Phase relations and structural properties of the ternary narrow gap semiconductors Zn5Sb4In2-δ (δ=0.15) and Zn9Sb6In2

Article abstract of DOI:10.1016/j.jssc.2010.05.001

A systematic study of the Zn-rich corner of the ternary system Zn-Sb-In revealed the presence of two ternary compounds: stable Zn₅Sb₄In_2-δ (δ=0.15) and metastable Zn₉Sb₆In₂ with closely related crystal structures. Their common motif is a tetragonal basic structure of 3²434 nets formed by the Sb atoms. The nets are stacked in antiposition to yield layers of square antiprisms sharing edges plus intervening tetracapped tetrahedra (tetreadersterns). The majority of Zn atoms occupy peripheral tetrahedra of such tetraedersterns, which produces frameworks with a composition ZnSb . These frameworks represent orthorhombic superstructures: (2×1×1) for Zn₅Sb₄In_2-δ (Z=4) and (2×3×1) for Zn₉Sb₆In₂ (Z=8) with respect to the tetragonal arrangement of Sb atoms. The In and remaining Zn atoms are distributed in the channels formed by the square antiprisms. Phase relations in the Zn-Sb-In system are complex. Crystals of metastable Zn₉Sb₆In₂ are regularly intergrown with various amounts of Zn₅Sb₄In_2-δ. Additionally, a monoclinic variant to orthorhombic Zn₉Sb₆In₂ could be identified. Zn₉Sb₆In₂ decomposes exothermically into a mixture of Zn₅Sb₄In_2-δ, Zn₄Sb₃ and elemental Zn at around 480 K. Both Zn₅Sb₄In_2-δ and Zn₉Sb₆In₂ are poor metals with resistivity values that are characteristic of heavily doped or degenerate semiconductors (0.2-3 m Ω cm at room temperature).

Full text of DOI:10.1016/j.jssc.2010.05.001

ARTICLE IN PRESS
Journal of Solid State Chemistry 183 (2010) 1574–1581
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Phase relations and structural properties of the ternary narrow gap
semiconductors Zn Sb In
(
d
b
¼0.15) and Zn Sb In
5
4
2ꢀ
d
9
6
2
a
b
a,n
Yang Wu , Andreas Teng a˚ , Sven Lidin , Ulrich H a¨ ussermann
a
Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA
b
Inorganic Chemistry, Stockholm University, SE-10691 Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
A systematic study of the Zn-rich corner of the ternary system Zn–Sb–In revealed the presence of two
ternary compounds: stable Zn Sb In
Received 12 February 2010
Received in revised form
d
(d¼0.15) and metastable Zn Sb In with closely related crystal
2
5
4
2ꢀ
9
6
2
structures. Their common motif is a tetragonal basic structure of 3 434 nets formed by the Sb atoms.
The nets are stacked in antiposition to yield layers of square antiprisms sharing edges plus intervening
tetracapped tetrahedra (tetreadersterns). The majority of Zn atoms occupy peripheral tetrahedra of
such tetraedersterns, which produces frameworks with a composition ‘‘ZnSb’’. These frameworks
2
7 April 2010
Accepted 2 May 2010
Available online 6 May 2010
Keywords:
represent orthorhombic superstructures: (2 ꢁ 1 ꢁ 1) for Zn
Z¼8) with respect to the tetragonal arrangement of Sb atoms. The In and remaining Zn atoms are
distributed in the channels formed by the square antiprisms. Phase relations in the Zn–Sb–In system are
complex. Crystals of metastable Zn Sb In are regularly intergrown with various amounts of
. Additionally, a monoclinic variant to orthorhombic Zn
decomposes exothermically into a mixture of Zn Sb In2ꢀ , Zn
Sb In2ꢀ and Zn Sb In
characteristic of heavily doped or degenerate semiconductors (0.2ꢀ3 m
2010 Elsevier Inc. All rights reserved.
5
Sb
4
In2ꢀ
d
(Z¼4) and (2 ꢁ 3 ꢁ 1) for Zn
9 6 2
Sb In
Zinc antimonides
Narrow gap semiconductors
Superstructures
(
9
6
2
Zn
Zn
5
Sb
Sb
4
In2ꢀ
In
d
9
Sb
6
In
2
could be identified.
and elemental Zn at
9
6
2
5
4
d
4
Sb
3
around 480 K. Both Zn
5
4
d
9
6
2
are poor metals with resistivity values that are
cm at room temperature).
O
&
1
. Introduction
of zinc and cadmium antimonides while maintaining a weak
polarity in the ternary compounds. Recently we reported on a
Binary zinc and cadmium antimonides are capable of remark-
ternary derivative of Cd
indium appears to replace
divalent cadmium [10]. This results in a framework structure
which in contrast with Cd Sb does not contain interstitial Cd
4
Sb
3
, Cd13ꢀxIn
y
Sb10, where trivalent
able structural complexity which expresses itself in large, some-
times even giant unit cells and various forms of disorder [1–5].
