As(2)—Cd(2ꢀꢀ)—As(1)
ꢀ
ꢃ
0.873 and !0.687
As(2)—As(2)
Cd(2ꢀ)—Cd(2ꢀꢀ) 0.603(7)
? w"1/pꢃ(F )ꢃ.
ꢀ

284
SHEVELKOV, RESHETOVA, AND POPOVKIN
As(2)—As(2) pairs. Such packing can be seen in Fig. 2, where
the Cd(2ꢀ) and Cd(2ꢀꢀ) atoms, each having partial occu-
pancies, are replaced by the Cd(2) atom lying in the ‘‘ideal’’
position, i.e., halfway between the neighboring Cd(2ꢀ) and
Cd(2ꢀꢀ) atomic sites. Figure 2 shows eight cubic voids: three
voids are filled with As(2)—As(2) pairs having different ori-
entation, one void is centered by the As(1) atom and an-
other by the Cl atom, and three voids remain unoccupied. If
half of the cadmium atoms were not split into two positions,
Cd(2ꢀ) and Cd(2ꢀꢀ), the As(1) and Cl atoms would possess
coordination numbers 8 (a highly distorted cube) and 4 (a
regular tetrahedron), respectively, with the following in-
teratomic distances: d
"2.73 and 3.28 A, d "
ꢊꢁ\!$
!ꢂ\!$
FIG. 1. Crystal structure of Cd As Cl: a view of the unit cell. Open
ꢈ
ꢉ
circles, cadmium; smaller filled circles, arsenic; larger filled circle, chlorine.
Note that the Cd(2ꢀ) and Cd(2ꢀꢀ) atoms cannot occupy the neighboring sites
simultaneously due to the short Cd(2ꢀ)—Cd(2ꢀꢀ) separation of 0.603 A.
surrounded by six cadmium atoms forming As Cd oc-
ꢃ
ꢅ
tahedra, the As—As distances ranging from 2.36 A in
Cd As I (8) to 2.40 A in Cd AsCl (9). In Cd As Cl, the
ꢂ
ꢃ ꢁ
ꢃ
ꢃ
ꢈ
ꢉ
As(2)—As(2) pairs are surrounded by eight cadmium atoms
forming a distorted cube. Such coordination of As pairs is
ꢃ
very rare. One example is the high-pressure phase CdAs, in
which the As—As pairs having d
surrounded by eight cadmium atoms (16).
"2.47 A are cubically
ꢊꢁ\ꢊꢁ
The As(1) atom is surrounded by four Cd(1) atoms and
four positions of the Cd(2ꢀꢀ) atoms. The latter atom occupies
randomly about 50% of its site; hence the As(1) atom
possesses a highly distorted octahedral coordination,
4Cd(1)#2Cd(2ꢀꢀ). The As(1)—Cd separation is rather large
(2.71 and 2.93 A); however, there is one example, Cd As Br
ꢃ
ꢁ
(4), where an arsenic atom completes its coordination with
one even more distant cadmium atom (d
"3.00 A).
ꢊꢁ\!$
The Cl atom is surrounded by four positions of the Cd(2ꢀ)
atoms. Again, since the Cd(2ꢀ) atom has an about 50%
occupied position, the ‘‘mean’’ coordination number of the
Cl atom is equal to 2. The Cd—Cl distance (2.72 A) is the
same as the separation in CdCl (17), but slightly longer
ꢃ
than in cadmium phosphide chlorides, 2.47—2.71 A in
FIG. 2. (Top) A view of the crystal structure of Cd As Cl. Origin at
Cd PCl (18) and 2.66 A in Cd P Cl (19).
ꢈ
ꢉ
ꢁ
ꢁ
ꢉ ꢂ ꢅ
ꢉ
(ꢀ, ꢀ, ꢀ). Arrows show the directions of the ‘‘ideal’’ Cd(2) atom split along
ꢂ ꢂ ꢂ
The crystal structure of Cd As Cl can be described as
the body diagonal of the cubic cell (cf. text). (Bottom) Coordination of the
ꢈ
a distorted primitive cubic packing of cadmium atoms, the
Cd(2ꢀ) and Cd(2ꢀꢀ) atoms. The small filled circle represents an ideal Cd(2)
cubic voids of which are filled by either Cl or As(1) atoms or position.

Cd As Cl: A NOVEL PNICTIDOHALIDE
285
ꢈ
ꢉ
3.02 A. Such coordination would be quite unusual for both
ꢁ\
3. A. V. Shevelkov, E. V. Dikarev, and B. A. Popovkin, J. Solid State
Chem. 104, 177 (1993).
4. A. V. Shevelkov, E. V. Dikarev, and B. A. Popovkin, J. Solid State
Chem. 113, 116 (1994).
5. A. V. Shevelkov, E. V. Dikarev, and B. A. Popovkin, Zh. Neorg. Khim.
40, 1496 (1995).
6. A. V. Shevelkov, E. V. Dikarev, and B. A. Popovkin, Z. Anorg. Allg.
Chem. 620, 389 (1994).
7. A. V. Shevelkov, E. V. Dikarev, and B. A. Popovkin, J. Solid State
Chem. 126, 324 (1996).
8. J. Gallay, G. Allais, and A. Deschanvres, Acta Crystallogr. Sect. B 31,
2274 (1975).
9. A. Rebbah, J. Yazbeck, A. Leclaire, and A. Deschanvres, Acta Crys-
tallogr. Sect. B 36, 771 (1980).
10. P. Labbe, M. Ledesert, and B. Raveau, Z. Kristallogr. 187, 177 (1989).
11. Enraf—Nonius: CAD-4 Operators Manual, Enraf—Nonius, Delft, The
Netherlands, 1984.
As
and Cl\ anions, and moreover, the ‘‘ideal’’ Cd(2)
atom, if not split into Cd(2ꢀ) and Cd(2ꢀꢀ) positions, would
possess an unusual trigonal-bipyramidal coordination
(Fig. 2). Thus, splitting of the cadmium atomic site into two
half-occupied positions affords a more convenient sur-
rounding of the As(1) and Cl atoms and provides two
tetrahedrally coordinated cadmium atoms, Cd(2ꢀ) and
Cd(2ꢀꢀ).
The voids occupied by the chlorine atoms are large and
seem to be suitable for incorporation of bromine atoms
(instead of chlorine atoms) without any splitting of the Cd(2)
atomic position. However, our attempts to prepare the
corresponding bromide and iodide failed. So far, we have no
reasonable explanation for this fact.
12. G. M. Sheldrick, in ‘‘Crystallographic Computing 3’’ (G. M. Sheldrick,
C. Kruger, and R. Goddard, Eds.), pp. 175—189. Oxford Univ. Press,
Oxford, 1985.
13. G. M. Sheldrick, in ‘‘Crystallographic Computing 6’’ (H. D. Flack, L.
Parkanyi, and K. Simon, Eds.), pp. 111—122. Oxford Univ. Press,
Oxford, 1993.
ACKNOWLEDGMENTS
This work is supported in part by the state program ‘‘Universities of
Russia’’ under Grant UNI-015-95 and by the St. Petersburg Center for
Basic Research under Grant 95-0-9.2-229.
14. A. Pietraszko and K. Lukaszewicz, Acta Crystallogr. Sect. B 25, 988
(1969).
15. L. Cervinka and A. Hruby, Acta Crystallogr. Sect. B 26, 457 (1970).
16. J. Clark and K. J. Range, Z. Naturforsch., B 31, 158 (1976).
17. A. F. Wells, ‘‘Structural Inorganic Chemistry,’’ 5th ed. Clarendon
Press, Oxford, 1986.
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