Structure and bonding of Bi4Ir: A difficult-to-access bismuth iridide with a unique framework structure

Article abstract of DOI:10.1021/ic502205k

Crystals of Bi₄Ir, a new intermetallic compound, were obtained by the reaction of an iridium-containing intermetallic precursor with liquid bismuth. X-ray diffraction on a single crystal revealed a rhombohedral structure [R3m, a = 2656.7(2) pm, and c = 701.6(4) pm]. Bi₄Ir is not isostructural to Bi₄Rh but combines motifs of the metastable superconductor Bi₁₄Rh₃ with those found in the weak topological insulator Bi₁₄Rh₃I₉. The two crystallographically independent iridium sites in Bi₄Ir have square-prismatic and skewed-square-antiprismatic bismuth coordination with Bi-Ir distances of 283-287 pm. By sharing common edges, the two types of [IrBi₈] units constitute a complex three-dimensional network of rings and helices. The bonding in the heterometallic framework is dominated by pairwise Bi-Ir interactions. In addition, three-center bonds are found in the bismuth triangles formed by adjacent [IrBi₈] polyhedra. Density functional theory based band-structure calculations suggest metallic properties.

Full text of DOI:10.1021/ic502205k

Article
pubs.acs.org/IC
Structure and Bonding of Bi Ir: A Difficult-to-Access Bismuth Iridide
4
with a Unique Framework Structure
†
,†,‡
§
§
,§
Anna Isaeva, Michael Ruck,* Konrad Schaf
̈
̈
er, Ute Ch. Rodewald, and Rainer Pottgen*
†
‡
§
Department of Chemistry and Food Chemistry, Technische Universitat
Max Planck Institute for Chemical Physics of Solids, Nothnitzer Straße 40, 01187 Dresden, Germany
Institut fur Anorganische und Analytische Chemie, Universitat Munster, Corrensstraße 36, 48149 Mu
S Supporting Information
̈
Dresden, 01062 Dresden, Germany
̈
̈
̈
̈
̈
nster, Germany
*
ABSTRACT: Crystals of Bi Ir, a new intermetallic compound, were
4
obtained by the reaction of an iridium-containing intermetallic
precursor with liquid bismuth. X-ray diffraction on a single crystal
revealed a rhombohedral structure [R3
̅
m, a = 2656.7(2) pm, and c =
7
01.6(4) pm]. Bi Ir is not isostructural to Bi Rh but combines motifs
4
4
of the metastable superconductor Bi Rh with those found in the weak
1
4
3
topological insulator Bi Rh I . The two crystallographically independ-
1
4
3 9
ent iridium sites in Bi Ir have square-prismatic and skewed-square-
4
antiprismatic bismuth coordination with Bi−Ir distances of 283−287
pm. By sharing common edges, the two types of [IrBi ] units constitute
8
a complex three-dimensional network of rings and helices. The
bonding in the heterometallic framework is dominated by pairwise Bi−
Ir interactions. In addition, three-center bonds are found in the
bismuth triangles formed by adjacent [IrBi ] polyhedra. Density
8
functional theory based band-structure calculations suggest metallic properties.
INTRODUCTION
ternary antimonides, we have repeatedly obtained crystals of
binary bismuth−iridium phases as byproducts. The crystal
structure and chemical-bonding peculiarities of the hitherto
■
The synthesis of bismuth-rich intermetallic compounds of high-
melting transition metals is strongly hampered by the dramatic
differences between the boiling point of bismuth (1883 K) and
unknown bismuth-richest phase, Bi Ir, are reported herein.
