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T-Shaped Nets of Antimony Atoms in the Binary Antimonide Hf5Sb9.

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Communications
Solid-State Structures
T-Shaped Nets of Antimony Atoms in the Binary
Antimonide Hf5Sb9**
Abdeljalil Assoud, Katja M. Kleinke, Navid Soheilnia,
and Holger Kleinke*
Regular planar square nets of antimony atoms occur in many
antimonides, for example, in the HfCuSi2[1–6] and SmSb2
types.[7] Less common types with antimony square nets
include LnGaSb2 (Ln = rare-earth element),[8] LnIn1xSb2[9]
and LaMSb3 (M = transition metal).[10, 11] Such square nets of
main-group elements have an ideal valence-electron count of
six, for example, formal Sb atoms, with four so-called half
(“hypervalent” one-electron) bonds per atom of roughly 300–
310 pm.[12, 13] These nets are prone to undergo Peierls distortions, for example to cis–trans chains (found in GdPS with
two single bonds per P atom[14]) or zigzag chains (in CeAsS
with two AsAs single bonds[15] and in CeSbTe with two Sb
Sb bonds[16]). Several defect variants of the ZrSiS type
(isopointal (pseudoisostructural) with PbFCl) are known,
including large commensurately modified superstructures, for
example, Gd8Se15, a 24-fold superstructure of the ZrSiS
type,[17] and GdS2x, a 144-fold superstructure.[18] Furthermore, incommensurately modified superstructures may form
as well, for example, in tellurides (recently reviewed by
Kanatzidis et al.[19]).
Our interest in nonclassical SbSb bonding of Group 4
antimonides dates back to 1998, when we found linear chains
of Sb atom in a metal-rich antimonide.[20] Our recent
[*] Dr. A. Assoud, K. M. Kleinke, N. Soheilnia, Prof. H. Kleinke
Department for Chemistry
University of Waterloo
Waterloo, Ontario, N2L 3G1 (Canada)
Fax: (+ 1) 519-746-0435
E-mail: kleinke@uwaterloo.ca
[**] We are indebted to the Natural Sciences and Engineering Research
Council of Canada for financial support.
5260
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
discovery, that the so-called “b-ZrSb2” with planar “Sb”
ribbons is actually a ternary silicide–antimonide,[46] motivated
us to turn our attention to orthorhombic HfSb2[21] and its
tetragonal high-temperature form, for which to date only the
lattice dimensions are known.[22] In Pearson=s handbook,[23]
the tetragonal high-temperature form is assigned to the Cu2Sb
type, isopointal with ZrSiS/PbFCl. If this is the case then the
Sb atoms would form an undistorted planar square net with
interatomic distances of 277 pm. This is unreasonably short
for SbSb half (“hypervalent”) bonds, which are typically
between 300 and 310 pm, as found in USb2 (302 pm)[24] and
ThSb2 (307 pm),[25] both forming the ZrSiS type. Moreover,
the density of the high-temperature hafnium–antimonide
would be about 6 % higher than that of the low-temperature
form of HfSb2, if the stoichiometry were indeed HfSb2.
Therefore we decided to study its crystal structure, expecting
both deficiencies and deviations from the ideal geometry.
We prepared orthorhombic HfSb2 by annealing elemental
hafnium and antimony in the stoichiometric 1:2 ratio at 650 8C
as described elsewhere.[26] Next, we performed a temperaturedependent combined differential scanning calorimetry (DSC)
and thermogravimetric (TG) measurement between room
temperature and 1080 8C.[27] An endothermic reaction was
observed during the heating process at 1020 8C. No reaction
was visible during the cooling process. Assuming that the
observed weight loss came exclusively from evaporation of
antimony, we calculated that 5.5 % of the antimony was lost in
the process. A powder diffraction diagram of the product
(INEL powder diffractometer) showed no trace of the
orthorhombic HfSb2 ; the experimentally obtained diagram
strongly resembled the postulated one of HfSb2 in the ZrSiS
type, accompanied by traces of elemental antimony.
