Synthesis and structural characterization of the ternary Zintl phases AE3Al2Pn4 and AE3Ga 2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As)

Article abstract of DOI:10.1016/j.jssc.2012.01.042

Ten new ternary phosphides and arsenides with empirical formulae AE ₃Al₂Pn₄ and AE₃Ga₂Pn ₄ (AE=Ca, Sr, Ba, Eu; Pn=P, As) have been synthesized using molten Ga, Al, and Pb fluxes. They have been structurally characterized by single-crystal and powder X-ray diffraction to form with two different structures - Ca₃Al₂P₄, Sr₃Al ₂As₄, Eu₃Al₂P₄, Eu ₃Al₂As₄, Ca₃Ga₂P ₄, Sr₃Ga₂P₄, Sr₃Ga ₂As₄, and Eu₃Ga₂As₄ crystallize with the Ca₃Al₂As₄ structure type (space group C2/c, Z=4); Ba₃Al₂P₄ and Ba ₃Al₂As₄ adopt the Na₃Fe ₂S₄ structure type (space group Pnma, Z=4). The polyanions in both structures are made up of TrPn₄ tetrahedra, which share common corners and edges to form [TrPⁿ²]32 layers in the phases with the Ca₃Al₂As₄ structure, and [TrP ⁿ²]31 chains in Ba₃Al₂P₄ and Ba ₃Al₂As₄ with the Na₃Fe ₂S₄ structure type. The valence electron count for all of these compounds follows the ZintlKlemm rules. Electronic band structure calculations confirm them to be semiconductors.

Full text of DOI:10.1016/j.jssc.2012.01.042

Journal of Solid State Chemistry 188 (2012) 59–65
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Synthesis and structural characterization of the ternary Zintl phases
AE3Al2Pn4 and AE3Ga2Pn4 (AE¼Ca, Sr, Ba, Eu; Pn¼P, As)
Hua He, Chauntae Tyson, Maia Saito, Svilen Bobev n
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Ten new ternary phosphides and arsenides with empirical formulae AE3Al2Pn4 and AE3Ga2Pn4 (AE¼Ca,
Sr, Ba, Eu; Pn¼P, As) have been synthesized using molten Ga, Al, and Pb fluxes. They have been
structurally characterized by single-crystal and powder X-ray diffraction to form with two different
structures—Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4
crystallize with the Ca3Al2As4 structure type (space group C2/c, Z¼4); Ba3Al2P4 and Ba3Al2As4 adopt the
Na3Fe2S4 structure type (space group Pnma, Z¼4). The polyanions in both structures are made up of
Received 14 November 2011
Received in revised form
19 January 2012
Accepted 23 January 2012
Available online 30 January 2012
TrPn4 tetrahedra, which share common corners and edges to form 21½TrPn2ꢀ layers in the phases with
32
Keywords:
1
1
Crystal structure
32
the Ca3Al2As4 structure, and ½TrPn2ꢀ chains in Ba3Al2P4 and Ba3Al2As4 with the Na3Fe2S4 structure
Single-crystal X-ray diffraction
Electronic structure calculations
Zintl phases.
type. The valence electron count for all of these compounds follows the Zintl–Klemm rules. Electronic
band structure calculations confirm them to be semiconductors.
& 2012 Elsevier Inc. All rights reserved.
1. Introduction
fruitful [13–16], not long ago, we embarked on investigations of
the corresponding arsenide and phosphide systems. For the
In recent years, there have been numerous reports on the
crystal chemistry and physical properties of ternary pnictides in
the systems AE–Tr–Pn (AE¼Ca, Sr, Ba, Eu, Yb; Tr¼Al, Ga, In; and
Pn¼P, As, Sb). Examples include BaGa2Sb2 [1], Yb5Al2Sb6 [2],
Ba2In5As5 [3], Eu3InP3 [4], Eu3In2P4 [5], EuIn2P2 [6], EuGa2As2 [7],
Ca3AlSb3 [8] and Ca5Al2Sb6 [9], to name just a few. Almost
exclusively, such compounds can be classified as Zintl phases
[10], where the alkaline-earth metals are the ‘‘cations’’ and they
donate their valence electrons to the post-transition elements,
which, in turn, form covalent bonds within (poly)anionic sub-
structure. The electron transfer is typically considered to be
‘‘complete’’, and all constituent atoms achieve closed-shell
configurations [10,11]. These are desirable characteristics in
thermoelectrics development, and many research groups are
turning their attention to Zintl phases as candidate materials for
solid-state energy conversion. Recent papers have already
demonstrated the favorable balance of charge and heat-transport
properties for the compounds Yb5Al2Sb6 [2], and Ca3AlSb3 [8];
EuIn2As2 [12] can be cited as an example showing colossal
magnetoresistance.
