Quaternary pnictides with complex, noncentrosymmetric structures. synthesis and structural characterization of the new zintl phases Na11Ca2Al3Sb8, Na4CaGaSb3, and Na15Ca3In5Sb12

Article abstract of DOI:10.1021/ic502822v

Three new Zintl phases, Na₁₁Ca₂Al₃Sb₈, Na₄CaGaSb₃, and Na₁₅Ca₃In₅Sb₁₂, have been synthesized by solid-state reactions, and their structures have been determined by single-crystal X-ray diffraction. Na₁₁Ca₂Al₃Sb₈ crystallizes with its own structure type (Pearson index oP48) with the primitive orthorhombic space group Pmn2₁ (No. 31). The structure is best viewed as [Al₃Sb₈]^15- units of fused AlSb₄ tetrahedra, a novel type of Zintl ion, with Na⁺ and Ca²⁺ cations that solvate them. Na₄CaGaSb₃ also crystallizes in its own type with the primitive monoclinic space group Pc (No. 7; Pearson index mP36), and its structure boasts one-dimensional [GaSb₃]^6- helical chains of corner-shared GaSb₄ tetrahedra. The third new compound, Na₁₅Ca₃In₅Sb₁₂, crystallizes with the recently reported K₂BaCdSb₂ structure type (space group Pmc2₁; Pearson index oP12). The Na₁₅Ca₃In₅Sb₁₂ structure is based on polyanionic layers made of corner-shared InSb₄ tetrahedra. Approximately one-sixth of the In sites are vacant in a statistical manner. All three structures exhibit similarities to the TiNiSi structure type, and the corresponding relationships are discussed. Electronic band structure calculations performed using the tight-binding linear muffin-tin orbital atomic sphere approximation method show small band gaps for all three compounds, which suggests intrinsic semiconducting behavior for these materials.

Full text of DOI:10.1021/ic502822v

Article
pubs.acs.org/IC
Quaternary Pnictides with Complex, Noncentrosymmetric Structures.
Synthesis and Structural Characterization of the New Zintl Phases
Na Ca Al Sb , Na CaGaSb , and Na Ca In Sb
11
2
3
8
4
3
15
3
5
12
Yi Wang, Stanislav Stoyko, and Svilen Bobev*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S Supporting Information
*
ABSTRACT: Three new Zintl phases, Na Ca Al Sb ,
1
1
2
3
8
Na CaGaSb , and Na Ca In Sb , have been synthesized by
4
3
15
3
5
12
solid-state reactions, and their structures have been determined
by single-crystal X-ray diffraction. Na Ca Al Sb crystallizes
1
1
2
3
8
with its own structure type (Pearson index oP48) with the
primitive orthorhombic space group Pmn2 (No. 31). The
1
1
5−
structure is best viewed as [Al Sb ]
units of fused AlSb
3
8
4
+
2+
tetrahedra, a novel type of Zintl ion, with Na and Ca cations
that solvate them. Na CaGaSb also crystallizes in its own type
4
3
with the primitive monoclinic space group Pc (No. 7; Pearson
index mP36), and its structure boasts one-dimensional
GaSb3]6 helical chains of corner-shared GaSb tetrahedra.
−
[
4
The third new compound, Na Ca In Sb , crystallizes with
1
5
3
5
12
the recently reported K BaCdSb structure type (space group Pmc2 ; Pearson index oP12). The Na Ca In Sb structure is
2
2
1
15
3
5
12
based on polyanionic layers made of corner-shared InSb tetrahedra. Approximately one-sixth of the In sites are vacant in a
4
statistical manner. All three structures exhibit similarities to the TiNiSi structure type, and the corresponding relationships are
discussed. Electronic band structure calculations performed using the tight-binding linear muffin-tin orbital atomic sphere
approximation method show small band gaps for all three compounds, which suggests intrinsic semiconducting behavior for
these materials.
INTRODUCTION
istics. The fact that in recent years many ternary antimonides
have gained interest as prospective materials for applications in
■
The compounds formed between the alkali or alkaline-earth
metals and the post-transition elements from groups 13−15 are
6
−8
thermoelectric energy conversion
is another reason why we
1
,2
were attracted to the ongoing research efforts in this general
area. Herein, we report the results from a new direction of this
project, which includes the identification and the character-
ization of three new Zintl phases: Na Ca Al Sb , Na CaGaSb ,
typically referred to as Zintl phases. Using the Zintl−Klemm
concept, the structures of such materials are readily rationalized
as cations (less electronegative elements) and anions (more
electronegative elements), so that the valence rules are satisfied.
Recently, our group has investigated quite a few new cases of
presumed Zintl phases, where the cations can be seen as
intimately involved in the chemical bonding; for example,
11
2
3
8
4
3
and Na Ca In Sb . They provide new examples for the
15
3
5
12
significant role of cations in chemical bonding and structure
formation in Zintl phases. The focus of this paper is on the
synthesis, the single-crystal X-ray diffraction studies, and the
electronic structures [calculated by the tight-binding linear
muffin-tin orbital atomic sphere approximation (TB-LMTO-
3
4
A Cd Sb (A = Eu, Ba), and A CdSb (A = Ca, Yb), are two
21
4
18
2
2
kinds of structures in which the cations proved to be important
structure-directing parameters (for the same valence electron
count). The series Ba ZnPn (Pn = As, Sb, Bi), not
surprisingly, demonstrated that for the same structure and
valence electron count mixing of the cation and anion states is
responsible for the transition from intrinsic semiconducting
behavior to metallic behavior as the sizes and electronegativities
of the atomic partners are varied. The cited examples are yet
another testament to the notion that “closed-shell species”,
9
5
ASA) method ] of the three new compounds. The structural
2
2
similarities among them are presented; the close structural
relationship to the TiNiSi structure type (also known as the
10
Co Si type ) is discussed as well.
2
EXPERIMENTAL SECTION
Synthesis. Because of the high sensitivity of elemental Na to air
and moisture, handling of the starting materials required extreme
caution. All manipulations were carried out inside an Ar-filled glovebox
■
“
electron donors/acceptors”, “space fillers”, etc., which are
traditional terminology in the Zintl−Klemm concept, are
somewhat far from reality.
The previous work motivated us to extend our research and
to seek more new structures with complex bonding character-
Received: November 25, 2014
©
XXXX American Chemical Society
A
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
with a controlled atmosphere or under vacuum. The metals were
purchased from Alfa or Aldrich (purity >99.9%) and were used as
received. The Na surface was scrapped off with a scalpel blade
immediately before use.
positions with the corresponding anisotropic displacement parameters.
Where applicable, site occupation factors were also refined. Additional
details pertaining to the structure refinements and the way the mixed-
occupied sites were handled, as well as structural drawings with
anisotropic displacement parameters at the 95% probability level, are
provided as Supporting Information (SI).