Because of the low electronegativity difference between the
divalent metal and antimony these structures represent weakly
polar frameworks. Characteristically constituting atoms have
coordination numbers larger than four while average valence
electron concentrations are lower than four. Nevertheless, such
frameworks realize a narrow band gap in the electronic structure at
or close to the Fermi level [6,7]. The combination of structural
complexity, weak polarity and a narrow band gap gives zinc and
cadmium antimonides interesting thermoelectric properties which
have been intensively investigated over the recent years [8].
The peculiar structural, bonding and electronic properties of
zinc and cadmium antimonides are not easily maintained in
ternary systems. For example, in conjunction with electropositive
metals (alkali, alkaline earth and rare earth) typically Zn/Cd and
Sb atoms form commonly polyanions in now rather polar Zintl
phases [9]. Reactions with indium, however, provide an interest-
ing possibility to modify the electron poor framework structures
a
considerable concentration of
4
3
atoms [5]. Similar attempts in the Zn–Sb–In system led to a
different result. Contrary to the Cd–Sb–In system, indium does
not produce a substitutional variant of a known zinc antimonide
structure, but segregates into distinct crystallographic positions in
new, ternary, crystal structures.
The exploration of the Zn–Sb–In system yielded hitherto two
5 4 d 9 6 2 5 4 d
new phases, Zn Sb In2ꢀ and Zn In In . Zn Sb In2ꢀ represents a
stable phase with highly interesting thermoelectric properties
[11]. Herein we report on the synthesis and structure of
metastable Zn
Zn Sb
9 6 2
Sb In and its phase and structural relation to
5
4
d
In2ꢀ .
2. Experimental section
2
.1. Synthesis and phase characterization
n
All samples for the synthesis were prepared in a dry-argon
Corresponding author.
E-mail address: Ulrich.Haussermann@asu.edu (U. H a¨ ussermann).
filled glove box. Pure Zn (granules, 99.99%), Sb (powder, 99.5%)
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.05.001

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Y. Wu et al. / Journal of Solid State Chemistry 183 (2010) 1574–1581
1575
˚
and In (ingot, 99.99%) were purchased from Sigma–Aldrich.
Powdered Sb was pressed into a pellet and all starting metals
were loaded into silica tubes specially prepared for flux synthesis
with monochromatic MoKa radiation (l¼0.71073 A) operated at
50 kV and 40 mA. The sample-to-detector distance was 50 mm.
Oxford Diffraction’s CrysAlis software was employed for data
reduction [13].
[
12]. Total sample masses varied between 0.5 and 1 g. Specific
sample compositions are compiled in Fig. 1 and discussed later.
The diffraction patterns of both the previously described room-
ꢀ
4
After flame sealing under vacuum (o10 mbar) ampoules
were placed into a well insulated stainless-steel container.
Reactions were carried out in a box furnace with a programmable
temperature controller. First, the temperature was raised to 923 K
and kept for 24 h to homogenize the metal mixture. Subsequently
the temperature was lowered to 598–748 K at rates between
5 4 d
temperature form of Zn Sb In2ꢀ [11] and the here presented
Zn
9
Sb
6
In
2
, can be interpreted as orthorhombic superstructures
˚
of a tetragonal, I-centered, basis lattice with atetE8.7 A and
tetE7.15 A. The room temperature structure of Zn
˚
c
5
Sb
4
In2ꢀ
d
has
space group symmetry Pbcn with aEctet, bE2 ꢁ atet, cEatet. For
Zn Sb In tet appears doubled and btet trebled. The symmetry of
the basic structure is I4/mcm. In orthorhombic setting this yields
the full set of symmetry operations (I (2,2 )/(c,b) (2,2 )/(c,a)
(2,2 )/(m,n)). Now, doubling or trebling an axis can never convert
9
6
2
a
2
4
and 5 K/h and the reaction mixture held at this temperature for
8 h. The actual holding temperature depended on the composi-
1
1
tion of the reaction mixture and was chosen high enough to
maintain flux conditions. This procedure resulted in mm-sized
specimens of agglomerated crystals which were separated from
the liquid excess metal mixture by centrifugation. The inter-
metallic products are air and moisture stable.
For phase analysis powder X-ray diffraction patterns were
collected on a Bruker D8 Advance diffractometer (CuKa radiation).
Thermal behavior was investigated by differential thermal
analysis (Shimadzu DTA-50). Samples constituting of crystal
pieces were sealed in stainless steel containers in the glove box.
The experiments were preformed under a flow of dry Ar in a
temperature interval from 300 to 830 K and with a temperature
increase rate of 5 K/min.
1
a glide operation to a reflection (while the reverse is possible), and
since there are no systematic absences for the h k 0, h 0 l or 0 k l
planes, the only possible reflection operation in the cell is the
mirror plane perpendicular to c. In orthorhombic symmetry, the
allowed combinations of two-fold axes and reflections are limited
to 2/m 2/m 2/m, mm2, m2m, 2 mm and 222, and hence, the lone
reflection perpendicular to c is disallowed as well. The orthor-
hombic space group must therefore be generated by a combina-
tion of 2-fold rotations and 2
1
screws. Inspection of the axial
reflections reveals systematic absences compatible with P2
1 1 1
2 2
and that was the space group used to solve the structure. To phase
the reflections, charge flipping [14], as implemented in the
program Superflip [15] was used. This yielded a set of atomic
positions that quickly refined to an acceptable agreement
2.2. Crystal structure characterization
2
between F data and model if twinning was properly modelled.