synthesis of new compounds with novel structures, by and large,
we have focused on the metal flux method [17,18], since the triel
elements Ga and In are particularly well-suited for such endea-
vors. Published structures from our prior systematic work include
BaGa2Pn2 (Pn¼P, As) [19], and CaGa2P2, CaGa2As2, and SrGa2As2
[20], which all crystallize with different structures. Attempts to
extend the ‘‘1–2–2’’ chemistry to the AE–Al–Pn system led to the
identification of two series of new compounds AE3Ga2Pn4 and
AE3Al2Pn4, which are the subject of this paper. Herein, we present
the ten newly synthesized compounds—Ba3Al2P4 and Ba3Al2As4 with
the Na3Fe2S4 structure type [21], and Ca3Al2P4, Sr3Al2As4, Eu3Al2P4,
Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4, isostruc-
tural with the previously reported Ca3Al2As4 [22] and Sr3Al2P4 [23].
Their bonding characteristics are elaborated and the topological
relationships to the structures of compounds with formulae AE3TrPn3
are discussed as well. The electronic band structures, calculated with
the aid of the TB-LMTO method [24], are also discussed.
2. Experimental
Our research group has previously explored considerable
sections of the ternary AE–Ga–Sb and AE–In–Sb phase diagrams
(AE¼Ca, Sr, Ba, Eu and Yb). Since these early studies had proven
2.1. Synthesis
All synthetic and post-synthetic manipulations were per-
formed inside an argon-filled glove box or under vacuum. The
elements with stated purity greater than 99.9 wt% were pur-
chased from either Alfa Aesar or Aldrich and used as received.
n Corresponding author. Fax: þ1 302 831 6335.
E-mail address: bobev@udel.edu (S. Bobev).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2012.01.042

60
H. He et al. / Journal of Solid State Chemistry 188 (2012) 59–65
All initial reactions were done in a manner, consistent with the
synthesis of CaGa2P2, CaGa2As2, and SrGa2As2 [20], where excess
Ga was utilized as a reactive flux; details of the metal flux method
can be found elsewhere [17,18]. Such reactions were aimed at the
‘‘missing’’ members of any of the three structures above, but by
serendipity afforded crystals of the new compounds Ca3Ga2P4 and
Sr3Ga2P4. Subsequent to the structure elucidation by single-crystal
X-ray diffraction, the syntheses of these three compounds were
replicated using mixtures of the corresponding elements with the
proper stoichiometric AE:Pn ratio of 3:4 and a 25-fold excess of Ga.
The mixtures of the elements were loaded in alumina crucibles,
which were then flame-sealed in fused silica tubes under vacuum.
The evacuated silica tubes were heated at 960 1C for 20 h, and then
cooled to 500 1C at a rate of 5 1C/h. The molten Ga was decanted at
this temperature. Previous successful application of Pb flux for the
crystal growth of other AE–Ga–Pn ternary compounds [20]
prompted us to undertake Pb flux reactions again. They were
done in a similar way, starting with the corresponding elements in
the 3:2:4 ratio and a large amount of Pb (20-fold excess)—large
single-crystals of Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4
were obtained from such experiments.
BaGa2P2 and BaGa2As2 [19], or Ba7Ga4P9 and Ba7Ga4As9 [25],
which are isotypic to Ba7Ga4Sb9 [26]. Ca3Ga2As4 and Eu3Ga2P4 are
also still unknown and could not be obtained under the studied
experimental conditions. In these cases, crystals of CaGa2As2 [19],
Ca14GaAs11 [27], and EuGa4 [28] were obtained. Analogous reac-
tions with Yb afforded only the binary phases YbP, YbAs, GaAs,
and GaP [28].
Switching to aluminum as a reactive flux proved to be somewhat
more challenging, although we identified six new compounds—
Ba3Al2P4, Ba3Al2As4, Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, and Eu3Al2As4.
First, the higher melting point of Al metal (660 1C) [29] required the
corresponding reactions to be equilibrated at 960 1C for 20 h, and
then cooled to 750 1C. Second, since Al is a strong reducing agent and
Al vapors are known to react quickly with SiO2, extreme care had to
be exercised when the ampoules were removed from the furnace
and the molten Al was decanted. Third, yields of the title compounds
were lower, and AlP or AlAs [28] were common side products; the
crystals were also smaller compared to the crystals obtained from
analogous Ga flux reactions.
The crystals exhibit characteristic colors, indicative of them
being intrinsic semiconductors: Ca3Al2P4 is orange; Sr3Al2As4,
Ca3Ga2P4, Sr3Ga2P4, and Ba3Al2P4 are red; Eu3Al2P4 is dark brown;
and Eu3Al2As4, Eu3Ga2As4, Sr3Ga2As4, and Ba3Al2As4 are black.