Single crystals of the title compounds were first identified from
exploratory reactions of the corresponding elements Na, Ca, Tr, and
Sb in the molar ratio of 3:3:1:4 (Tr = Triel, i.e., a group 13 element).
The metals were weighed in the glovebox (total weight ca. 500 mg)
and loaded into Nb tubes, which were then closed by arc welding
under high-purity Ar gas. The sealed Nb containers were subsequently
jacketed within evacuated fused-silica tubes. The reaction mixtures
were heated to 1173 K at a rate of 100 K/h and equilibrated at this
temperature for 24 h. Following a cooling step to 773 K at a rate of 5
K/h, the products were brought to room temperature over 12 h. On
the basis of analysis of the single-crystal and powder X-ray diffraction
data, multiple binary, ternary, and quaternary phases were obtained in
each case. Besides Na Ca Al Sb , Na CaGaSb , and Na Ca In Sb ,
Selected crystallographic information and refinement parameters for
Na Ca Al Sb , Na CaGaSb , and Na Ca In Sb are summarized in
11
2
3
8
4
12
15
3
5
12
Table 1. Final positional and equivalent isotropic displacement
Table 1. Selected Single-Crystal Data Collection and
Structure Refinement Parameters for Na Ca Al Sb ,
1
1
2
3
8
Na CaGaSb , and Na Ca In Sb
4
3
15
3
5
12
a
empirical formula
Na Ca Al Sb
Na CaGaSb3
4
Na CaIn1.66Sb2
5
1
1
2
3
8
fw
1387.99
567.01
200(2)
833.78
1
1
2
3
8
4
3
15
12
3
5
12
13
temperature (K)
radiation, λ (Å)
space group
Z
11
we were also able to identify Na Ca AlSb , Ca Ga Sb , Na InSb ,
3
3
4
5
2
6
3
2
14
15
Mo Kα, 0.71073
Pc (No. 7)
4
Ca Sb , and NaSb, among some other, yet unidentified phases.
11
10
Pmn2 (No. 31)
Pmc2 (No. 26)
1
1
After the structures and chemical compositions were established by
single-crystal work, new reactions with the proper stoichiometry were
loaded. They yielded the desired compounds as major products. The
produced samples contained small amounts of impurity phase(s), but
the bulk consisted of small, shiny crystals with a dark-metallic luster
that were the targeted phases. The materials were brittle and air-
sensitive.
We speculated that the three newly identified phases might have
isostructural “neighbors” within the arsenides or bismuthides. Such
reactions were set up, and the corresponding products were
analyzedno isostructural arsenides or bismuthides were found;
instead, we uncovered new phases with different structures, which will
be the subject of a forthcoming publication. We also explored periodic
trends by changing the alkali and/or alkaline-earth metal and will detail
the results from this work in another follow-up article.
2
1
a (Å)
18.9690(15)
8.8533(7)
7.9304(7)
9.1513(9)
8.9852(9)
12.2305(12)
97.840(1)
996.3(2)
3.78
4.7609(5)
9.1680(10)
7.8898(8)
b (Å)
c (Å)
β (deg)
V (Å3)
1331.8(2)
3.46
344.37(6)
4.02
ρcal (g/cm3)
μ (cm−1
)
86.4
113.3
109.6
1.106
0.08(9)
1058
GOF on F2
1.051
0.02(4)
3716
0.994
Flack parameter
unique reflns
refined param
0.04(1)
5260
119
165
38
b
R1 [I > 2σ(I)]
0.0288
0.0598
0.0210
0.0301
0.0617
b
Caution! At the aforementioned conditions, Na metal will evaporate
wR2 [I > 2σ(I)]
0.0411
and could create high pressure. It is therefore important to use as long as
a
Refined formulas with freely refined occupancies of the Na/Ca mixed
positions: Na10.90(2)Ca2.10Al Sb , Na3.98(1)Ca1.02GaSb , and
3
1
possible Nb containers ( / in. in diameter and ca. 2 / to 3 in. in length)
8
2
7
3
8
3
and long fused-silica tubes ( / in. in diameter and ca. 10−12 in. in
b
8
Na5.06(2)Ca0.94In
Sb , respectively. R1 = ∑||F | − |F ||/∑|F |;
1.58(2)
2
o
c
o
+
length), which must be left protruding outside the furnace. Doing so
minimizes the risk of serious accidents because, in the case of a leak, the Na
vapors will diffuse and condense at the cold end of the fused-silica tube. If
this happens, the furnace must be stopped immediately.
2
2 2 2 2 1/2 2 2
wR2 = [∑[w(F − F ) ]/∑[w(F ) ]] , where w = 1/[σ F
o
c
o
o
2
2
2
(
AP) + (BP)], and P = (Fo + 2F )/3; A, B = weight coefficients.
c
Powder X-ray Diffraction. The as-synthesized samples were
ground in the glovebox to fine powders. Then, powder X-ray
diffraction patterns were taken at room temperature on a Rigaku
MiniFlex powder diffractometer using filtered Cu Kα radiation (λ =
parameters are given in Tables 2−4. Relevant interatomic distances
and angles are listed in Table 5. CIFs are provided as SI; the files have
Table 2. Atomic Coordinates and Equivalent Isotropic
1.54056 Å), also operated inside the glovebox. Data analysis was done
a
Displacement Parameters Ueq for Na Ca Al Sb
11
2
3
8
using the JADE 6.5 software package.
Because of the limited capabilities of the MiniFlex, combined with
the low symmetry and complex nature of the structures, the patterns
had many peaks that could not be well-resolved. Therefore, the
powder X-ray diffraction patterns were only used for phase
identification; all reported cell parameters were refined by single-
crystal X-ray diffraction data.
