Cd P Cl AND Cd P Cl CRYSTAL STRUCTURES
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be regarded as bonding (2.62, 2.68, and 2.86 A) and another
three that are much distant (3.56, 3.65, and 3.75 A). Figure
2 shows a fragment of a cadmium atoms packing and
phosphorus and chlorine atoms which occupy the inter-
stices. An arrangement of phosphorus and chlorine atoms
provides a tetrahedral environment for cadmium atoms, 3Cl
#1P for the Cd(1) atoms, and 2P#2Cl for the Cd(2) ones.
Again, as in the case of (I), the structure of (II) can be
alternatively described by subtracting [P Cd ] octahedra
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based on the P(1) atoms and [PCd ] tetrahedral centered
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with the P(2) atoms. Two types of polyhedra comprise the
three-dimensional of (II) (4).
DISCUSSION
Compounds (I) and (II) possess quite different crystal
structures, though both crystallize in a cubic space group
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Pa with close unit cell dimensions. The crystallochemical
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formulas [Cd (P )] (CdCl ) (I) and Cd (P ) (P )Cl
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(II) show that the two compounds differ chemically, as well
as structurally. In (I) all phosphorus atoms are joined into
FIG. 3. Coordination of cadmium atoms: the Cd(1) atom in I (top left);
the Cd(2) atom in I (top right); the Cd(1) atom in II (bottom left); and the
Cd(2) atom in II (bottom right). Cadmium, large open circles; phosphorus,
small open circles; chlorine, filled circles. Thin lines represent distant (more
than 3 A) Cd—Cl separations.
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P
dumbbells which are surrounded octahedrally by six
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cadmium atoms. In (II) such dumbbells coexist with tet-
rahedrally coordinated phosphorus atoms in the form of
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P
anions. The most striking difference is the coordination
of cadmium atoms (Fig. 3). Being tetrahedrally coordinated
in (II), cadmium atoms possess two different coordination
polyhedra in (I), octahedral and one which is described
above as triangular with the Cd atom in the plane.
ing phases, mercury atoms in partially occupied 4b positions
are believed to possess a zero oxidation state (4). The re-
spective position is empty in (I); the highest Fourier peak of
It is worthy to note that the P—P distances are the same in
both structures, d "2.19 A, and that this distance is
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0.386 e/A leaves no doubt about possibly unidentified
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shorter than the interatomic distance 2.21 A corresponding
atoms, and this is not a surprise. Cadmium atoms, if they
had a zero oxidation state, would reduce phosphorus atoms
having oxidation state !2 according to the reaction
Cd P Cl #Cd"2Cd P Cl . Our preliminary results
to a single P—P bond (12). Inspection of data in the literature
shows that in all known cadmium and mercury phos-
phidehalides the P—P pairs if present have the P—P distances
shorter than 2.21 A, the only exception is Hg P Br , where
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show that this reaction can be easily conducted at 620 K,
but this means only that at least at 620 K a thermodyn-
amical equilibrium is achieved, and the conditions favor
formation of (II). Unlike cadmium, mercury might be in-
serted in into (I) to occupy partially the 4b position. The
relevant experiments are underway.
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a distance of 2.21 A which corresponds to a single bond is
observed (3). In Hg P I these values are extremely low,
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2.10 and 2.13 A (13), and that may raise a question about the
bond order.
The structure of (II) looks much like the structures of
other cadmium and mercury pnictidehalides having a 4 : 2 :3
stoichiometry (1). Even Cd P I (14), which exhibits an
orthorhombic distortion, has the same structural motif and
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ACKNOWLEDGMENTS
coordination of all atoms in the structure. On the contrary,
the structure of (I) differs distinctly from the structures of
Hg P Br (3), Hg Sb Br (5), and Hg As Cl (4), the only
The authors thank Dr. Eugeny V. Dikarev (Texas A&M University) for
fruitful discussions. This work is supported in part by the state program
‘‘Universities of Russia’’ under Grant UNI-015-95 and by the St. Peter-
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known pnictidehalides of the 7 : 4 : 6 stoichiometry. While sburg Center for Basic Research under Grant 95-0-9.2-229.
Hg P Br has its special type of structure (3), Hg Sb Br
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and Hg As Cl crystallize in a cubic space group Pa3 and
REFERENCES
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are composed each of the R [Hg Z ] cationic nets and
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[HgX ] octahedral anions (X"Cl, Br; Z"As, Sb) (4, 5).
1. H. Rebbah and A. Rebbah, J. Solid State Chem. 113, 1 (1994).
2. A. Rebbah, J. Yazbeck, and A. Deschanvres, Rev. Chim. Miner. 17, 96
(1980).
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They would be isotypical with (I) if the latter had an addi-
tional metal atom in a 4b position (15). In mercury contain-
Shevelkov, Andrei V.