For this the program JANA2000 was employed [16]. The final
model treats all atomic positions with anisotropic thermal
Crystals suitable for single crystal X-ray diffraction were
obtained by crushing specimens obtained by flux synthesis.
Intensity data collection was performed at room temperature
displacement parameters. In Zn
stantial occupational deficiency for the In positions [11], and the
structural model for Zn Sb In was tested for In deficiency as well.
5 4 d
Sb In2ꢀ , we detected a sub-
(
295 K) on an Oxford Diffraction Xcalibur 3 CCD diffractometer
9
6
2
Allowing occupational deficiency of the In positions did not result
in any change of the overall composition, although two In
positions became over-occupied and the other two under-
occupied. The effect is slight, and the difference in fit between
model and data is negligible. Consequently, the structure was
treated as stoichiometric with all atomic positions fully occupied.
Table 1 lists a summary of the refinement results of orthorhombic
In
5
0
5
0
6
9 6 2
Zn Sb In and a monoclinic variant, which is addressed later on.
Details are supplied as supporting information, or can be obtained
from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-
Leopoldshafen, Germany (fax: +49 7247 808 666; E-mail:
crysdata@fiz-karlsruhe.de) on quoting the depository numbers
4
0
6
0
5
98
1
3
0
CSD-421504 (orthorhombic Zn
9
Sb
6
In
2
)
and CSD-421505
7
0
23
3
623
(
9 6 2
monoclinic Zn Sb In ).
8
0
673
723
20
6
23
2.3. Electrical resistivity measurements
9
0
673
698
10
7
23
748
The electrical resistivity of Zn Sb
9
6
In
2
was measured on a
Physical Property Measurement System (PPMS) from Quantum
Design using a four-point probe in-line configuration. Needle-
shaped crystals were selected from different synthesis batches
and gently polished to remove any excess metal residual on the
2
Zn
10
20
30
40
50
Sb
3
surface. Typical specimen dimensions were 0.5 ꢁ 0.5 ꢁ 5.0 mm .
5 4 d 9 6 2
Fig. 1. Synthesis diagram for Zn Sb In2ꢀ and Zn Sb In . Colored symbols demark
Gold bonding wires (0.03 mm) were painted on the sample and
electrodes on the resistivity puck using an air-drying silver paint
(Demetron D200) for contacts. The puck was subsequently loaded
into the PPMS chamber and then evacuated and sealed for the
measurement. A constant current of 1 mA was applied and the
resistance recorded from 10 to 350 and 350 to 10 K (one heating/
cooling cycle) at a rate of 1 K/min.
investigated compositional areas with temperatures inscribed that are necessary
for maintaining flux conditions. Product regions: 1—(red, upward triangles):
Zn
triangles): product mixture of Zn
—gray area: Zn Sb In upon quenching. Compositions on axes are in at%. The red
2
and gray stars mark the compound compositions for Zn Sb In2ꢀ and Zn Sb In ,
5
4
Sb In2ꢀ
d
4 3
with traces of InSb; 2—(blue, squares): Zn Sb ; 3—green (downward
5
Sb In2ꢀ and Zn Sb
4
d
4
3
upon radiant cooling;
3
9
6
2
5
4
d
9
6
respectively. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)

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Y. Wu et al. / Journal of Solid State Chemistry 183 (2010) 1574–1581
Table 1
Selected crystallographic data for the orthorhombic and monoclinic form of
Zn Sb In
virtually phase pure and in the form of mm-sized crystals.
Depending on synthesis temperature and composition the
product may contain small amounts of InSb. Fig. 2a shows
representatively the powder pattern of the product from the
synthesis mixture Zn50Sb20In30 annealed at 623 K.
9
6
2
.
Crystal data
Orthorhombic
Monoclinic
Formula
Zn
1548.6
P2
9
Sb
6
In
2
Zn
1548.6
P2 11
9
Sb
6
In
2
A second ternary compound, later established as Zn
Sb
6 2
In ,
9
Formula weight
Space group
a ( A˚ )
was first recognized in
a
reaction mixture Zn62.5Sb25In12.5
1
2
1
2
1
1
7
1
2
.142(2)
7.142
17.146
51.439
90
b ( A˚ )
7.146(5)
5.719(7)
c ( A˚ )
Beta (deg)
Z; V ( A˚ 3
calc (g cm
Temp (K)
(MoK
) ( A˚ )
Absorption coeff.
8; 3149.4(15)
16; 6299
)
ꢀ
3
D
)
6.53
293
6.53
293
0.71069
0.71069
l
a
26.347
26.347
ꢀ
1
(mm
)
F(000)
5391
10,784
o
min
–
o
max (deg)
2.38–28.37
4.43–26.89
Index ranges
ꢀ9rhr9, ꢀ22rkr22, ꢀ8rhr8, ꢀ18rkr21,
ꢀ
34rlr34
ꢀ63rlr54
25,164
Total reflns collected 21,287
Independent reflns
Refinement method
7183
17,525
Full-matrix least-squares Full-matrix least-squares
2
2
on F
7183/0/308
on F
17,525/0/224
Data/restraints/
params
Final R indices
R
1
¼0.0408, wR
2
2
¼0.0837
¼0.1028
R
1
¼0.1259, wR
2
2
¼0.1354
¼0.1393
a,b
[I43s(I)]
a,b
R indices (all data)
R
1
¼0.1885, wR
R
1
¼0.1611, wR
10
20
30
40
50
60
70
80
Largest diff. peak and 3.21, ꢀ3.11
7.40, ꢀ9.01
hole (e A˚
ꢀ3
)
2
GOF on F
1.6
3.85
a
R
1
¼99F
o
9ꢀ9F
c
99/99F
o
99.
b
2
2
2
2 2 1/2
) ]
wR
2
¼[w(F
o
ꢀF
c
) ]/[w(F
o
.