Single-Crystal X-ray Diffraction. The single-crystal X-ray
diffraction experiments were done on a Bruker SMART CCD-based
diffractometer with a three-circle goniometer, employing monochrom-
atized Mo Kα radiation (λ = 0.71073 Å). During the data collection,
Ueq (Å2)
atom
site
x
y
z
b
Na1/Ca1
Na2
Na3
Na4
Na5
Na6
Na7
Ca2
Al1
4b
4b
4b
4b
4b
2a
2a
2a
4b
2a
4b
4b
4b
2a
2a
0.7560(1)
0.9466(2)
0.5935(4)
0.6654(3)
0.5715(4)
0.8027(3)
0.9484(4)
0.3384(4)
0.0809(2)
0.8351(2)
0.6862(3)
0.8633(1)
0.3311(1)
0.8778(1)
0.6814(1)
0.4016(1)
0.9845(2)
0.9494(5)
0.6288(4)
0.9546(4)
0.3255(4)
0.0000(5)
0.1289(6)
0.4897(3)
0.3371(3)
0.6153(4)
0.6853(1)
0.7350(1)
0.7205(1)
0.2679(1)
0.7268(1)
0.0245(4)
0.0377(7)
0.0286(7)
0.0341(7)
0.0289(7)
0.0264(8)
0.0285(9)
0.0284(5)
0.0207(4)
0.0199(6)
0.0208(1)
0.0242(1)
0.0201(1)
0.0197(1)
0.0215(1)
0.8817(2)
0.7499(2)
0.6307(2)
0.6242(1)
0
0
0
the temperature was kept at 200 K by a cold N stream. Maintaining
2
0.8772(1)
an inert atmosphere during the experiments was critical because the
crystals deteriorate very quickly if exposed to the laboratory air (all
preparations were done in a glovebox with an optical microscope and
under dried Paratone N oil). Data acquisition and integration were
completed using the programs SMART and SAINT, supplied by
Al2
0
Sb1
0.8852(1)
Sb2
0.7309(1)
Sb3
0.6265(2)
16
Sb4
0
0
Bruker. Semiempirical absorption correction based on equivalent
17
reflections was applied using SADABS. The structures were solved by
direct methods and refined with a full-matrix least-squares method on
Sb5
2
18
a
F , as implemented in SHELXTL. The refinements converged readily
to low conventional residuals and final difference Fourier maps, which
were flat. Refined parameters included the scale factors and atomic
Ueq is defined as one-third of the trace of the orthogonalized U
ij
b
tensor. Refined freely as 55(1)% Ca and 45% Na; in the final
refinement model, for simplicity, constrained to 50% Ca and 50% Na.
B
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
5
using the tetrahedron method, and the Fermi level was set at 0 eV as
the energy reference.
Table 3. Atomic Coordinates and Equivalent Isotropic
a
Displacement Parameters Ueq for Na CaGaSb
4
3
atom
site
x
y
z
Ueq (Å2)
RESULTS AND DISCUSSION
b
c
■
Na1/Ca1
Na2/Ca2
Na3
Na4
Na5
Na6
Na7
Na8
Na9
Ca3
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
0.0000(2)
0.6808(1)
0.9566(3)
0.9153(2)
0.8502(3)
0.5697(2)
0.5443(2)
0.2392(2)
0.2021(3)
0.3427(1)
0.5977(1)
0.2770(1)
0.8087(1)
0.6897(1)
0.5012(1)
0.3573(1)
0.2093(1)
0.0430(1)
0.9406(2)
0.9585(2)
0.8047(2)
0.6726(2)
0.6032(3)
0.3371(2)
0.5925(3)
0.6631(2)
0.4396(3)
0.0471(1)
0.8254(1)
0.8274(1)
0.3456(1)
0.8937(1)
0.6875(1)
0.8778(1)
0.6650(1)
0.8705(1)
0.0001(1)
0.6713(1)
0.2811(2)
0.5412(2)
0.8155(2)
0.7297(2)
0.5054(2)
0.8959(2)
0.6606(2)
0.8451(1)
0.9572(1)
0.6249(1)
0.5671(1)
0.4131(1)
0.7601(1)
0.0831(1)
0.4413(1)
0.7380(1)
0.0207(3)
0.0182(3)
0.0263(5)
0.0236(5)
0.0346(6)
0.0275(5)
0.0286(5)
0.0248(5)
0.0418(6)
0.0184(2)
0.0176(1)
0.0170(1)
0.0206(1)
0.0151(1)
0.0176(1)
0.0145(1)
0.0183(1)
0.0162(1)
Structure and Chemical Bonding of Na Ca In Sb .
15
3
5
12
We begin our discussion on the chemical bonding by describing
the structure of Na Ca In Sb , which is the simplest among
15
3
5
12
the three (Figure 1a). Na Ca In Sb appears to be the first
15
3
5
12
structurally characterized compound in the quaternary Na−
Ca−In−Sb phase diagram; it crystallizes in the orthorhombic
space group Pmc2 (Pearson index oP12).
1
Na Ca In Sb (i.e., Na (Na Ca )In Sb ) is best
1
5
3
5
12
2
0.5
0.5
0.83
2
described as a substitution derivative of the recently reported
26
Ga1
Zintl phase K BaCdSb , whereby the trivalent In (partially
2 2
2
7
Ga2
occupied) takes the role of the divalent Cd. Because the
important features of this structure have already been
Sb1
26
Sb2
discussed, in the following paragraphs we will focus on a
few details and on the structural relationship to the other two
compounds.
Sb3
Sb4
Sb5
There are six crystallographically unique atoms in the
asymmetric unit (Table 2), which include two Na sites, one
mixed-occupied Ca/Na site, one In site, and two Sb sites. The
In site, as mentioned already (and elaborated on in the SI), is
not fully occupied, with one-sixth of the positions being
randomly vacant. When the disorder is ignored, the structure
Sb6
a
Ueq is defined as one-third of the trace of the orthogonalized Uij
b
tensor. Refined freely as 60(1)% Ca and 40% Na; in the final
refinement model, for simplicity, constrained to 60% Ca and 40% Na.
c
Refined freely as 37(1)% Ca and 63% Na; in the final refinement
model, for simplicity, constrained to 40% Ca and 60% Na.
2−
can be described as infinite [InSb2] polyanionic layers made
of corner-shared InSb tetrahedra (Figures 1a, 2a, and S3 in the
4
Table 4. Atomic Coordinates and Equivalent Isotropic
SI). Defects in the polyanions of the Zintl phases are rare but
not without precedent, as demonstrated on the example of
a
Displacement Parameters Ueq for Na Ca In Sb
15
3
5
12
28
atom
Na1
site
x
y
z
Ueq (Å2)
Eu Cd Sb . In the latter structure (own type), one-fourth of
5 3 6
1/
the Cd positions are randomly vacant.
2b
2a
2a
2b
2b
2a
0.4152(5)
0.3262(5)
0.0613(4)
0.1786(1)
0.1197(1)
0.6559(1)
0.0489(6)
0.3630(6)
0.0000(4)
0.6558(1)
0.2868(1)
0.2618(1)
0.036(1)
0.027(1)
0.0215(6)
0.0234(2)
0.0180(1)
0.0344(2)
2
There are no direct homoatomic interactions. The In−Sb
distances range from 2.924(7) to 2.960(8) Å (Table 5). Within
the tetrahedra, the Sb−In−Sb angles range from 100.18(3)° to
112.41(2)°, attesting to a slight distortion from the ideal
tetrahedral geometry. The numerical values for the In−Sb
bonds and Sb−In−Sb angles compare well with those reported
for other ternary A−In−Sb phases (A = alkali or alkaline-earth
Na2
0
b
Na3/Ca3
In
0
1
/2
1/
Sb1
2
Sb2
0
a
Ueq is defined as one-third of the trace of the orthogonalized Uij
b
tensor. Refined freely as 53(1)% Ca and 47% Na; in the final
29
13
metal), such as Ca In Sb
and Na InSb . The In−Sb
5
2
6
3
2
refinement model, for simplicity, constrained to 50% Ca and 50% Na.
interatomic distances (Table 5) are only ca. 4−6% longer than
the sum of their single-bonded Pauling radii (e.g., r + r
=
Sb
In
30
1
.421 + 1.391 = 2.812 Å), suggesting appreciable covalent
also been deposited with Fachinformationszentrum Karlsruhe, 76344
Eggenstein-Leopoldshafen, Germany [fax (49) 7247-808-666; e-mail
crysdata@fiz.karlsruhe.de] with depository numbers CSD 428753 for
Na Ca Al Sb , CSD 428754 for Na CaGaSb , and CSD 428752 for
bonding between them.