Attempts to synthesize isotypic Ba3Ga2P4 and Ba3Ga2As4
compounds were unsuccessful—the outcome, depending on the
AE:Pn ratio (1:2 or 3:4), and the choice of flux (Ga or Pb) were
2.2. Powder X-ray diffraction
Table 1
X-ray powder diffraction patterns were collected at room
Selected single-crystal data collection and structure refinement parameters for
Ba3Al2Pn4 (Pn¼P, As).
temperature on a Rigaku MiniFlex powder diffractometer using
˚
filtered CuK
a radiation (l¼1.5418 A). The diffractometer was
Empirical formula
Ba3Al2As4
765.66
Ba3Al2P4
enclosed and operated inside a glove-box. The observed peak-
positions and the peaks’ relative intensities, analyzed using the
JADE 6.5 software package, matched well with those calculated
from the single-crystal work. According to powder patterns
collected for specimens kept under inert atmosphere and to those
collected after being exposed to air, all of the title compounds are
unstable at ambient conditions. The air-sensitivity was found to
vary considerably, with the Ba-containing samples being the most
reactive and the Eu-containing samples being the least sensitive,
respectively.
Formula weight
Temperature
589.86
200(2) K
MoK
Radiation,
l
˚
a
, 0.71073 A
Space group, Z
Pnma (No. 62), 4
7.2540(5)
7.425(2)
˚
a (A)
11.784(3)
11.842(3)
1036.0(5)
11.5416(8)
11.5770(8)
969.26(12)
˚
b (A)
˚
c (A)
3
˚
V (A )
rcal (g/cm3)
4.909
4.042
m
(cm–1
)
240.49
128.13
Goodness-of-fit on F2
1.079
1.106
a
R1 (I42sI
wR2 (I42sI
Largest diff. peak/hole (eꢁ/A
)
0.0348
0.0189
2.3. Single-crystal X-ray diffraction
a
)
0.0664
0.0394
ꢁ3
2.133/ꢁ1.453
1.143/ꢁ0.777
˚
)
Single-crystal X-ray diffraction data were collected on a Bruker
P
P
P
P
R1¼ 99Fo9–9Fc99/ 9Fo9; wR2¼[ [w(F2o–Fc2)2]/ [w(Fo2)2]]1/2
,
where w¼1/
SMART CCD-based diffractometer, employing monochromated MoK
a
a
[
s
2F2o þ(AP)2], and P¼(Fo2þ2F2c)/3; A—weight coefficient.
˚
radiation (
l
¼0.71073 A). Crystals from freshly prepared samples
Table 2
Selected single-crystal data collection and structure refinement parameters for AE3Al2Pn4 (AE¼Ca, Sr, Eu; Pn¼P, As).
Empirical formula
Ca3Al2P4
298.08
Sr3Al2As4
616.5
Eu3Al2P4
633.72
Eu3Al2As4
Ca3Ga2P4
Sr3Ga2P4
526.18
Sr3Ga2As4
701.98
Eu3Ga2As4
895.00
Formula weight
Temperature
809.52
383.56
200(2) K
MoK , 0.71073 A
C2/c (No. 15), 4
13.404(2) 12.623(3)
Radiation,
l
˚
a
Space group, Z
12.6236(11)
9.8358(8)
6.4469(6)
13.5017(16)
10.4565(13)
6.8109(8)
13.0304(17)
10.0974(13)
6.5857(8)
13.1522(13)
10.1989(10)
6.6290(7)
13.497(3)
10.497(2)
6.7903(16)
13.386(4)
10.426(3)
6.732(2)
˚
a (A)
10.3935(17
6.7502(11)
9.870(2)
˚
b (A)
6.4435(15)
˚
c (A)
90.454(1)
90.457(2)
961.5(2)
90.607(2)
90.023(2)
940.4(3)
91.050(3)
802.6(3)
90.314(1)
90.275(4)
962.0(4)
90.742(4)
939.5(5)
b
(1)
3
800.44(12)
866.45(19)
889.19(16)
˚
V (A )
rcal (g/cm3)
2.474
29.82
1.053
0.0219
0.0464
4.259
4.858
5.718
3.174
93.24
1.037
0.0187
0.0419
3.931
4.847
6.327
m
(cm–1
)
303.42
0.993
222.97
1.195
338.31
1.038
244.90
1.000
356.25
1.001
392.88
1.159
Goodness-of-fit on F2
a
R1 (I42sI
wR2 (I42sI
)
0.0325
0.0528
0.0161
0.0362
0.0164
0.0352
0.0248
0.0507
0.0222
0.0492
0.0185
0.0430
a
)
–
–3
0.429/ꢁ0.361 1.102/ꢁ1.139 0.773/ꢁ1.075 0.878/ꢁ0.795 0.437/ꢁ0.467 0.832/ꢁ0.875 0.934/ꢁ0.900 0.969/ꢁ1.004
˚
Largest diff. peak and hole (e /A
)
P
P
P
P
a
R1¼ 99Fo9–9Fc99/ 9Fo9; wR2¼[ [w(F2o ꢁFc2)2]/ [w(F2o)2]]1/2, where w¼1/[
s
2F2oþ(AP)2þ(BP)], and P¼(F2o þ2F2c)/3; A,B—weight coefficients.