3
. Results and discussion
The Zn-rich corner of the ternary phase diagram was system-
atically investigated. Fig. 1 summarizes the applied reaction
mixtures used and corresponding temperatures where flux
conditions are maintained. (Note that synthesis temperatures
increased with increasing Sb concentrations.) Additionally the
products are indicated. With respect to reaction products one can
discern three regions: (1) High In concentrations yield the ternary
compound Zn
panying higher temperatures) favor the formation of Zn
3) A mixture of Zn Sb and Zn Sb is obtained in the
intermediate region.
Importantly, attempts to synthesize Zn
chiometric melts always produced a mixture of Zn
Zn Sb and InSb, which indicates that the ternary compound
melts incongruently. In contrast, flux synthesis with higher In
and Zn) concentrations (i.e. region (1)) yields Zn Sb
5
Sb
4
In2ꢀ
d
. (2) High Sb concentrations (and accom-
1
0
20
30
40
50
60
70
80
4
Sb .
3
(
5
4
In2ꢀ
d
4
3
5
Sb
4
In2ꢀ
d
from stoi-
Sb
5
4
d
In2ꢀ ,
4
3
(
5
4
In2ꢀ
d
Fig. 2. (a) Powder XRD pattern (CuK
a) for the product of sample Zn50Sb20In30
(
top). Square and triangle symbols mark reflections from the byproducts InSb and
Zn, respectively. The calculated pattern for Zn Sb In2ꢀ is shown below. (b)
Powder XRD patterns for the product of sample Zn60Sb25In15 (top). This product
represents a macroscopic mixture of Zn Sb In and a small amount of byproduct
Zn Sb In2ꢀ The arrow marks reflection from Zn Sb In2ꢀ Discrepancies
between measured and calculated pattern around 2
¼331 are also attributed to
this byproduct. Additionally, crystals of Zn Sb In contain various amounts of
microscopically intergrown Zn Sb In2ꢀ . The calculated pattern for Zn Sb In is
shown below. (c) Powder XRD pattern of the product mixture obtained after
5
4
d
9
6
2
5
4
d
.
a
5
4
d
.
y
9
6
2
5
4
d
9
6
2
annealing Zn
9
Sb
6
In
Sb
2
at 573 K (top). Zn
and elemental Zn, indicated with yellow, green and red
-Zn Sb . (For
9 6 2
Sb In decomposes quantitatively into
Zn Sb In2ꢀ , Zn
5
4
d
4
3
reflection markers, respectively. The calculated pattern below is for
b
4
3
1
0
20
30
40
50
60
70
80
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
2
θ (°)

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Y. Wu et al. / Journal of Solid State Chemistry 183 (2010) 1574–1581
1577
annealed at 723 K, but could subsequently not be reproduced.
This raised the suspicion that the compound may be metastable
and decomposing if not quenched. In the routine way of finishing
off reactions, crystalline products were left in the insulated
stainless steel container after centrifugation to cool radiantly to
room temperature (cf. Section 2). Indeed, when silica jackets were
taken out of the steel container immediately after centrifugation
and quenched in ice water the crystalline product corresponded
2
1
1
0
5
0
5
0
reproducibly to Zn
reactions were repeated and finished off by quenching. This
procedure yielded Zn Sb In , together with minor amounts of
Zn Sb for the compositional region that previously
afforded a product mixture of Zn Sb and Zn Sb (i.e. region
2), Fig. 1). Fig. 2b shows representatively the powder pattern of
9 6 2
Sb In . Subsequently a larger number of
9
6
2
5
4
d
In2ꢀ ,
5
4
In2ꢀ
d
4
3
(
the product from the synthesis mixture Zn60Sb25In15 annealed at
698 K and subsequently quenched. The great similarity to the
exo
powder pattern of Zn
similarity.
5
Sb
4
In2ꢀ
d
indicates a very close structural
The metastable nature of Zn
experiments (Fig. 3). The broad exothermic event in the trace for
Zn Sb In with a maximum around 485 K is attributed to its
decomposition into the thermodynamically stable mixture.
Powder X-ray diffraction analysis of sample Zn Sb In
annealed at 573 K for 1 h identifies the constituents of this
mixture as Zn Sb , Zn Sb and elemental Zn (Fig. 2c).