The structure also contains Na and Ca cations, which fill
+
2+
the space between the polyanionic layers. As discussed in a
11
2
3
8
4
3
2
6
1,2
Na Ca In Sb .
previous paper, following the Zintl−Klemm concept, the
15
3
5
12
+ 2+
Energy-Dispersive X-ray Spectroscopy (EDX). To verify the
refined compositions, selected single crystals from each sample were
subjected to EDX analysis. A JEOL 7400 F electron microscope,
equipped with an Oxford INCA spectrometer, was employed. The
analyses were severely hindered by the air sensitivity of the studied
samples, and the obtained data could only confirm the elemental
makeup; no quantitative interpretation of the results was possible.
Electronic Structure Calculations. The electronic structure
parent K BaCdSb structure can be rationalized as (K ) (Ba )-
2 2 2
Cd )(Sb ) . In the same manner, taking into account the
2+
3−
(
2
disorder, Na Ca In Sb can also be viewed as a Zintl phase,
15
3
5
12
the formula of which can be expressed as
+
2+
3+
3−
(
Na ) (Ca ) (In ) (Sb ) . This argument is nicely sup-
15 3 5 12
ported by the electronic structure calculations, which show
opening of a band gap right at the energy where the bonding
interactions are optimized (vide infra).
9
calculations were carried out with the TB-LMTO method using the
1
9−21
LMTO47 program.
The total and partial density of states (DOS)
Structure and Chemical Bonding of Na CaGaSb .
4
3
22
and crystal orbital Hamilton population (COHP) curves of selected
interactions are presented and discussed. Exchange and correlation
terms were treated by the local density approximation. All relativistic
effects except spin−orbit coupling were taken into account by using a
scalar relativistic approximation. The basis sets including 3s, 3p, and 3d
orbitals for Na and Al, 4s, 4p, and 3d orbitals for Ca and Ga, and 5s,
Upon replacement of the heavier In with the lighter and
smaller Ga, the same reaction scheme that produced
Na Ca In Sb allowed for the quantitative synthesis of the
23
15
3
5
12
new phase Na CaGaSb (=Na Ca Ga Sb ). As can be inferred
4
3
16
4
4
12
from the close compositions, the two structures are similar,
although there are distinct differences. The structure is
projected in Figure 1b; some important similarities and
5
p, 5d, and 4f orbitals for In and Sb were treated by the Lo
̈
wdin
24
downfolding technique. The k-space integrations were conducted
C
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 5. Selected Interatomic Distances (Å) and Angles (deg) in Na Ca Al Sb , Na CaGaSb , and Na Ca In Sb
1
1
2
3
8
4
3
15
3
5
12
Na Ca Al Sb
Na CaGaSb3
4
Na Ca In Sb
15 3 5 12
1
1
2
3
8
Al1−Sb2
Al1−Sb3
Al1−Sb4
Al1−Sb1
2.650(2)
2.706(2)
2.753(2)
2.777(2)
2.670(3)
2.739(1)
2.756(3)
3.071(3)
3.078(3)
3.165(3)
3.202(3)
3.154(3)
3.230(4)
3.238(4)
Ga1−Sb1
Ga1−Sb2
Ga1−Sb3
Ga1−Sb4
Ga2−Sb5
Ga2−Sb6
Ga2−Sb3
Ga2−Sb4
Na3−Sb5
Na3−Sb6
Na3−Sb1
Na3−Sb2
2.6812(7)
2.7374(7)
2.7461(7)
2.8913(7)
2.6792(7)
2.7310(7)
2.7568(7)
2.8142(7)
3.086(2)
3.087(2)
3.089(2)
3.207(2)
In−Sb1
In−Sb2 (×2)
In−Sb1
Na2−Sb1 (×2)
Na2−Sb2
Na2−Sb2
2.924(1)
2.9436(8)
2.961(1)
3.101(3)
3.127(5)
3.150(5)
Al2−Sb5
Al2−Sb1 (×2)
Al2−Sb4
Na5−Sb5
Na5−Sb2
Na5−Sb1
Na5−Sb3
Na7−Sb3 (×2)
Na7−Sb4
Na7−Sb5
Sb1−Al1−Sb2
Sb1−Al1−Sb3
Sb1−Al1−Al4
Sb2−Al1−Sb3
Sb2−Al1−Sb4
Sb3−Al1−Sb4
Sb1−Al2−Sb1
Sb1−Al2−Sb4
Sb1−Al2−Sb5
Sb4−Al2−Sb5
113.44(7)
104.95(7)
101.32(7)
113.39(8)
108.53(9)
114.58(6)
105.45(2)
102.22(5)
118.22(7)
108.22(6)
Sb1−Ga1−Sb2
Sb2−Ga1−Sb3
Sb3−Ga1−Sb4
Sb1−Ga1−Sb4
Sb1−Ga1−Sb3
Sb2−Ga1−Sb4
Sb3−Ga2−Sb4
Sb3−Ga2−Sb5
Sb3−Ga2−Sb6
Sb4−Ga2−Sb5
Sb4−Ga2−Sb6
Sb5−Ga2−Sb6
114.02(7)
108.31(2)
111.35(2)
111.98(3)
107.74(6)
103.39(2)
110.28(2)
108.83(2)
109.36(4)
113.38(3)
101.58(4)
103.20(4)
Sb1−In−Sb1
Sb2−In−Sb2
Sb1−In−Sb2
Sb2−In−Sb1
100.18(3)
107.94(4)
112.41(2)
111.93(2)
Figure 1. Polyhedral views of the structures of Na Ca In Sb (a), Na CaGaSb (b), and Na Ca Al Sb (c). The common atomsCa, Na, and
15
3
5
12
4
3
11
2
3
8
Sbare drawn as blue, green, and orange spheres, respectively. The atoms centering the Sb tetrahedraIn, Ga, and Alare shown in light blue,
pink, and red, respectively.
differences between Na Ca In Sb and Na CaGaSb3 are
GaSb and NaSb tetrahedra can be derived (Figure S5 in the
4 4
SI). Such hybrid layers would have the same topology as the
1
5
3
5
12
4
shown in Figures 2 and S5 in the SI.