4
the much higher melting points of the transition metals, e.g.,
1
EXPERIMENTAL SECTION
2683 K for iridium or 2239 K for rhodium. Only small
■
amounts of rhodium and iridium are soluble in liquid bismuth,
and flux synthesis is slow. Early investigations of the Bi−Ir
system by melt reactions and subsequent annealing yielded the
Synthesis. Single crystals of Bi Ir are accessible from the reaction
4
of an iridium-containing intermetallic precursor with liquid bismuth. In
a typical experiment, a sample of the nominal composition Sm Ir Sb
3
3
7
1
5
2
,3
was synthesized by arc-melting from the elements [samarium pieces
Smart Elements), iridium powder (Allgemeine Pforzheim), and
antimony lumps (Johnson Matthey), all with purities >99.9%] under
00 mbar of argon (purified with titanium sponge at 900 K, with silica
gel, and with molecular sieves). The crushed Sm Ir Sb regulus was
binary phases Bi Ir and Bi Ir. The structure types β-Bi Rh
2
3
2
(
and Bi Ni were assigned on the basis of Debye−Scherrer
3
4
diffractograms. Subsequent studies by Kjekshus could not
7
reproduce Bi Ir either by annealing experiments or by chemical-
2
3
3
7
transport reactions. Phase-pure Bi Ir nanoparticles were only
2
subsequently mixed with 40 mol equiv of bismuth shots (ABCR;
>99.9%). The mixture was placed in a corundum crucible, which
afterward was sealed in an evacuated silica tube. The ampule was
rapidly heated up to 1370 K in a muffle furnace and kept at that
temperature for 1 week. Then the temperature was decreased to 1070
K by radiative heat loss, and the sample was annealed at that
temperature for another 1 week, followed by slow cooling to room
temperature within 60 h. The bismuth excess was slowly dissolved in
an equimolar solution of H O (Acros; 35%) and glacial acetic acid
5
recently synthesized via a microwave-assisted polyol process.
Bi Ir is a weakly paramagnetic semimetal. Bi Ir nanoparticles
2
3
6
can be obtained by the same technique; these nanoparticles
reversibly intercalate oxygen up to the composition Bi IrO .
7
3
2
The latter material appears to be a unique metallic oxide ion
conductor and is also the first one that operates at room
temperature.
2
2
Single crystals of Bi (Ir0.77Cu0.17Ni ) were recently
3
0.06
(VWR International). The resultant sample was washed with
demineralized water. The reaction products were SmSb, elemental
7
obtained from intermetallic precursor compounds. In metal
flux synthesis, the broadly applied approach to intermetal-
8
−10
lics,
bismuth has only scarcely been used for the growth of
Special Issue: To Honor the Memory of Prof. John D. Corbett
Received: September 11, 2014
phosphides and germanides. Recent examples include
1
1
12
13
14
Lu Ir P , ScRh P , CeRh Ge , and Ce Rh Ge . In the
3
7
5
6
4
6
4
2
3
5
pursuit of expanding our explorative bismuth flux technique to
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic502205k | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
iridium, Bi Ir, and Bi Ir as well as thin fibers of a samarium−iridium
3
4
Table 1. Crystal Data and Structure Refinement for Bi Ir
4
antimonide−bismuthide with an approximate composition SmIrSb Bi
5
2
16
[
energy-dispersive X-ray (EDX) data]. Various phases were easily
empirical formula
cryst syst
Bi
trigonal, rhombohedral
R3
4
Ir
separated from each other mechanically based on their crystal habits.
Certainly, the synthesis of Bi Ir was not targeted in the way
space group
̅
m
4
presented. We repeatedly observed Bi Ir from bismuth flux experi-
4
Pearson symbol
fw, g mol−1
hR135
ments on related ternary antimonides. These data underline that Bi Ir
4
1028.1
is achievable and reproducible through this technique. This does,
however, not mean that such a strategy can generally by applied to
related binaries.
unit cell dimensions, pm
cell volume, nm3
a = 2656.7(2), c = 701.6(4)
V = 4.2881
Z = 27
formula units per cell
calcd density, g cm−
diffractometer type
EDX Analyses. The composition of the Bi Ir crystals was analyzed
4
3
10.75
by EDX using a ZEISS EVO MA10 scanning electron microscope in
variable pressure mode. Elemental iridium and bismuth were used as
standards. No impurity elements heavier than sodium (detection limit
of the instrument) were observed. The crystals exhibited the
composition of 80 ± 1 atom % bismuth and 20 ± 1 atom % iridium,
IPDS-II
detector distance, mm
exposure time, min
ω range; increment, deg
60
3
0−180, 1.0°
13.3, 3.1, 0.013
20 × 20 × 60
131.2
in excellent agreement with the proposed formula Bi Ir.