Thermal events at higher temperatures (above 600 K) must be
inherent to this mixture. Zn Sb undergoes several high
9
Sb In was confirmed in DTA
6
2
Zn Sb In
2
5
4
9
6
2
Zn Sb In
9
6
2
a
9
6
2
500
600
700
800
5
4
In2ꢀ
d
4
3
Temperature (K)
5
4
In2ꢀ
d
Fig. 3. DTA heating trace for Zn
9 9
Sb In
2
(red line) and Zn Sb
5 4
In2ꢀ
d
(black line). (For
temperature phase transitions above 573 K, which are not well
investigated yet, and decomposes at a temperature between
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
Fig. 4. Diffraction patterns corresponding to h k 1 ‘ of (a) orthorhombic Zn
Zn Sb In . The reflections corresponding to the basic tetragonal lattice are highlighted. The diffraction pattern shown in (d) represents an enlargement of (c) where the
reciprocal unit cells of orthorhombic and monoclinic Zn Sb In have been highlighted.
5 4 d 9 6 2
Sb In2ꢀ , (b) orthorhombic Zn Sb In and (c) a monoclinic and orthorhombic two-phase crystal
9
6
2
9
6
2

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Y. Wu et al. / Journal of Solid State Chemistry 183 (2010) 1574–1581
7
50 and 800 K. Events in the trace for Zn
resemble that for pure Zn Sb
Single crystal X-ray diffraction studies revealed further
intricacies in the phase relation between Zn Sb In , Zn Sb
and also Zn Sb . Structurally, Zn Sb In turns out to be quite
challenging. As for the previously reported Zn Sb there is a
basic tetragonal pattern (Fig. 4a) arising from the Sb substructure
I4/mcm). In Zn Sb the systematic extinction associated
9
Sb
6
In
2
above 700 K
substantially when Zn
ment as a second phase. It appears that substantial fractions of
Zn Sb intergrow regularly with metastable Zn Sb In . The
reverse situation is observed as well. However, the volume
fractions of Zn Sb In in crystals of thermodynamically stable
Zn Sb are significantly smaller (up to 8%) [11].
One particular crystal showed a distinctly different diffraction
pattern. The six-fold super-structuring is much more pronounced
than in Zn Sb In and interestingly, this effect is exhibited only in
every second row of reflections (Fig. 4c). The obvious interpreta-
tion is a further doubling of the long axis in Zn Sb In but this
5 4 d
Sb In2ꢀ was introduced into the refine-
5
4
d
In2ꢀ .
5
4
In2ꢀ
d
9
6
2
9
6
2
5
4
d
In2ꢀ ,
4
3
9
6
2
9
6
2
5
4
In2ꢀ
d
5
4
In2ꢀ
d
(
5
4
In2ꢀ
d
with the I-centering is broken, and additionally both the a and b
directions exhibit satellite reflections that indicate a doubling
of the corresponding axes. There is however a conspicuous lack
9
6
2
9
6
2
of reflections that double the tetragonal
simultaneously, and this led us to conclude that the doubling
only applies to one direction, while the second set of satellites is
a
and
b
axes
approach proved unsuccessful in providing a structural solution.
After abandoning this first interpretation, it was realized that the
diffraction pattern may be generated by a two-phase crystal,
generated by pseudomerohedral twinning. The space group
˚
9 6 2
where one phase is orthorhombic Zn Sb In and the other is a
symmetry of Zn
˚
5
Sb
4
In2ꢀ
d
is Pbcn with aE7.15 A (ctet),
monoclinic variant that generates a new set of reflections along
the long axis that neatly interdigitates the reflections from the
former (Fig. 4d). The symmetry of this new monoclinic phase was
˚
bE17.15 A (2 ꢁ atet) and cE8.7 A (atet).
9 6 2
The diffraction pattern of Zn Sb In is more complex, and a
cursory inspection indicates a tetragonal unit cell with a six-fold
superstructure along the a and b directions (Fig. 4b). A very large
number of reflections are however systematically absent, indicat-
ing twinning. With few exceptions the non-crystallographic
absences are in agreement with a model where atet is doubled
and btet is trebled. Pseudomerohedral twinning generates the rest
of the reflections observed. The violations of this simple model are
a weak set of reflections that double atet and btet simultaneously,
and those reflections will be dealt with separately at a later stage.
1
assumed to be P2 11, and the structure was solved by charge
flipping. It comes out as a simple variation of the orthorhombic
form. The structure of the monoclinic form may be generated by a
simple shift of a unit cell sized block of the orthorhombic phase by
1
2
1
2
the vector ð0 Þ. To facilitate the simultaneous treatment of the
1
2
1
2
two structures, the shift vector ð0 Þ was employed as a centering
vector in the monoclinic structure. This allows a description in a
metrically orthorhombic cell, twice the size of the cell of the
orthorhombic phase (doubling the long b-axis). Thus, both
Zn
9
Sb
A large number of crystals were studied, and in many samples
the fit between data and model was quite poor, but improved
6
In
2
was assigned space group symmetry P2
1
2
1
2
1
.