H. He et al. / Journal of Solid State Chemistry 188 (2012) 59–65
61
were selected under a microscope in the glove-box and cut to suitable
Table 5
Selected interatomic distances (A) and angles (1) of Ba3Al2Pn4 (Pn¼P, As).
˚
sizes (ca. 0.1 mm in all dimensions). The crystals were then mounted
on glass fibers using Paratone-N oil. The operating temperature was
200(2) K, maintained by a cold nitrogen stream. This method for
handling the crystals alleviated the issue with their air-sensitivity.
After ensuring the crystal quality, full spheres of data were collected
Ba3Al2P4
Ba1–
Ba3Al2As4
Ba1–
P3
3.1412(10)
3.2254(11)
3.3127(10)
3.3979(11)
3.4656(10)
3.5080(10)
3.1513(13)
3.1708(13)
3.1991(9)
3.3718(9)
2.3718(15)
2.3802(14)
2.4314(15)
2.4444(14)
2.888(2)
As3
3.2085(12)
P3
As3
3.2857(12)
3.4007(12)
3.4593(12)
3.5316(12)
3.5510(12)
3.2138(15)
3.2340(14)
3.2636(11)
3.4047(11)
2.456(3)
P1
As1
in four batch runs with frame width of 0.41 for
o and y. Data
P2
As2
collection and data integration were done using SMART and SAINT-
plus programs, respectively [30]. SADABS was used for semi-empirical
absorption correction based on equivalent reflections [31]. The
structures were solved by direct methods and refined by full matrix
least squares on F2 using SHELXL [32]. In the last refinement cycles,
the unit cell axes and the atomic coordinates were standardized with
the aid of the Structure TIDY program [33]. All crystal data and
refinement parameters are summarized in Tables 1 and 2; positional
and equivalent isotropic displacement parameters for representative
structures are listed in Tables 3 and 4. Selected interatomic distances
and angles are tabulated in Tables 5 and 6. CIFs have also been
deposited with Fachinformationszentrum Karlsruhe, 76344 Eggen-
stein-Leopoldshafen, Germany (fax: þ49 7247-808-666; e-mail:
hcrysdata@fiz.karlsruhe.de) with depository numbers: CSD-423778
for Ba3Al2P4, CSD-423779 for Ba3Al2As4, CSD-423780 for Ca3Al2P4,
CSD-423781 for Ca3Ga2P4, CSD-423782 for Sr3Ga2P4, CSD-423783 for
Eu3Al2P4, CSD-423784 for Eu3Ga2As4, CSD-423785 for Sr3Ga2As4,
CSD-423786 for Eu3Al2As4, and CSD-423787 for Sr3Al2As4.
P1
As1
P1
As1
Ba2–
Al–
P2
Ba2–
Al–
As2
P2
As2
P1 ꢂ (2)
P1 ꢂ (2)
P3
As1 ꢂ (2)
As1 ꢂ (2)
As3
P1
As1
2.470(2)
P2
As2
2.521(3)
P1
As1
2.539(2)
Al
Al
2.929(5)
Al
3.122(2)
Al
3.220(4)
P3–Al–P1
P3–Al–P2
P1–Al–P2
P3–Al–P1
P1–Al–P1
P2–Al–P1
115.24(5)
100.16(5)
115.08(5)
111.45(6)
99.36(5)
As3–Al–As1
As3–Al–As2
As1–Al–As2
As3–Al–As1
As1–Al–As1
As2–Al–As1
115.05(9)
101.71(9)
113.71(9)
110.84(10)
100.01(9)
116.15(9)
116.33(5)
Table 6
˚
Selected interatomic distances (A) and angles (1) of Sr3Al2As4.