To obtain single crystals of our target compound, we
placed the elements in the Hf:Sb ratio of 1:4 (using Sb as a
reactive flux) into a small ceramic crucible, placed it in a fused
silica tube, and sealed the tube under dynamic vacuum of
103 mbar. The tube was heated in a resistance furnace under
dynamic vacuum at 1075 8C for 32 h, and then cooled to
500 8C at a rate of 5 8C min1, that is, within two hours.
Thereafter, the furnace was switched off to maximize the
cooling rate. Small beads of excess antimony were found at
the top of the tube, while the ceramic crucible at the bottom of
the tube contained the majority of the sample, mostly small
black platelike crystals. A powder diffraction diagram of
these crystals showed almost exclusively reflections belonging
to tetragonal “HfSb2”. Hence, the cooling rate was fast
enough to prevent the formation of orthorhombic HfSb2,
which is thermodynamically preferred below 1020 8C in the
presence of excess antimony. Qualitatively the same results,
albeit with worse crystal quality, are obtained by arc-melting
HfSb2 under Argon. Energy dispersive spectroscopy (EDS)[28]
analyses on selected crystals of the tube reaction using
orthorhombic HfSb2 as a standard gave an Hf:Sb ratio of
37(1):63 atomic percent (at %).
The single-crystal structure study performed on a platelike crystal revealed that the so-called high-temperature
p p p
“HfSb2” is actually Hf5Sb9, which is a 5 I 5 I 1 superstructure of the originally reported cell,[29] hence a new
structure type. The Hf:Sb ratio (36:64 in at %) concurs well
DOI: 10.1002/anie.200460488
Angew. Chem. Int. Ed. 2004, 43, 5260 –5262
Angewandte
Chemie
with the EDS results, and is quite similar to the Zr:Sb ratio in
Zr11Sb18 (38:62) which contains a 3D antimony network.[30]
The projection of the crystal structure along [100] confirms
that the Hf5Sb9 structure is a distorted (deficient) superstructure variant of the ZrSiS type (Figure 1).
Figure 3. Antimony nets of Hf5Sb9 projected along [001] formed exclusively by the Sb3 atoms (large square: unit cell). Left: hypothetical
undistorted square net (dashed square = ZrSiS unit cell); right: experimentally observed T-shaped net. Every fifth Sb positions is unoccupied
(open circles in the left part, arrows indicate change in atom positions
required to generate the observed net from the hypothetical net).
Figure 1. Projection of the Hf5Sb9 structure along [010]. Small, white
circles = Hf; large, gray circles = Sb. The numbers indicate the atomic
labels (Figure 2).
Of the two symmetry-independent hafnium sites, one
(Hf1) is coordinated by nine antimony atoms, and the second
(Hf2) by eight (noting that the multiplicities of Hf1 and Hf2
are two and eight, respectively), whereas the coordination
number of the M atoms in the ZrSiS type is nine. Both
coordination polyhedra are best described as distorted monocapped square antiprisms, in the case of Hf2 one corner is
unoccupied (Figure 2). All these HfSb bonds of Hf5Sb9 (289–
Figure 2. Hf1Sb9 (left) and Hf2Sb8 (right) polyhedra.
319 pm) are comparable to those of HfSb2 (293–339 pm), in
which the coordination numbers of the Hf atoms are eight and
nine as well.
The most interesting structural feature is the unique Tshaped net formed by the Sb3 atoms, in which each atom has
the T-shaped coordination environment found in the molecular interhalides BrF3 and ClF3. A significant deviation from
the orthogonal T geometry is apparent, with angles of 70.58,
908, and 160.58 (instead of 2 I 908 and 1808). As pointed out in
a recent overview of T nets given by Hoffmann et al., this net,
albeit the simplest 2D net of this family, was not realized
before.[31] Figure 3 shows how the observed Sb3 net is related
to the undistorted square net: after removing every fifth
atom, the four atoms closest to the resulting hole are shifted
Angew. Chem. Int. Ed. 2004, 43, 5260 –5262
slightly towards each other, and the four next-nearest shifted
away from each other. This results in three SbSb contacts of
299 and 2 I 303 pm per Sb3 atom, instead of four times 277 pm
in the hypothetical undistorted fully occupied net.