Na CaGaSb crystallizes in its own structure type with the
previously described [InSb ] layers made solely of corner-
4
3
2
primitive monoclinic space group Pc (Pearson symbol mP36).
The structure contains eight Na, two Ca, two Ga, and six Sb
atoms in the asymmetric unit. In analogy to what we already
discussed for Na Ca In Sb , the two Ca sites here are also
shared InSb tetrahedra.
4
The corresponding Ga−Sb distances range from 2.679(3) to
2.891(5) Å (Table 5). The range is somewhat wider than
expected, and the sum of the Pauling single-bonded radii (e.g.,
1
5
3
5
12
30
mixed-occupied (Table 3). The Na CaGaSb structure can be
rGa + rSb = 1.246 + 1.391 = 2.637 Å) confirms that there will
4
3
6
−
31
viewed as one-dimensional [GaSb3] helical chains made up of
corner-shared GaSb4 tetrahedra (Figure 2b). The chains
propagate along the crystallographic c direction, with Na and
Ca cations filling the space between them. Importantly, if one
considers some of the Na cations, Na3 in the current
nomenclature, as part of the polyanionic structure (vide
infra), hypothetical [Ga Na Sb ] layers of corner-shared
be significant variations in the Ga−Sb bonding. The Sb−Ga−
Sb angles, which range from 101.58(4)° to 114.02(7)°, also
indicate that the GaSb tetrahedra are somewhat more distorted
4
than the aforementioned InSb tetrahedra, which could be a
4
subtle feature of the Ga−Sb bonding. It appears the former
speculation is more likely because this observation is not
unprecedented; other ternary A−Ga−Sb phases, such as
2
/3
1/3
2
D
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. Detailed drawings in different orientations of a [In Sb ] cutout from the layers of the Na Ca In Sb structure (a), a [Ga Sb ] cutout
4
12
15
3
5
12
4
13
from the chains in the Na CaGaSb structure (b), and the [Al Sb ] cluster in the Na Ca Al Sb structure (c). The atoms are labeled, and the
4
3
3
8
11
2
3
8
relevant distances and angles are listed in Table 5
1
2
32
Ca Ga Sb and Na Ga Sb , whose structures are also based
considering the vacant In sites that could account for ribbons if
long-range-ordered), Na CaGaSb features one-dimensional
5
2
6
2
3
3
on GaSb tetrahedra, show a comparatively wide range of Ga−
Sb distances. Just like Ca Ga Sb and Na Ga Sb , the new
compound Na CaGaSb is also a Zintl phase, which can be
rationalized as (Na ) Ca Ga (Sb ) .
4
4
3
12
32
6−
[GaSb3]
helical chains of GaSb tetrahedra, and
5
2
6
2
3
3
4
15−
Na Ca Al Sb has trimeric [Al Sb ] zero-dimensional frag-
4
3
11
2
3
8
3
8
+
2+
3+
3−
ments of fused AlSb tetrahedra.
4
4
3
Structure and Chemical Bonding of Na Ca Al Sb .
At first glance, there is no connection between the three
structures. However, after careful consideration, striking
similarities among the three compounds become apparent
upon studying the cations’ coordination. For example, one can
see that in the three structures there are two types of Na atoms:
interlayer and intralayer Na atoms, i.e., the atoms residing
between and within the layers, respectively. The nuances of the
interlayer and intralayer bonding, and the subtly different roles
the two types of cations play, have already been discussed on
the examples of Ba Cd Sb and its Sr-substituted variant
11
2
3
8
The last, and the most dimensionally reduced, structure of all
three is that of Na Ca Al Sb . This new compound crystallizes
1
1
2
3
8
with the noncentrosymmetric orthorhombic space group
Pmn2 (Figures 1c and S1 in the SI). Inspection of Pearson’s
1
1
0
Handbook and the Inorganic Crystals Structure Database
3
3
(
ICSD) reveals that there is no precedent of a structure with
the same sequence of Wyckoff letters, suggesting that the
Na Ca Al Sb structure represents yet another novel structure
11
2
3
8
type (Pearson symbol oP48). The structure contains six Na,
two Ca, two Al, and five Sb atoms in the asymmetric unit. One
of the two Ca sites here is mixed-occupied also (Table 2).
As emphasized in Figure 2c, the most prominent feature of
3
2
4
36
Ba3−xSr Cd Sb . This discussion was subsequently extended
x
2
4
along the lines of the coloring of the cation sites by cations with
26
different charges, as in K BaCdSb , which is the prototype of
2
2
1
5−
this structure is the [Al Sb ]
subunit. This molecular-like
the herein-presented Na Ca In Sb . In the Na CaGaSb and
3
8
15 3 5 12 4 3
fragment is first-of-a-kind in the chemistry of the Zintl phases
Na Ca Al Sb structures, which are not layered per se, the
11
2
3
8
1
,2
and ions and is built of three AlSb tetrahedra, which share
“intralayer” Na atoms will be those located beside the
4
[GaSb3]6 helical chains or the [Al Sb ] trimers. In Figure
−
15−
edges with each other in a cis-arrangement. The trimers are
composed of two tetrahedra that are crystallographically
unique; the third one is a symmetry-equivalent image,
generated by the mirror plane perpendicular to the a direction
and passing through the Al atom #2. All Al−Sb distances fall in
the range of 2.650(2)−2.777(2) Å (Table 5). The range is
much narrower than the interval observed for the Ga−Sb
distances in Na CaGaSb (vide supra), and the numerical
3 8
3a, the Na2 atom is shown as surrounded by four Sb atoms in
distorted tetrahedral geometry and is positioned between two
adjacent helical chains, connecting them by Na−Sb bonds. In
Figure 3b, Na5 in the Na Ca Al Sb structure is shown in its
11
2
3
8
tetrahedral coordination of four Sb atoms. Two such tetrahedra
form a dimer, which is connected to surrounding Al Sb
3
8
trimers.
4
3
values match well with the sum of the corresponding Pauling
Clearly, the common denominator here is the ability of these
Na atoms to participate in covalent-like bonding with the
polyanions. In both structures, the intralayer Na is tetrahedrally
coordinated with the Na−Sb bonds showing “normal”
distances. The Na−Sb distances range from 3.086(2) to
3.207(2) Å in Na CaGaSb and from 3.078(3) to 3.202(3) Å
30
radii (e.g., r + r = 1.248 + 1.391 = 2.639 Å). The Sb−Al−
Al
Sb
Sb angles vary from 101.32(7)° to 118.22(7)°. All structural
metrics compare well with those reported for other structures
34
based on AlSb tetrahedra, for instance, Ca Al Sb
and
Na Al Sb . Given the absence of homoatomic interactions,
4
5
2
6
3
5
7
2
5
4
3
Na Ca Al Sb can also be classified as a Zintl phase based on
in Na Ca Al Sb , respectively. The range is similar to the
reported distances in Na YbCdSb (ordered quaternary variant
2 2
11
2
3
8
11 2 3 8
+
2+
3+
3−
the formulation (Na ) (Ca ) (Al ) (Sb ) .