2.4. Electronic structure calculations
Sr1–
As2
3.0649(9)
3.1035(10)
3.1912(10)
3.2764(9)
3.3487(10)
3.5317(10)
3.535(2)
Al–
As2
As2
As1
As1
Al
2.461(2)
2.471(2)
2.505(2)
2.533(2)
3.127(2)
101.36(7)
114.84(8)
111.78(7)
107.34(7)
118.70(8)
103.27(7)
The Stuttgart TB-LMTO 4.7 program [34] was used to calculate
band structures of Ba3Al2P4, Ba3Al2As4, and Sr3Al2As4. Local
density approximation (LDA) was used to treat exchange and
correlations [35]. All relativistic effects except for spin–orbital
As2
As1
As1
As1
As1
As2–Al–As2
As2–Al–As1
As2–Al–As1
As2–Al–As1
As2–Al–As1
As1–Al–As1
Al
Table 3
Sr2–
As2 ꢂ (2)
As2 ꢂ (2)
As1 ꢂ (2)
Al ꢂ (2)
3.0700(7)
3.1054(9)
3.2315(9)
3.5907(19)
a
2
˚
Atomic coordinates and equivalent isotropic displacement parameters Ueq (A ) of
Ba3Al2Pn4 (Pn¼P, As).
Atom
Wyckoff Site
x
y
z
Ueq
Ba3Al2P4
Ba1
Ba2
Al
coupling were taken into account by the scalar relativistic
approximation [36]. The basis set included the 5d, 6s, and 6p
orbitals for Ba; 4d, 5s, and 5p orbitals for Sr, 3s, 3p, and 3d orbitals
for Al; 4s, 4p, and 4d orbitals for As, and 3s, 3p, and 3d orbitals for
P. The 6p orbital of Ba, 5p orbital of Sr, 3d orbital of Al, 4d orbital
of As, and 3d orbital of P were treated with the down-folding
technique [37]. The k-space integrations were performed by the
tetrahedron method [38], using 225-294 irreducible k-points in
the Brillouin zone. The total and partial density of states (DOS)
were computed and studied. To interrogate the chemical bonding,
crystal orbital Hamilton populations (COHP) [39] of selected
interactions were also analyzed.
8d
4c
8d
8d
4c
4c
0.0788(1)
0.0961(1)
0.0591(1)
0.1537(1)
0.3084(2)
0.4101(2)
0.0775(1)
1/4
0.1387(1)
0.7656(1)
0.4638(1)
0.3871(1)
0.5282(1)
0.1693(1)
0.012(1)
0.011(1)
0.010(1)
0.011(1)
0.011(1)
0.010(1)
0.1249(1)
0.5571(1)
1/4
P1
P2
P3
1/4
Ba3Al2As4
Ba1
8d
4c
8d
8d
4c
4c
0.0812(1)
0.0929(1)
0.0598(3)
0.1587(1)
0.3154(2)
0.4084(2)
0.0760(1)
1/4
0.1377(1)
0.7612(1)
0.4623(2)
0.3866(1)
0.5284(1)
0.1745(1)
0.013(1)
0.012(1)
0.011(1)
0.012(1)
0.011(1)
0.011(1)
Ba2
Al
0.1257(2)
0.5594(1)
1/4
As1
As2
As3
1/4
a
Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
3. Results and discussion
Table 4
a
2
˚
Atomic coordinates and equivalent isotropic displacement parameters Ueq (A ) of
Sr3Al2As4. The refined coordinates for the isostructural AE3Al2Pn4 (AE¼Ca, Eu;
Pn¼P, As) and Sr3Ga2As4 are provided as supporting information.
3.1. Structure of Ba3Al2P4 and Ba3Al2As4
Ba3Al2P4 and Ba3Al2As4 are isotypic, and crystallize with the
orthorhombic space group Pnma (No. 62, Pearson symbol oP36)
[28]. This structure formally belongs to the Na3Fe2S4 structure
type, which has been discussed in an earlier publication [21]. The
asymmetric unit of the structure contains two Ba, one Al, and
three pnictogen atoms, located at either the general site 8d or the
special position 4c (Table 3). Al and Pn atoms constitute AlPn4
Atom
Wyckoff Site
x
y
z
Ueq
Sr3Al2As4
Sr1
8f
4e
8f
8d
8f
0.1219(1)
0
0.1295(1)
0.4015(1)
0.1227(2)
0.1755(1)
0.0822(1)
0.5383(1)
1/4
0.011(1)
0.009(1)
0.009(1)
0.009(1)
0.009(1)
Sr2
Al
0.2952(1)
0.1163(1)
0.3412(1)
0.0811(3)
0.0246(1)
0.4258(1)
As1
As2
1
1
32
tetrahedra, which by sharing common edges form ½AlPn4=2
chains, as shown in Fig. 1. These chains of edge-shared tetrahedral
ꢀ
a
Ueq is defined as one third of the trace of the orthogonalized Uij tensor.