4
integration param (A, B, EMS)
Single-Crystal X-ray Diffraction. Small block-shaped single
cryst size, μm3
crystals of Bi Ir were isolated from the flux by mechanical
abs coeff, mm−1
4
fragmentation. In order to check the crystal quality, Laue photographs
of the crystals were collected on a Buerger precession camera (white
molybdenum radiation, imaging plate technique, and Fujifilm BAS-
transmn ratio (max/min)
F(000), e
3.54
11043
θ range for data collection, deg
range in hkl
3−30
±36, ±36, ±8
1
800). Intensity data were collected at room temperature using a Stoe
IPDS-II imaging plate diffractometer (graphite-monochromatized Mo
Kα radiation; λ = 71.073 pm) in the oscillation mode. Numerical
total no. of reflns
5339
−1
absorption correction (μ = 131.2 mm ) was applied to the data set.
Lattice metrics and systematic reflection conditions first suggested the
monoclinic space groups C2/m, Cm, and C2. The centrosymmetric
group was chosen for the first structure refinement. The starting
atomic parameters were found by means of the SUPERFLIP
indep reflns/Rint
959/0.088
477/0.120
959/41
reflns with I ≥ 2σ(I)/Rσ
no. of data/param
GOF on F2
0.74
1
7
18
R1/wR2 for I > 2σ(I)
0.028/0.044
0.058/0.092
38(8) × 10−
5.84/−5.17
program, implemented in the JANA2006 package. Critical
inspection of the resultant structure model gave strong hints for
higher, presumably rhombohedral, symmetry. Evaluation of the
R1/wR2 (all data)
5
extinction coeff
largest diff peak/hole (e Å−3)
19
monoclinic structural model with the PLATON routine readily led
to the correct space group, R3m. Examples of overlooked trigonal
symmetry are well-known for diverse structures. The setting
̅
2
0
obtained for Bi Ir from PLATON was subsequently standardized
4
2
1,22
with the TIDY routine,
and the structure refinement has smoothly
converged to the residuals listed in Table 1. The final positional
parameters and interatomic distances are listed in Tables 2 and 3. All
sites are fully occupied. Further details on the structure determination
are available from Fachinformationszentrum Karlsruhe, D-76344
Eggenstein-Leopoldshafen, Germany, by quoting CSD 428458.
Quantum-Chemical Calculations. Scalar-relativistic and fully
relativistic density functional theory (DFT)-based calculations were
performed using the full-potential linearized augmented plane wave
23
(
LAPW) method within the local density approximation. Chemical
bonding was characterized via topological analysis of the real-space
σ
24,25
electron localizability indicator (ELI-D, γ )
that was performed in
D
26
the DGrid program package and visualized with the Paraview
software. Formal atomic charges were calculated via integration of
27
the electron density (ρ) in basins according to the quantum theory of
28
atoms in molecules (QTAIM) developed by Bader.
RESULTS AND DISCUSSION
■
Synthesis and Structure. The fundamental obstacles that
hamper direct crystal growth of bismuth−iridium phases have
been ameliorated by activating iridium via a prereaction with
antimony and samarium with the subsequent addition of
bismuth to the system. The Bi Ir crystals are one of the reaction
the aforementioned cases, the local connectivity is the same.
Each polyhedron is linked to four others, defining two trigonal
prisms of bismuth atoms; i.e., there are two trigonal prisms per
three polyhedra.
4
products (see the Experimental Section) that grew out of the
quaternary reaction mixture. The formation of byproducts
cannot be avoided, and pure-phase samples cannot be
synthesized by this method.