1
1
2
3
8
Structural Relationships. Having discussed the important
features of the three structures, we now turn our attention to
the structural relationships among them. Briefly, let us recall
that Na Ca In Sb boasts two-dimensional [InSb2]2 poly-
of the Yb CdSb structure), where the tetrahedral Na is located
ca. 3.1 Å away from its closest neighbors. In another structure
2 2
26
type, adopted by at least five more Zintl phases KA Cd Sb (A
2
2
3
−
37
= Ca, Sr, Ba, Eu, Yb), all K cations are in tetrahedral
coordination also, which is not common for such a large ion;
15
3
5
12
anionic layers of corner-shared InSb tetrahedra (again, not
4
E
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
The participation of different amounts of Na atoms in the
formation of “hybrid polyanionic layers” could also explain the
variations of the Ga−Sb and Al−Sb bond lengths. As we
discussed before, the shortest Ga−Sb bond measures 2.679(3)
Å (between Ga2 and Sb5). The second shortest is 2.681(6) Å
between Ga1 and Sb1 (Table 5). Considering the proximity of
the intralayer Na2 atom, which impacts both Ga1 and Ga2
through Sb1 and Sb5, respectively, we can argue that the
corresponding Ga−Sb distances ought to be on the short side
in order for the structure to “make room” for the more spatially
demanding Na. In the Na Ca Al Sb structure, the shortest
11
2
3
8
Al−Sb bond is 2.650(2) Å and also occurs in the [Al Na Sb ]
2
2
12
subunit (Figure S4 in the SI) between Al1 and Sb2 (Table 5).
Here, Na5 is affecting Al1 through the Sb2 vertex (Figure 3). In
analogy with the [Ga NaSb ] fragment (Figure S4 in the SI), it
3
12
is again reasonable to expect that Al1−Sb1 will be “squeezed”
Figure 3. Polyanionic substructures of Na CaGaSb3 (a) and
and would be the shortest bond with the AlSb tetrahedra.
4
4
Na Ca Al Sb (b) viewed as tetrahedral “hybrid layers”, made up of
11
2
3
8
Relationship to the Common TiNiSi Structure Type. A
different description of these structures can be proposed on the
basis of their close relationship with the ubiquitous TiNiSi
Ga, Na, Sb and Al, Na, Sb, respectively. The atoms are labeled, and the
relevant distances are listed in Table 5
1
0
structure type (also referred to as the Co Si structure type).
2
+
The compound Na In Bi [NaIn0. 67 Bi = (Na )-
In ) (Bi )] is known to form with this structure (Figure
3
2
3
they are also tightly coordinated with K−Sb distances as short
3+
3− 38
(
3
7
0.67
as 3.36 Å. All of the above-mentioned interatomic distance
match well with the sum of the corresponding Pauling single-
bonded radii (e.g., rNa + r = 1.572 + 1.391 = 2.963 Å; r + r
5
a), and it can be described as a polyanionic InBi framework
Sb
K
Sb
=
2.025 + 1.391 = 3.416 Å).
Including the intralayer Na atoms in the consideration of the
polyanions in Na CaGaSb and Na Ca Al Sb , one can notice
4
3
11
2
3
8
that they can be viewed as “hybrid-layered” structures (Figure
). For the Na CaGaSb structure, the ratio of Ga/Na is 2:1
4
4
3
within the “hybrid layers”, while in Na Ca Al Sb , the ratio of
Al/Na is 1:1. In the Na Ca In Sb structure, the [InSb2]
1
1
2
3
8
2
∞
1
5
3
5
12
layers are “free-standing”; i.e., the Na atoms here are positioned
between adjacent layers (see the SI).
Figure 5. Structural relationships among Na In Bi (TiNiSi structure
3
2
3
3
6
type) and Na Ca In Sb , Na CaGaSb , and Na Ca Al Sb . Panel a
15
3
5
12
4
3
11
2
3
8
shows the three-dimensional framework of the parent structure with
Na shown in yellow, In shown in blue, and Bi shown in orange. Panels
b−d depict the structures of Na Ca In Sb , Na CaGaSb , and
15
3
5
12
4
3
Na Ca Al Sb , where the frameworks include the tetrahedrally
1
1
2
3
8
coordinated Na atoms. To guide the eye, the In−Sb, Ga−Sb, and
Al−Sb bonds are colored with the respective elemental colorsblue,
pink, and redand the Na−Sb “bonds” are drawn as gray cylinders.
(
note that, for simplicity, the defects on the In sites are not
+
considered here) with Na cations filling the channels within it.
Figure 5b depicts the structure of Na Ca In Sb , where the
15
3
5
12
Na−Sb bonds (tetrahedral ones) are shown to guide the eye, as
well as the bonds between In and Sb (differentiated in light
blue). By comparing the two images, one can see that
Na Ca In Sb , reformulated as Na Ca (In Na )Sb , or
1
5
3
5
12
9
3
5
6
12
Figure 4. Structural relationships among Na Ca In Sb ,
1
5
3
5
12
notwithstanding the defects on the In site as (Na0.75Ca0.25)-
In Na )Sb, is a variant of the same structure; the “coloring”
Na CaGaSb , and Na Ca Al Sb . Panel a shows the idealized,
4
3
11
2
3
8
(
0.5
0.5
defect-free [InSb ] polyanionic layers in Na Ca In Sb ; panel b
2
15
3
5
12
of the atomic positions describing the framework is different.
Figure 5c shows Na CaGaSb drawn in a similar manner [can
depicts the hypothetical hybrid [Ga2/3Na1/3Sb ] layers in Na CaGaSb ;
2
4
3
panel c shows the imaginary [Al1/2Na1/2Sb ] layers in Na Ca Al Sb .
4
3
2
11
2
3
8
In all drawings, two different orientations are shown, with the
be reformulated as Na Ca(GaNa )Sb , or rather (Na0.67Ca0.33)-
2 2 3
tetrahedra rendered in green corresponding to the NaSb units.
(Ga0.33Na0.67)Sb]. Figure 5d is a representation of the
4
F
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. Calculated total and partial DOS curves for Na Ca In Sb (a), Na CaGaSb (b), and Na Ca Al Sb (c). The Fermi level is the energy
15
3
5
12
4
3
11
2
3
8
reference at 0 eV.