62
H. He et al. / Journal of Solid State Chemistry 188 (2012) 59–65
units run along the b axis with Ba2þ cations separating them. The
chains are similar to those commonly found in the structures of
the silicon dichalcogenides [40]; they are also seen in Zintl
compounds, such as Ba2ZnPn2 [41] and Na3AlAs2 [42]. However,
symmetry site (4c) than the Ba1 site (8d). On average, the Ba2–Pn
˚
distances are slightly shorter than the Ba1–Pn distances—3.244(1) A
˚
˚
˚
vs. 3.342(1) A in Ba3Al2P4 and 3.297(1) A vs. 3.406(1) A in Ba3Al2As4.
In the second coordination sphere of the Ba atoms, the nearest Al
the ½AlPn2ꢀ32 chains in Ba3Al2P4 and Ba3Al2As4 are not isosteric
˚
neighbors are located more than 3.7 A away, which is longer than the
1
1
˚
˚
sum of their Pauling’s atomic radii (rBa¼2.215 A and rAl¼1.429 A
with the ½SiS2ꢀ chains in SiS2 [40] or the ½ZnPn2ꢀ42 chains in
1
1
1
1
[44]), implying negligible Ba–Al interactions.
Ba2ZnPn2 [41], although they are isoelectronic. Instead, the
½AlPn2ꢀ32 chains ‘‘swivel’’ in space, as if the centers of the AlPn4
1
3.2. Structure of AE3Al2P4 and AE3Ga2P4 (AE¼Ca, Sr, Eu; Pn¼P, As)
1
tetrahedra form a zigzag chain [43], while the Si and Zn atoms in
the former are arranged in straight lines.
The rest of the title compounds, Ca3Al2P4, Sr3Al2As4, Eu3Al2P4,
Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4 all
crystallize with the monoclinic space group C2/c (No. 15, Pearson
symbol mS36) [28]. They are isostructural with the previously
reported Ca3Al2As4 [22] and Sr3Al2P4 [23]. For the sake of
simplicity in the context of direct comparison with Ba3Al2As4,
one of these compounds, Sr3Al2As4, will be discussed as a
representative.
The Al–P and Al–As distances are in the range of 2.372(2)–
˚
2.444(2) and 2.456(3)–2.539(2) A, respectively, which match
closely with the sum of the corresponding Pauling’s single-bond
˚
˚
˚
radii of Al (1.248 A) and P (1.10 A) and As (1.210 A) [44]. These
values also compare well with those reported for other phos-
phides and arsenides with similar bonding patterns: dAl–P
¼
¼
˚
˚
2.376 A in Na3AlP2 [45], dAl–As¼2.507 A in Na3AlAs2 [42], dAl–P
˚
˚
2.377–2.443 A in Sr3Al2P4 [23], dAl–As¼2.503–2.540 A in Ca3AlAs3
[46], suggesting strong covalent bonding character.
The structure of Sr3Al2As4 is based on AlAs4 tetrahedra, which by
2
1
32
both corner- and edge-sharing form ½AlAs2ꢀ
layers (Fig. 2), not
1
1
32
Ba1 and Ba2 atoms are both surrounded by six pnictogens in
distorted octahedral fashion, although the more regular octahedra for
Ba2 should be noted. This is not unusual since Ba2 is a higher
½AlPn2ꢀ chains, as in Ba3Al2As4. The layers feature 4-membered
and 12-membered rings, which are similar to the layers found in the
EuGa2S4 structure [47]. The Al–As distances are again indicative of
Fig. 1. Combined ball-and-stick and polyhedral representations of the crystal structure of Ba3Al2As4, projected along both the a axis and the b axis. The unit cell is outlined.
On-line colors: Al atoms are represented as cyan spheres, the pnictogens are drawn as red spheres, and the Ba atoms are shown as green spheres, respectively. (For
interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 2. Combined ball-and-stick and polyhedral representations of the crystal structure of Sr3Al2As4, projected along both the b axis and the c axis. On-line colors: Al atoms
are represented as cyan spheres, the pnictogens are drawn as red spheres, and the Sr atoms are shown as green spheres, respectively. The unit cell is outlined. (For
interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

H. He et al. / Journal of Solid State Chemistry 188 (2012) 59–65
63
the covalent character of the Al–As bonding. They are comparable
with those in Ba3Al2As4. The Al–As distances in the series Sr3Al2As4–
˚
Eu3Al2As4–Ca3Al2As4 fall in the narrow range 2.415(5)–2.533(2) A,
showing a slight contraction in the presented order. Such decrease
can be correlated with the decrease in the corresponding atomic
˚
˚
˚
sizes (rSr¼2.148 A; rEu¼2.084 A and rCa¼1.970 A [44]).