The polyhedral network of Bi Ir combines structural motifs
4
As by now, Bi Ir represents the bismuth-richest phase in the
that resemble those in the topological insulator Bi Rh I (six-
membered rings of polyhedra stacked along 0, 0, z) with those
4
14
3 9
2
9,30
Bi−Ir system. Bi Ir is not isostructural to Bi Rh
but
crystallizes in a novel rhombohedral structure type with 18
formula units per unit cell [R3m, a = 2656.7(2) pm, and c =
4
4
found in the superconductor Bi Rh (helices of alternating
14
3
1
1
̅
cubes and antiprisms around / , / , z and, with opposite
3 3
B
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Inorganic Chemistry
Table 2. Atomic Coordinates and Anisotropic Displacement Parameters (pm ) for Bi Ir
Article
2
a
4
atom
site
x
y
z
U11
U22
U33
U12
U13
U23
Ueq
Bi1
Bi2
Bi3
Bi4
Ir1
36i
36i
18h
18h
18g
9e
0.06061(4)
0.13027(5)
0.60481(3)
0.75189(3)
0.16937(5)
1/2
0.24008(4)
0.18365(13)
0.03919(15)
0.0325(2)
0.08165(19)
1/2
133(4)
229(5)
129(4)
177(5)
123(4)
131(4)
106(4)
U11
147(4)
195(5)
179(7)
131(7)
151(7)
140(9)
49(4)
60(4)
53(5)
97(6)
57(3)
35(4)
−6(4)
−80(4)
−14(3)
6(3)
20(4)
−17(4)
−U13
−U13
−10(5)
145(3)
187(4)
151(4)
158(5)
130(4)
122(6)
0.38324(4)
−x
−x
0
U11
114(6)
71(8)
−5(3)
−2(3)
Ir2
0
0
135(6)
−5(7)
a
2
2
The anisotropic displacement factor exponent takes the form −2π [(ha*) U + ... + 2hka*b*U ]. U = U23 = 0.
11
12
12
a
2
2
Table 3. Interatomic Distances (pm) in Bi Ir
chirality, around / , / , z). In contrast to Bi Rh X (X = Bi,
3 3 12 3 2
Cl, and Br),
4
3
1−33
thorough analysis of the residual electron
Bi1−Ir1 (×1)
Bi1−Ir2 (×1)
Bi1−Bi1 (×1)
Bi1−Bi3 (×1)
Bi1−Bi2 (×1)
Bi1−Bi2 (×1)
Bi1−Bi2 (×1)
Bi1−Bi4 (×1)
Bi1−Bi1 (×1)
Bi2−Ir1 (×1)
Bi2−Ir2 (×1)
Bi2−Bi4 (×1)
Bi2−Bi2 (×1)
Bi2−Bi1 (×1)
Bi2−Bi1 (×1)
Bi2−Bi1 (×1)
Bi2−Bi2 (×1)
283.2
286.3
315.8
321.5
327.0
344.7
361.5
379.3
379.7
284.1
286.9
318.3
326.0
327.0
344.7
361.5
369.0
Bi3−Ir1 (×2)
Bi3−Bi1 (×2)
Bi3−Bi4 (×1)
Bi3−Bi4 (×2)
Bi4−Ir1 (×2)
Bi4−Bi2 (×2)
Bi4−Bi3 (×1)
Bi4−Bi3 (×2)
Bi4−Bi1 (×2)
Ir1−Bi1 (×2)
Ir1−Bi2 (×2)
Ir1−Bi4 (×2)
Ir1−Bi3 (×2)
Ir2−Bi1 (×4)
Ir2−Bi2 (×4)
284.9
321.5
331.8
352.7
284.6
318.3
331.8
352.7
379.3
283.2
284.1
284.6
284.9
286.3
286.9
density inside the helices of the Bi Ir structure does not reveal
any additional “interstitial” atoms X on their central axes.
4
Likewise, the hexagonal prisms defined by six [IrBi ] cubes are
8
not centered by isolated atoms, as has been found in the above-
mentioned layered subiodides. Consistently, the inner diame-
ters of the unfilled structural motifs in Bi Ir are about 50−100
4
pm smaller. As in all of the other structures, the lone pairs of
the bismuth atoms, which decorate the surface of the
framework, are pointing into these voids (cf. Figure 3 and
the discussion below).
a
All distances within the first coordination spheres are listed. Standard
deviations are all equal or less than 0.2 pm.