structures are described in
a
metrically orthorhombic cell
˚
1
2
˚
˚
measuring 7.14 A ꢁ 17.14 A ꢁ 51.44 A. (The orthorhombic phase
1
2
possesses a centering vector of ð0 Þ in this setting.) In addition to
b
b
c
c
c
b
Erro
a
a
c
b
2
Fig. 5. (a) The framework ‘‘ZnSb’’ in Zn
black, thick, lines. The unit cell of the Sb atom arrangement is outlined in red, the one of the Zn
formed by 8 Sb atoms. The central tetrahedron (empty) is marked in gray. (c) Arrangement of tetraedersterns in Zn
by Zn atoms (marked in green). (d) Filling pattern of peripheral tetrahedra in Zn Sb In . (e) The framework ‘‘ZnSb’’ in Zn
For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5
Sb
4
In2ꢀ
d
. Sb atoms (red) form 3 434 nets (thin lines) stacked in antiposition orientation. Atom pairs Zn–Zn (cyan) are connected by
Sb In2ꢀ structure in black. (b) Tetracapped tetrahedron (‘‘tetraederstern’’)
Sb In2ꢀ with half of the peripheral tetrahedra occupied
Sb In . Ellipsoids are drawn at a 90% probability.
5
4
d
5
4
d
9
6
2
9
6
2
(

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Y. Wu et al. / Journal of Solid State Chemistry 183 (2010) 1574–1581
1579
the biphasic nature of the crystal, both phases are twinned by
pseudomerohedry because of the underlying tetragonal symme-
try. The final agreement between model and data is modest (R is
1
ca. 12%, cf. Table 1), but considering the complexity of the sample
this is not unexpected.
The most prominent satellite reflections of the monoclinic
phase are those that simultaneously double the a and b axes of the
basic tetragonal cell. It is interesting to note that these reflections
are absent both in Zn
These are the unexplained weak reflections present in the
diffraction pattern of Zn Sb In referred to earlier. It would seem
that metastable Zn Sb In occurs in different structural variations
5 4 d 9 6 2
Sb In2ꢀ and in orthorhombic Zn Sb In .
9
6
2
9
6
2
depending on melt composition and the thermal history of sample
preparation. The same holds for the intergrowth with
b
Zn
identified crystals were Zn
intergrown with the binary phase Zn
5
Sb
4
In2ꢀ
d
. Finally, it should be mentioned that we also
Sb In appears to be epitaxially
Sb . This shows the
9
6
2
c
4
3
complexity of phase relations in the compositional region (2),
even nominal ‘‘single’’ crystals may consist of several phases.
In the following, we establish the crystal structure relation-
5 4 d 9 6 2
ships between orthorhombic Zn Sb In2ꢀ and Zn Sb In (Figs. 5
2
and 6). The characteristic features of both structures are 3 434
nets formed by the Sb atoms that are stacked in antiposition
orientation (Fig. 5a). This arrangement is body centered
tetragonal (the unit cell is indicated in Fig. 5a) and also yields
rows of face-sharing square antiprisms that are connected in the
tetragonal plane by sharing triangle edges. Such an arrangement
2
Fig. 6. (a) The total structure of Zn
9
Sb
434 net. Cyan, red and gray ellipsoids denote Zn, Sb and In atoms, respectively.
Atom pairs Zn–Zn are connected by black, thick lines. (b) Filling of four symmetry
independent tetragonal channels in Zn Sb In . Ellipsoids are drawn at a 90%
6 2
In projected along the stacking direction of
of 3 434 nets occurs in many intermetallic compounds; most
2
3
2
prominent is probably the CuAl type where nets are formed by Al
atoms while Cu atoms center square antiprisms [17]. In addition
to square antiprisms the Sb atom substructure generates
also intervening tetracapped tetrahedra usually termed
tetreadersterns (Fig. 5b) [17].
9
6
2
probability. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
In Zn
5
Sb
4
In2ꢀ
d
and Zn
9
Sb
6
In
2
Zn atoms occupy two of the four
channels provided by the rows of square antiprisms in the ‘‘ZnSb’’
peripheral tetrahedra in Sb
8
tetreadersterns. Since tetrahedra
frameworks ([11] and Fig. 6a). For Zn
antiprisms are occupied alternately by pairs of Zn and In atoms,
and single In atoms. The In positions in Zn Sb display some
occupational deficiency (5–10%) which is indicated as in the
compound formula. The value of is 0.15 and does not appear to
be variable, suggesting that Zn Sb has no or only a very
small homogeneity range. Instead of pairs Zn–In and single In
atoms for Zn Sb triangles of Zn atoms (Zn ) and In atoms
in a ratio 1:2 occur for Zn Sb In (Fig. 6a). Their arrangement
5 4 d
Sb In2ꢀ centers of square
share edges, this results in rather short ZnꢀZn distances (below
˚
or close to 3 A). The distribution of Zn atoms (or ‘‘Zn pairs’’) is
5
4
In2ꢀ
d
shown in Fig. 5c. In Zn
5
Sb
4
In2ꢀ
d
, Zn atoms of a pair are always at
d
2
different heights with respect to the stacking direction of 3 434
nets and in the same orientation, which gives rise to a ‘‘parallel’’
pattern in the projection along the stacking direction (cf. Fig. 5a).
d
5
4
In2ꢀ
d
In Zn
9
Sb
6
In
2
this distribution is more complex (Fig. 5d and e): 1/3
5
4
In2ꢀ
d
3
of the pairs are formed by Zn atoms at the same height, giving rise
to a ‘‘cross’’ pattern, whereas 2/3 of the pairs are arranged in the
9
6
2
within square antiprisms is shown in Fig. 6b. The Zn–Zn contacts
‘
1
‘parallel’’ fashion, however, with alternating orientation (ratio
:1). Note, that in our previous paper on Zn Sb , where we
preliminarily addressed Zn Sb In , we erroneously stated that the
within triangles correspond to the shortest interatomic distances
˚
9 6 2
in the structure of Zn Sb In (2.5–2.6 A). As aforementioned, in
5
4
In2ꢀ
d
9
6
2
contrast with Zn
could not be detected for Zn
monoclinic form of Zn Sb In is shown in Fig. 7. It is simply
5
Sb
4
In2ꢀ
d
occupational deficiency of In atoms
distribution of Zn atoms in tetrahedral interstices is the same in
both structures [11]. It is, however, clearly different.