Figure 7. COHP curves for In−Sb, Ga−Sb, and Al−Sb (black lines) and for selected Na−Sb interactions (red lines). Panel a refers to
Na Ca In Sb , panel b refers to Na CaGaSb , and panels c and d refer to Na Ca Al Sb . In plots of the COHP curves, the COHP values were
15
3
5
12
4
3
11
2
3
8
inverted (i.e., −COHP is shown) so that the positive and negative regions represent bonding and antibonding states, respectively. The Fermi level is
the energy reference at 0 eV.
Na Ca Al Sb structure, where the relationship to the “parent”
of the conduction band, which indicates small-gap semi-
conductor properties. Compared with that, the band gap in
Na CaGaSb is larger, as expected for a compound with the less
11
2
3
8
is the most complicated among all three, yet composing similar
Al−Na−Sb hybrid layers allows for the structure to be viewed
as Na Ca (Al Na )Sb , or (Na Ca )(Al Na )Sb if
4
3
30
electronegative Ga (χGa = 1.6; χ = 1.7 on the Pauling scale ).
6
2
3
5
8
0.75
0.25
0.375
0.625
In
normalized to the 1:1:1 stoichiometry. Note also that, for
ease of viewing, the ordering of the Na and Ca atoms filling the
channels (in all cases) has been exaggerated.
Similarly, the band gap widens a bit more for Na Ca Al Sb
1
1
2
3
8
30
(χAl = 1.5 on the Pauling scale ). Such a comparison of the
calculated band gaps, of course, should be taken with caution
because the three compounds under consideration are not
isotypic and there are marked differences in their structures. We
also need to acknowledge the relatively small amount of the Tr
element in the structures and the fact that Sb and Na/Ca
electronic states dominate on both sides of the band gap; in
that sense, the notion that the choice of the Tr element will
strongly influence the size of the band gap can be easily refuted.
The calculated band gaps vary from 0.05 to 0.75 eV, which
could mean that some of these compounds (or their properly
designed solid solutions) could be potentially good thermo-
electric materials, a speculation that comes about from
recognizing that similar antimonide compounds are well-
Electronic Structure. The calculated DOS are presented in
Figure 6; the COHPs for selected atom pairs are projected in
Figure 7. For all three structures, idealized models were
considered, whereby the mixed-occupied Na/Ca positions were
treated as either Na or Ca, and then the Fermi level was
adjusted to the correct number of valence electrons per formula
unit. For Na Ca In Sb , where in addition to the Na/Ca
15
3
5
12
disorder partial occupancy on the In site is also observed (vide
supra), we used the idealized, stoichiometric formula
26
Na CaInSb (as in the archetype K BaCdSb ) and then
2
2
2
2
integrated the DOS for the experimental number of valence
electrons and placed the Fermi level accordingly.
6
−8
As shown in Figure 6, the DOS plot of Na Ca In Sb is
known as high-efficiency thermoelectric materials.
The states just below the Fermi levels in Na Ca In Sb are
1
5
3
5
12
much simpler than those of Na CaGaSb and Na Ca Al Sb ,
4
3
11
2
3
8
15
3
5
12
which is apparently due to the greater complexity of the latter
two structures. As discussed before, there are only six unique
atoms in the asymmetric unit of the Na Ca In Sb structure,
predominately from the contribution of the Sb 5p orbital.
Judging from the COHPs, the contribution from the Ca and Na
states is limited, which means their roles are largely of electron
donors, as in the classical Zintl description. This is not quite
true for Na CaGaSb and Na Ca Al Sb , where the Sb 5p
1
5
3
5
12
while Na CaGaSb and Na Ca Al Sb have nearly three times
4
3
11
2
3
8
as many. Notwithstanding the differences in the general
appearance of the curves, one can notice that there is a small
energy gap between the top of the valence band and the bottom
4
3
11
2
3
8
states still have the major contribution; however, the relative
contributions from Na and Ca to the total DOS appear to
G
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
increase. Given that there are Na/Ca atoms in tetrahedral voids
Author Contributions
(
recall the interlayer Na vs intralayer Na atoms; Figure 3), the
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
implied higher degree of covalence is not difficult to
understand. We can relate this finding to the previously
published Ba Cd Sb and Ba ZnSb , for example, in which
the Ba atoms, despite their very low electronegativity (χ = 0.9
on the Pauling scale ), also have important contributions to
the DOS near the Fermi levels.
39
3
5
2
2
2
2
Notes
Ba
The authors declare no competing financial interest.
30
ACKNOWLEDGMENTS
■
Last, we turn our attention to the COHP plots for
S.B. gratefully acknowledges financial support from the U.S.
Department of Energy, Basic Energy Sciences, through Grant
DE-SC0008885. We thank N.-T. Suen for discussions and help
with the electronic structure calculations.
Na Ca In Sb , Na CaGaSb , and Na Ca Al Sb presented
15
3
5
12
4
3
11
2
3
8
in Figure 7. From the graphs, it can be surmised that all In−Sb,
Ga−Sb, and Al−Sb interactions are fully optimized at the Fermi
level. This is expected because optimization of the interactions
(
at least within the anionic substructure) is a typical feature of
REFERENCES
■
the classic Zintl compounds. Considering the Na−Sb
interactions involving the intralayer Na atoms, it becomes
obvious that, for both Na CaGaSb and Na Ca Al Sb , the
(
1) Schaf
̈
er, H.; Eisenmann, B.; Muller, W. Angew. Chem., Int. Ed.
Engl. 1973, 12, 694−712. (b) Corbett, J. D. Angew. Chem., Int. Ed.
4
3
11
2
3
8
2000, 39, 670−690.
Na−Sb bonding states are also filled at the Fermi level; i.e., the
bonding is again optimized. Compared with the Ga−Sb and
Al−Sb COHPs, however, it is noticeable that the Na−Sb
interactions are much weaker. For instance, in Na CaGaSb , the
integrated COHPs of the Na−Sb bonds (within the Na2
tetrahedra) range from 0.024 to 0.036, more than 5 times
smaller than the integrated COHPs of the Ga−Sb bonds, which
range from 0.143 to 0.217.
(2) (a) Kauzlarich, S. M., Ed. Chemistry, Structure and Bonding in Zintl
Phases and Ions; VCH: New York, 1998; and references cited therein.
(
b) Miller, G. J. Eur. J. Inorg. Chem. 1998, 523−536.
(
(
(
(
3) Xia, S.-Q.; Bobev, S. Inorg. Chem. 2008, 47, 1919−1921.
4) Xia, S.-Q.; Bobev, S. J. Am. Chem. Soc. 2007, 129, 4049−4057.
5) Saparov, B.; Bobev, S. Inorg. Chem. 2010, 49, 5173−5179.
6) Brown, S. R.; Kauzlarich, S. M.; Gascoin, F.; Snyder, G. J. Chem.