Figure 3. Fragment of the Bi Ir structure with both types of iridium
4
coordination polyhedra and trigonal prisms of bismuth atoms
connecting them. Thicker lines signify the strongest bonds in the
framework. ELI-D localization domains given at γ = 1.01 [1, pairwise
Bi−Ir bonds (0.8−0.9 electrons); 2, three-center bond slightly off the
base of the trigonal prism formed by bismuth atoms (0.4 electrons)]
and at γ = 1.08 [3, Bi 6s lone pairs (1.6−2.4 electrons)].
Figure 1. View of the Bi Ir structure along the c axis. The edge-sharing
Electronic Structure and Chemical Bonding. The
slightly higher electronegativity of iridium in comparison to
rhodium, as evidenced by the estimated values of the ionization
4
[
IrBi8/2] cubes (Ir2, yellow) and square antiprisms (Ir1, blue) are
emphasized.
38
potentials, suggests that the iridium atom in Bi Ir should bear
4
a larger negative charge than the rhodium atom in Bi Rh. The
4
higher electronic withdrawal toward iridium, opposite to
rhodium, is further confirmed by formal atomic charges
(
calculated according to the QTAIM): 1.0− for iridium and
0
.2+ to 0.3+ for bismuth in Bi Ir but 0.7− for rhodium and 0.3+
4
34
to 0.35+ for bismuth in Bi Rh I .
14
3 9
Apart from monosynaptic basins, which correspond to
bismuth lone pairs and closed shells or the iridium penultimate
and core shells, there exist two types of ELI-D maxima that
designate bonding interactions: (a) eight pairwise Bi−Ir bonds
per polyhedron; (b) three-center Bi−Bi bonds that reside in the
upper and lower bases of every trigonal prism defined by edge-
Figure 2. Cutout of the Bi Ir structure with edge-sharing [IrBi8/2]
4
cubes (Ir2, yellow) and square antiprisms (Ir1, blue), emphasizing the
mixed helix (central axis) and the attached six-membered rings of the
antiprisms.
sharing [IrBi ] polyhedra (Figure 3). The latter bonds manifest
8
themselves also through the shorter Bi−Bi distances in the
prisms (ca. 3.27 Å), as opposed to the other Bi−Bi contacts
C
dx.doi.org/10.1021/ic502205k | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
within the framework (3.4−3.6 Å). The bonding pattern
CONCLUSION
■
34
strongly resembles that of Bi Rh I , in which the respective
1
4
3 9
The bismuth-rich intermetallic compound Bi Ir is accessible in
2
8
4
bonding basins comprise 1.1 and 0.8 electrons. The
the form of block-shaped single crystals from bismuth flux
reactions of different quaternary Sm−Ir−Sb−Bi starting
compositions. The basic building units of the Bi Ir structure
are square-prismatic and skewed-square-antiprismatic [IrBi8]
units with Ir−Bi distances of 283−287 pm, which condense to a
complex three-dimensional network that is dominated by
strong Ir−Bi bonding.
corresponding electron count for Bi Ir yields about 0.8−0.9
4
and 0.4 electrons, respectively.
4
The conductivity pathway provided by the bonding network
propagates along the entire framework and thus accounts for
metallic properties predicted by the DFT-based calculations
(Figures 4 and 5). Strongly mixed Ir d and Bi p states provide
ASSOCIATED CONTENT
■
*
S
Supporting Information
Crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
E-mail: michael.ruck@tu-dresden.de.
E-mail: pottgen@uni-muenster.de.
*
*
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
Figure 4. Band structure of Bi Ir in the scalar-relativistic
approximation. The Fermi level resides at 0 eV.
4
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Deutsche Forschungsgemein-
schaft.
DEDICATION
■
In memory of John Corbett.
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