9
Sb In . The structure of the
6
2
9
6
2
The filling patterns of Zn atoms in peripheral tetrahedra result
in a symmetry lowering of the originally tetragonal arrangement
generated by a shift of unit cell sized blocks of the orthorhombic
structure by half of the translational period in the b direction
(cf. Fig. 6a).
Previously it has been shown that Zn Sb In2ꢀ has promising
5 4 d
thermoelectric properties [11]. As a matter of fact, in the
of Sb
Zn atoms in Zn
2 ꢁ 3 ꢁ 1) superstructures, respectively, with respect to the
tetragonal basis structure of Sb atoms. It has a composition
‘ZnSb’’, with 16 and 48 formula units, respectively, in the unit
cell. The frameworks ‘‘ZnSb’’ are highly unusual when considering
the structures of Zn Sb and Zn Sb In as derivatives of the
CuAl type. None of the ubiquitous structures based on antiposi-
8
tetraedersterns to orthorhombic. The framework of Sb and
5
Sb and Zn Sb In
4
In2-
d
9
6
2
represent (2 ꢁ 1 ꢁ 1) and
(
investigated temperature interval 10–350 K it is superior to
‘
state-of-the-art Zn
remarkable is the thermal conductivity of Zn
4
Sb
3
as a thermoelectric material. Especially
Sb : low
5
4
In2ꢀ
d
5
4
In2ꢀ
d
9
6
2
magnitudes (around 1 W/m K) occur over the whole range of
temperatures, and a peak at low temperatures characteristic for
crystalline materials is absent [18,19]. This peculiar behavior has
been attributed to the presence of intergrown domains of
2
2
tion stackings of 3 434 nets and different fillings of square
antiprisms and tetrahedra display this kind of distribution [17].
However, when focusing on shortest interatomic distances in the
frameworks, Zn–Zn and Zn–Sb, structural fragments characteristic
Zn
9
Sb
6
In
2
in Zn
5
Sb
4
In2ꢀ
d
, turning this material to a phonon glass.
Despite the inclusion of small fractions Zn
9
Sb In , Zn Sb In2ꢀ
6
2
5
4
d
for binary zinc antimonides (ZnSb and Zn
The fraction of remaining atoms (‘‘ZnIn
‘Zn Sb ’’ and ‘‘Zn In Sb In ꢀ‘‘Zn Sb ’’) is located in the
’’¼Zn
4
Sb
3
) emerge.
crystals and bulk samples can be considered as compositionally
rather homogenous. Diffraction patterns and measured properties
do not significantly vary for specimens from different samples.
2
’’¼Zn
5
Sb
4
In
2
ꢀ
‘
4
4
3
2
9
6
2
6
6

ARTICLE IN PRESS
1
580
Y. Wu et al. / Journal of Solid State Chemistry 183 (2010) 1574–1581
Fig. 7. Structure of monoclinic Zn
9 6
Sb In
2
presented in an A-centered, metrically orthorhombic, unit cell. The color code of atoms is the same as in Fig. 6.
an ideal situation with fully occupied In positions (
for Zn Sb In . The electronic density of states (DOS) of both
compounds has been recently calculated [7]. The DOS for
idealized Zn Sb In displays a narrow band gap with a size of
0.2 eV, and the Fermi level would be situated at this gap if the In
positions were fully occupied. For more electron-poor Zn Sb In
d¼0), and 3.18
9
6
2
1
Zn65Sb20In15
5
4
2
Zn54Sb31In15
9
6
2
a
similar situation is obtained, however, the gap, where the Fermi
level is located, appears just closed.
0
.1
Although detailed features of the bonding interactions between
In and the ‘‘ZnSb’’ framework in these ternary compounds are not
yet clear, it is interesting to note that despite of the low values of
electron concentration narrow band gaps at or close the Fermi level
can still be realized. The electrical resistivity behavior of both
Zn54Sb31In15
Zn57.5Sb31In17.5
Zn60Sb25In15
0
.01
Zn70Sb15In15
materials is compatible with a stoichiometric Zn
Fermi level is at a closed band gap, and a non-stoichiometric
Zn Sb possessing a proper band gap but having the Fermi
level shifted slightly into the valence band through the In deficiency.
9 6 2
Sb In where the
In2ꢀ
4 d
0
100
200
Temperature (K)
300
5
9 6 2
Fig. 8. Electrical resistivity of various specimens of Zn Sb In . The composition of
their synthesis mixture is indicated in the plot.