4
3
Mater. 2006, 18, 1873−1877.
(7) Sales, B. C.; Mandrus, D.; Williams, R. K. Science 1996, 272,
1
(
325−1328.
8) Gascoin, F.; Ottensmann, S.; Stark, S.; Haïle, S. M.; Snyder, G. J.
CONCLUSIONS
■
Adv. Funct. Matter. 2005, 15, 1860−1864.
The three new compounds Na Ca In Sb , Na CaGaSb , and
(9) Andersen, O. K. Phys. Rev. B: Condens. Matter 1975, 12, 3060−
1
5
3
5
12
4
3
Na Ca Al Sb are the first structurally characterized phases in
3083.
1
1
2
3
8
(
10) Villars, P., Calvert, L. D., Eds. Pearson’s Handbook of
the respective quaternary Na−Ca−Tr−Sb systems (Tr = Al,
Ga, In). Their structures boast different structure types and
feature the following: two-dimensional [InSb ] layers in
Crystallographic Data for Intermetallic Compounds, 2nd ed.; American
Society for Metals: Materials Park, OH, 1997.
2
(11) Wang, Y.; Bobev, S. Unpublished results. The corresponding
Na Ca In Sb ; one-dimensional [GaSb ] helical chains in
1
5
3
5
12
3
CIF is provided as SI.
Na CaGaSb . In the Na Ca Al Sb structure, one finds isolated
4
3
11
2
3
8
̈
(12) Cordier, G.; Schafer, H.; Stelter, M. Z. Naturforsch. 1985, B40,
[
Al Sb ] trimeric species. All three structures are related to the
5−8.
3
8
ubiquitous TiNiSi structure type. The study of the close
structural relationships is greatly facilitated by considering some
of the Na atoms in ambivalent roles, i.e., as contributing to the
covalent bonding and forming complex Na−Al−Sb and Na−
Ga−Sb slabs. The computed electronic structures show narrow
band gaps at the Fermi level, suggesting that the title
compounds could be considered as Zintl phases. The band
gaps open in order of reduced dimensionality on the order
from the In to the Ga to the Al compound. The expected
intrinsic semiconducting properties could be of relevance to the
development of new thermoelectric materials.
(13) Cordier, G.; Ochmann, H. Z. Kristallogr. 1991, 195, 107−108.
(
(
(
14) Deller, K.; Eisenmann, B. Z. Naturforsch. 1976, B31, 29−34.
15) Cromer, D. T. Acta Crystallogr. 1959, 12, 41−45.
16) (a) SMART; Bruker AXS Inc.: Madison, WI, 2003. (b) SAINT;
Bruker AXS Inc.: Madison, WI, 2003.
(
(
17) SADABS; Bruker AXS Inc.: Madison, WI, 2003.
18) SHEXLTL; Bruker AXS Inc.: Madison, WI, 2003.
(19) Jepsen, O.; Burkhardt, A.; Andersen, O. K. The TB-LMTO-ASA
Program, version 4.7; Max-Planck Institut fur Festkorperforschung:
Stuttgart, Germany, 1999.
20) Andersen, O. K.; Jepsen, O. Phys. Rev. Lett. 1984, 53, 2571−
574.
21) Andersen, O. K.; Pawlowska, Z.; Jepsen, O. Phys. Rev. B:
Condens. Matter 1986, 34, 5253−5269.
(22) Dronskowski, R.; Blochl, P. E. J. Phys. Chem. 1993, 97, 8617−
624.
23) Barth, U.; Hedin, L. J. Phys. C: Solid State Phys. 1972, 5, 1629−
1642.
(24) Andersen, O. K.; Jepsen, O.; Glo
̈
̈
(
2
(
ASSOCIATED CONTENT
̈
■
8
*
S
Supporting Information
(
X-ray crystallographic files in CIF format, additional details of
the structural work, and figures showing structural representa-
tions with anisotropic displacement parameters and other
structural relationships. This material is available free of charge
via the Internet at http://pubs.acs.org.
̈
tzel, D. In Highlights of
Condensed Matter Theory; Bassani, F., Fumi, F., Tosi, M., Eds.; North-
Holland: New York, 1985.
(
25) Blo
Matter 1994, 49, 16223−16233.
26) Saparov, B.; Saito, M.; Bobev, S. J. Solid State Chem. 2011, 184,
432−440.
27) The one-electron difference between Cd and In, while
̈
chl, P. E.; Jepsen, O.; Andersen, O. K. Phys. Rev. B: Condens.
(
AUTHOR INFORMATION
■
*
8
(
Corresponding Author
E-mail: bobev@udel.edu. Phone: (302) 831-8720. Fax: (302)
31-6335.
maintaining an ideal valence electron count, is compensated for by
defects on the In site and the concomitant nearly equal cooccupation
by Ca and Na on one of the cation sites. See the SI for further details.
H
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
(
28) Xia, S.-Q.; Bobev, S. Chem.Asian J. 2007, 2, 619−624.
29) Cordier, G.; Schafer, H.; Stelter, M. Z. Naturforsch. 1985, B40,
̈
8
(
58−871.
30) Pauling, L. The Nature of the Chemical Bond; Cornell University
Press: Ithaca, NY, 1960.
31) The integrated COHP values nicely correlate with the bond
(
distances. For example, for the shortest Ga1−Sb1 bond, the integrated
COHP value is calculated to be 0.217. Considering Ga2, the Ga2−Sb5
bond is comparably short and has an integrated COHP value of 0.211.
The longer bonds are weaker, and their integrated COHP values are
0
(
2
(
2
(
1
(
1
(
1
(
.143 for Ga1−Sb4 and 0.160 for Ga2−Sb4.
32) Cordier, G.; Ochmann, H.; Schafer, H. Mater. Res. Bull. 1986,
1, 331−336.
33) ICSD Database; Fachinformationszentrum: Karlsruhe, Germany,
014.
34) Cordier, G.; Czech, E.; Jakowski, M.; Schaf
981, 18, 9−18.
35) Cordier, G.; Ochmann, H.; Schaf
984, 517, 118−124.
36) Saparov, B.; Xia, S.-Q.; Bobev, S. Inorg. Chem. 2008, 47, 11237−
1244.
37) Saparov, B.; Broda, M.; Ramanujachary, K. V.; Bobev, S.
̈
̈
er, H. Rev. Chim.
̈
er, H. Z. Anorg. Allg. Chem.
Polyhedron 2010, 29, 456−462.
(
(
38) Bobev, S.; Sevov, S. C. J. Solid State Chem. 2001, 163, 436−448.
39) Saparov, B.; He, H.; Zhang, X. H.; Greene, R.; Bobev, S. Dalton
Trans. 2010, 39, 1063−1070.
I
DOI: 10.1021/ic502822v
Inorg. Chem. XXXX, XXX, XXX−XXX