Overall this net is composed of undistorted square (Sb3)4
planes of ideal D4h symmetry, which are interconnected by
four Sb3Sb3 bonds per square to the neighboring squares.
The neighboring squares are shifted by 3.6 pm relative to each
other, so that the Sb3 layer is slightly puckered. To date,
antimony atoms in a T-coordination environment were only
found in 1D substructures, that is, ladders as in FeSb2,[32]
MoSb2S,[33] and (Zr,Ti)Sb.[34] A 2D net with Te atoms in a Tcoordination environment is found in Cs3Te22[35] (and
Cs4Te28[36]) in which the Te4 squares are interconnected by
an additional bridging, linearly coordinated Te atom. This
arrangement results in the formation of Te12 squares, each
with one Cs atom in its center. The TeTe bonds in Cs3Te22 of
300–308 pm are comparable to the SbSb bonds in Hf5Sb9 (Sb
and Te atoms have almost the same covalent radii (139 pm vs.
137 pm)).[37] Because of the unique nature of the CsTe6 net, its
electronic structure was studied in detail both by Extended
HKckel[38] and by LMTO[39, 40] calculations.
Our LMTO studies on Hf5Sb9 and its hypothetical
undistorted variant with the vacancies as indicated in
Figure 3 (left), showed that the experimentally detected
distortion leads to a lower total energy of 2.1 eV per unit
cell. The densities of states (DOS, Figure 4, left) indicate
Figure 4. Densities of states (left) and Sb3Sb3 COHP curves (right)
of Hf5Sb9.
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5261
Communications
metallic properties, with the Fermi level (EF) lying close to a
local nonzero minimum. The area below EF is dominated by
the antimony contributions.
The crystal orbital Hamilton population curves
(COHP)[41] of the two different Sb3Sb3 interactions are
shown in Figure 4(right). The integrated COHP values
(ICOHPs) of 0.72 and 0.71 eV per bond reveal overall
bonding character, significantly weaker than expected for an
SbSb single 2-electron 2-center bond. The shortest SbSb
bond in Zr11Sb18 (311 pm in an anchorlike Sb6 unit) has an
ICOHP of 0.41 eV, and the shortest SbSb bond in ZrSb2
(289 pm, in a 2D strip), which is classified as a regular single
bond,[42, 43] has an ICOHP of 1.21 eV.[30] The fact that
antibonding Sb3Sb3 states start to become filled at
1.5 eV below EF indicates that the Sb3 net comprises more
than the ideal number of valence electrons. Both curves show
a striking qualitative resemblance to the corresponding curves
for the TeTe bonds of Cs3Te22.[39]
A detailed electronic structure investigation of the T net
of Sb3 atoms of Hf5Sb9, accompanied by temperaturedependent physical property measurements, will show
whether this unique net of main-group elements undergoes
a Peierls distortion at lower temperatures. Moreover, what is
to be expected upon changing the valence-electron concentration? Thus far, three different binary Group 4 antimonides
with M:Sb ratios between 1:1 and 1:2 are known, namely
Ti5Sb8,[44] Zr11Sb18, and Hf5Sb9. Ti5Sb8 can accommodate the
other Group 4 elements (both Zr and Hf to large extents),[45]
while V atoms can be incorporated in Zr11Sb18.[30] We will
therefore study the possibilities (and consequences) of
replacing part of Hf with M = Ti, Zr, V, Nb, Mo, as well as
Sb with Se and Te, in Hf5Sb9.
Received: April 28, 2004
.
Keywords: antimony · electronic structure · hafnium ·
solid-state structures
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Angew. Chem. Int. Ed. 2004, 43, 5260 –5262
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