Two different Sr sites exist in the structure—Sr1 (between the
layers) and Sr2 (within the layers). Sr2 is located in an irregular
octahedron of As atoms with an average Sr–As distance of
˚
3.136 A. On the other hand, Sr1 is only 5-coordinated to As if
only the first coordination sphere is considered. However, if
another As and the closest Al atom are considered, which are
˚
about the same distance away from Sr1, 3.532(1) A and
˚
3.535(2) A, respectively, the coordination environment of Sr1
can be described as a monocapped octahedron. This subtle
difference in the cationic coordination, which seems to be related
to the sizes of the cations (and perhaps to their electronegativities
too [44]), could be a reason for Sr3Al2As4 and Ba3Al2As4 to
crystallize with different structure types. Notice that only
Ba3Al2P4 and Ba3Al2As4 crystallize with the Na3Fe2S4 structure,
and all of the lighter alkaline-earth metal analogs, including
Eu3Al2P4, Eu3Al2As4, Eu3Ga2P4, and Eu3Ga2As4, adopt the
Ca3Al2As4 structure.
Fig. 3. Comparative representation of fragments from the polyanionic sub-
structures of Ba2Al2As4 (Na3Fe2S4 structure type), Sr3Al2As4 (Ca3Al2As4 structure
type), and Sr3In2P4 (own structure type).
Tight-binding linear-muffin-tin-orbital (TB-LMTO-ASA) elec-
tronic structure calculations were carried out for Ba3Al2P4,
Ba3Al2As4, and Sr3Al2As4. The plots of the computed density of
states (DOS) and the crystal orbital Hamilton populations (COHP)
for each compound are shown in Figs. 4 and 5, respectively. Band
gaps can be noticed at the Fermi level in each DOS plot, suggest-
ing that these compounds are intrinsic semiconductors, as
expected for Zintl phases. The sizes of the band gaps in Ba3Al2P4
and Ba3Al2As4 are calculated to be on the order of 1.6 and 1.3 eV,
respectively, which is in good agreement with the difference in
electronegativities of P and As [44]. The band gap of Sr3Al2As4 is
also ca. 1.6 eV. Considering that the LMTO calculations usually
underestimate the band gap in semiconductor, these values
actually correlate reasonably well with the colors of the crystals
(Sr3Al2As4 and Ba3Al2P4 being red; Ba3Al2As4 being dark-to-
black).
In spite of the different structure types, contributions from the
elements to the states around the Fermi level are similar. In the
energy window from ꢁ3.5 eV to the Fermi edge (0 eV), the states
originate predominately from the p orbitals of pnictogens and Al;
a substantial contribution from the d orbitals of the alkaline-earth
metal is also noted. This suggests that the Zintl formalism over-
simplifies the interactions between cations and anions, and the
actual bonding pictures in these compounds could be far more
complicated. The states at lower energy, from ꢁ6 to ꢁ4 eV, are
mainly contributed from the Al s orbitals and pnictogen p orbitals.
However, comparing with states in Ba3Al2P4 and Ba3Al2As4 that
are compressed in very narrow energy range, the states in
Sr3Al2As4 are more dispersive. Since the Al s and pnictogen p
states are dominating in this range, such difference signifies
different overlapping of the Al s and pnictogen p orbitals that
could be attributed to the different anionic structures of the
compounds, i.e., the different connectivity of the AlPn4 tetrahedra.
The COHP diagrams are also projected in the same energy
window (Fig. 5). As seen from the plots, the strongest bonding
interactions are those between Al and pnictogen, in agreement
with their covalent bonding character, as discussed above. The
COHP curves for the AE–Pn interactions also show appreciable p–d
mixing, which is indicative of some degree of covalency of the
bonding between the pnictogen and alkaline-earth metal, i.e., the
cations are more than just spectators and/or space fillers. Both
Al–As and Sr–As interactions are optimized at the Fermi level for
Sr3Al2As4. For Ba3Al2P4 and Ba3Al2As4, while the Al–Pn interac-
tions are optimized at the Fermi level, the Ba–Pn interactions
remain slightly bonding character just above the Fermi level.