4. Conclusions
The exploration of the Zn-rich section of Zn–Sb–In system
This is not the case for Zn
variable amounts of intergrown Zn
Zn Sb ). Additionally, Zn Sb In can occur in different modifica-
tions with slightly different structures.
Indeed, electrical resistivity ( ) measurements between 10 and
50 K on a large number of Zn Sn In specimens showed a rather
broad range of resistivity values (Fig. 8). The extremes differ by an
order of magnitude. We do not find an apparent correlation of
resistivity values and synthesis compositions used for producing
9
Sb
6
In
2
where specimens contain
yielded two ternary intermetallic compounds, stable Zn
¼0.15) and metastable Zn Sb In with closely related crystal
structures. Their common features are 3 434 nets of Sb atoms that
are stacked in antiposition orientation. Although this is
prominent structural motif of intermetallic compounds, the
distribution of Zn and In atoms in the square antiprismatic and
tetrahedral voids provided by the Sb atom arrangement is unique
and results in novel structure types. The electronic structure of
5 4 d
Sb In2ꢀ
5
Sb (and possibly also
4
In2ꢀ
d
(d
9
6
2
2
4
3
9
6
2
a
r
3
9
6
2
Zn
5
Sb
4
In2ꢀ
d
and Zn
9
Sb
6
In
2
resembles that of binary zinc
the Zn
of the resistivity is metal-like and magnitudes (m
typical of poor metal or heavily doped semiconductor.
Specimens with high resistivity seem to show weaker
temperature dependence. As a matter of fact, the slope of (T)
and also magnitudes of up to temperatures around 220 K are
very similar to Zn Sb [11]. After this temperature, however,
(T) for Zn Sb changes into semiconductor-like [11]. It may
be assumed that specimens displaying a high resistivity contain a
large(r) fraction of intergrown Zn Sb , and that Zn Sb In
has a lower intrinsic resistivity than Zn
The average valence electron concentration of the ternary
Zn–In–Sb frameworks is low, 3.27 for Zn Sb when considering
9
Sb
6
In
2
specimens. Generally, the temperature dependence
antimonides in that a narrow band gap, or pronounced pseudo-
gap, is realized at or close to the Fermi level. The electrical
resistivity of both materials is typical of poor metals or
degenerate semiconductors.
O
cm range) are
a
a
r
r
In2ꢀ
d
Acknowledgments
5
4
r
5
4
In2ꢀ
d
This work was supported by the Swedish Research Council (VR)
and the National Science Foundation through Grants DMR-0638826,
DMR-1007557and CHE-0742006. We are grateful to Dr. N. Newman,
ASU, for supporting this research by generously making his PPMS
available.
5
4
In2ꢀ
d
9
6
2
5
Sb
4
In2ꢀ
d
.
5
4
In2ꢀ
d

ARTICLE IN PRESS
Y. Wu et al. / Journal of Solid State Chemistry 183 (2010) 1574–1581
1581
Appendix A. Supporting information available
(b) P.J. Shaver, J. Blair, Phys. Rev. 141 (1966) 649;
(c) T. Caillat, J.-P. Fleurial, A. Borshchevsky, J. Phys. Chem. Solids 58 (1997)
1
119;
Crystallographic data in CIF format for orthorhombic and
monoclinic Zn Sb In2. Tables of atomic position parameters and
(
d) V.L. Kuznetsov, D.M. Rowe, J. Alloys Compd. 372 (2004) 103;
(e) S.C. Ur, I.H. Kim, P. Nash, J. Mater. Sci. 42 (2007) 2143;
f) G. Nakamoto, N. Akai, M. Kurisu, J. Alloys Compd. 437 (2007) 151;
g) B.L. Pedersen, B.B. Iversen, Appl. Phys. Lett. 92 (2008) 161907.
9
6
(
(
9 6 2
interatomic distances for orthorhombic Zn Sb In .
[
9] Villars, P.; Calvert, L.D. Pearsons Handbook of Crystallographic Data for
Intermetallic Compounds, second ed., ASM International, Materials Park, OH,
1991; Desk Edition, 1997. Prominent representatives are AZnSb (A¼alkali
metal, e.g. LiZnSb (G. Schr o¨ der, H.U. Schuster, Z. Naturforsch B 30 (1975) 978)
Appendix B. Supplementary material
and AeZn
2
Sb
2
(Ae¼alkaline earth metal, e.g. CaZn
2
Sb
2
(A. Mewis,
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2010.05.001.
Z. Naturforsch. B 33 (1978) 382). More complex structures and their bonding
properties are analyzed in e.g. S. Bobev, J.D. Thompson, J.L. Sarrao, M.M.
Olmstead, M. Hope, S.M. Kauzlarich, Inorg. Chem. 43 (2004) 5044; B. Saparov,
S.Q. Xia, S. Bobev, Inorg. Chem. 47 (2008) 11237.
[
[
10] A. Teng a˚ , S. Lidin, J.-P. Belieres, N. Newman, Y. Wu, U. H a¨ ussermann,
Chem.-Eur. J. 15 (2009) 6704.
11] Y. Wu, S. Lidin, T.L. Groy, N. Newman, U. H a¨ ussermann, Inorg. Chem. 48
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