3.3. Structural relationships
Although the Ba3Al2As4 and Sr3Al2As4 structures are distinctly
different, they share common building motifs – the AlAs4 tetra-
hedra – which through different edge- and corner-sharing form
two different polyanionic sub-structures. A search of the ICSD
database reveals
a third structure type adopted by some
AE3Tr2Pn4 compounds, exemplified by the Sr3In2P4 structure
(Pnnm, No. 58, Pearson symbol oP18) [48]. This structure contains
another type of edge-shared tetrahedral motifs, which are dimer-
ized and further interconnected by corner sharing (see supporting
information). It is interesting to note that breaking down the
anionic parts of these three structures results in TrPn3 motifs,
which are building blocks in the structures of some AE3TrPn3
1
1
32
compounds [46,49,50]. For example, the ½AlAs2ꢀ
chain in
Ba3Al2As4 could be cleaved into dimers of edge-shared AlAs4
tetrahedra—the very same isolated [Al2Sb6]12ꢁ units are found to
exist in the structure of Ba3AlSb3 [46]. Both Sr3Al2As4 and Sr3In2P4
structures can also be cut into edge-shared Al2As6 and In2P6
fragments if the corner-sharing is removed. On the other hand,
the 21½AlAs2ꢀ layer and the 11½InP2ꢀ chain can also be ‘‘broken’’
along the shared-edges, which will result in infinite 1-D chains of
corner-shared tetrahedra—these again have prototypes among
some other AE3TrPn3 structures (Fig. 3). For example, the
chain derived from Sr3In2P4 is a simple one repeating
unit chain, which is similar to those found in the Ca3InP3 structure
32
32
1
1
62
½InP3ꢀ
1
1
62
[49]. The ½AlAs3ꢀ
fragment excised from Sr3Al2As4 is a two-
1
1
62
unit repeating chain that is similar to ½GaSb3ꢀ
seen from
Sr3GaSb3 [50]. The same arrangement could be seen in the KPO3
structure [51] too.
3.4. Electron count and electronic structure
Despite the different connectivity of the TrPn4 tetrahedra,
these structures are free of Tr–Tr or Pn–Pn bonds, and the Tr–Pn
distances suggest that the corresponding interactions are simple
2-center-2-electron bonds. Therefore, following the valence rules
and keeping in mind that all of the pnictogen atoms are 2-bonded,
the formulae of the title compounds can be readily rationalized as
(AE2þ)3(4b-Tr1ꢁ)2(2b-Pn1ꢁ)4, i.e., they are Zintl phases [11].
Electronic structure calculations (below) confirm this reasoning.

64
H. He et al. / Journal of Solid State Chemistry 188 (2012) 59–65
Fig. 4. DOS diagrams for (a) Ba3Al2P4, (b) Ba3Al2As4, and (c) Sr3Al2As4. The Fermi level is set as the energy reference at 0 eV. On-line colors: Total DOS is shown with a black
curve; partial DOS of alkaline-earth metal, Al, and the pnictogen are represented by green, blue, and red curves, respectively. (For interpretation of the references to color in
this figure, the reader is referred to the web version of this article.)
Fig. 5. COHP diagrams for (a) Ba3Al2P4, (b) Ba3Al2As4, and (c) Sr3Al2As4. The Fermi level is set as the energy reference at 0 eV. On-line colors: The COHP curves of the Ba–Pn
(or Sr–Pn), Al–Pn, and Ba–Al (or Sr–Al) interactions are shown in blue, red, and green, respectively. Since the ‘‘inverted’’ COHP values are plotted, the positive regions
represent the bonding interactions, while the negative regions denote antibonding interactions. (For interpretation of the references to color in this figure, the reader is
referred to the web version of this article.)
The interactions between alkaline-earth metal and Al in three
compounds are all very weak.
Acknowledgments
Svilen Bobev acknowledges financial support from the US
Department of Energy through a grant DE-SC0001360. Maia Saito
acknowledges NSF REU fellowship.
4. Conclusions
Ten new Zintl compounds from the series of AE3Tr2Pn4 (AE¼Ca,
Sr, Ba, Eu; Tr¼Al, Ga; Pn¼P, As) have been synthesized from flux
reactions. While eight of them, Ca3Al2P4, Sr3Al2As4, Eu3Al2P4,
Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Eu3Ga2As4, and Sr3Ga2As4 crystallize
with the Ca3Al2As4 structure type [22], the heavier alkaline-earth
metal analogs, Ba3Al2P4 and Ba3Al2As4, adopt the Na3Fe2S4 type
structure [21]. Both structures feature corner- and edge-shared TrPn4
tetrahedra, forming 21½TrPn2ꢀ layers or 11 ½TrPn2ꢀ chains. The title
compounds are semiconductors, as it can be inferred from the
valence electron count (AE3Tr2Pn4¼[AE2þ]3[Tr3þ]2[Pn3–]4), as well
as the electronic structure calculations. Although Ba3Ga2P4,
Ba3Ga2As4, or Yb3Tr2Pn4 (Tr¼Al, Ga; Pn¼P, As) could not be
synthesized as part of this study, further exploratory work, possibly
involving different synthetic approaches, will be worthwhile
pursuing.
Appendix A. Supplementary materials
Supplementary materials associated with this article can be
found in the online version at doi:10.1016/j.jssc.2012.01.042.
32
32
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The information consists of a figure showing the crystal
structure of Sr3In2P4; figures showing the experimental and
simulated X-ray powder diffraction patterns of Ba3Al2As4 and
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Eu3Ga2As4; as well as tables with selected interatomic distances
and angles.

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