Influence of the cation size on the framework structures and space group centricities in AMo2O5(SeO3)2 (A = Sr, Pb, and Ba)

Article abstract of DOI:10.1021/ic3003564

Two new quaternary mixed-metal selenites, SrMo₂O ₅(SeO₃)₂ and PbMo₂O ₅(SeO₃)₂, have been synthesized as crystals and pure polycrystalline phases by standard solid-state reactions using SrMoO ₄, PbO, MoO₃, and SeO₂ as reagents. The crystal structures of the reported materials have been determined by single-crystal X-ray diffraction. SrMo₂O₅(SeO₃)₂ and PbMo₂O₅(SeO₃)₂ are isostructural and crystallized in the triclinic centrosymmetric space group P1 (No. 2). The reported materials exhibit chain structures consisting of MoO₆ octahedra and asymmetric SeO₃ polyhedra. Complete characterizations including IR spectroscopy and thermal analyses for the compounds are also presented, as are dipole moment calculations. In addition, the powder second-harmonic-generating (SHG) properties of noncentrosymmetric polar BaMo₂O₅(SeO₃)₂ have been measured using 1064 nm radiation. Through powder SHG measurement, we are able to determine that BaMo₂O₅(SeO₃)₂ has a SHG efficiency of approximately 80 times that of α-SiO₂. Additional SHG measurements reveal that the material is phase-matchable (type 1). A detailed cation size effect on the symmetry and framework structure is discussed.

Full text of DOI:10.1021/ic3003564

Article
pubs.acs.org/IC
Influence of the Cation Size on the Framework Structures and Space
Group Centricities in AMo O (SeO ) (A = Sr, Pb, and Ba)
2
5
3 2
Seung-Jin Oh, Dong Woo Lee, and Kang Min Ok*
Department of Chemistry, Chung-Ang University, 221 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea
S Supporting Information
*
ABSTRACT: Two new quaternary mixed-metal selenites,
SrMo O (SeO ) and PbMo O (SeO ) , have been synthesized as
2
5
3 2
2
5
3 2
crystals and pure polycrystalline phases by standard solid-state
reactions using SrMoO , PbO, MoO , and SeO as reagents. The
4
3
2
crystal structures of the reported materials have been determined by
single-crystal X-ray diffraction. SrMo O (SeO ) and
2
5
3 2
PbMo O (SeO ) are isostructural and crystallized in the triclinic
2
5
3 2
̅
centrosymmetric space group P1 (No. 2). The reported materials
exhibit chain structures consisting of MoO6 octahedra and
asymmetric SeO polyhedra. Complete characterizations including
3
IR spectroscopy and thermal analyses for the compounds are also
presented, as are dipole moment calculations. In addition, the powder
second-harmonic-generating (SHG) properties of noncentrosymmet-
ric polar BaMo O (SeO ) have been measured using 1064 nm radiation. Through powder SHG measurement, we are able to
2
5
3 2
determine that BaMo O (SeO ) has a SHG efficiency of approximately 80 times that of α-SiO . Additional SHG measurements
2
5
3 2
2
reveal that the material is phase-matchable (type 1). A detailed cation size effect on the symmetry and framework structure is
discussed.
INTRODUCTION
important to understand factors determining the space group
symmetry to increase the incidence of crystallographic NCS in
■
The demand for advanced new functional materials containing
noncentrosymmetric (NCS) crystal structures remains very
strong because the NCS materials exhibit technologically
important properties such as second-order nonlinear optical
15−18
any new material.
In fact, there are several reports
exhibiting factors that influence the overall centricity, in
which the size of the alkali-metal cations and the hydrogen-
1
9−22
bonding effect played very important roles.
With these
(
NLO), piezoelectric, pyroelectric, and ferroelectric proper-
2+
6+
4+
1
−5
ideas in mind, we decided to investigate the A −Mo −Se −
oxide system. As SOJT-distortive cations, both Mo and Se
cations exhibit highly asymmetric coordination environments.
ties. Although the relationships between the NCS structures
6+ 4+
6
−8
and the above-mentioned properties are well established, the
development of strategies for rationally designed superior
performing NCS materials is still challenging. Thus far, one of
the most successful strategies to increasing the crystallographic
NCS behavior is introducing cations with asymmetric
coordination environments and larger polarizability to the
framework structures. With oxide materials, significant numbers
of NCS crystal structures have been observed in materials
containing cations susceptible to second-order Jahn−Teller
6+
Mo has been known to be located at the highest magnitude of
23
4+
the out-of-center distortion scale. Also, Se inherently
possesses asymmetric structural geometry attributable to the
nonbonded electron pair. In fact, several molybdenum selenite
materials such as A (MoO ) (SeO ) (A = NH , Ag, Rb, Tl, and
2
3 2
27
3
4
2
4−26
27
Cs),
(
BaMoO SeO , Ba(Mo O )(SeO ) , A MoSeO
3 3 2 5 3 2 2 6
2
8
29,30
A = Na, K, and Rb), A MoSe O (A = Nd or Ga),
2
2
10
2
9,31
32
9
−12
A MoSe O (A = Gd or Bi),
Ni (Mo O )(SeO ), and
3 2 8 3
(
SOJT) distortions.
SOJT distortions exhibiting asymmet-
2
3
12
3
3
Cu (MoO )(SeO ) have been reported. Here we report
ric coordination environments are normally found in two
2
4
3
0
solid-state syntheses and characterizations of a series of
quaternary mixed-metal selenites, AMo O (SeO ) (A = Sr,
families of cations: octahedrally coordinated d transition
4+
5+
6+
metals (Ti , V , Mo , etc.) and cations with stereoactive
2
5
3 2
3+
4+ 5+
10
Pb, and Ba). We will demonstrate that the cation size and
coordination number influence the framework architecture and
the space group symmetry of the materials. With the NCS
lone pairs (Sb , Se , I , etc.). In addition, d transition-metal
cations showing polar displacement in the center of the
coordination environment, and asymmetric π-orbital systems
found in the borate groups are another example of important
BaMo O (SeO ) , detailed second-harmonic-generating
2
5
3 2
1
3,14
(
SHG) properties will also be reported.
NCS chromophores.
All of the local acentricities are
necessary but not sufficient for generating NCS because the
local asymmetric units often align in an antiparallel manner and
crystallize in a centrosymmetric (CS) structure. Thus, it is more
Received: February 17, 2012
Published: April 18, 2012
©
2012 American Chemical Society
5393
dx.doi.org/10.1021/ic3003564 | Inorg. Chem. 2012, 51, 5393−5399

Inorganic Chemistry
Article
EXPERIMENTAL SECTION
Table 1. Crystallographic Data for SrMo O (SeO ) and
2 5 3 2
■
PbMo O (SeO )
2
5
3 2
Reagents. PbO (Junsei, 99%), SrCO (Aldrich, 99.9%), BaCO
3
3
(
Duksan, 99.5%), MoO (Hayashi, 99.5%), and SeO (Aldrich, 98%)
3
2
formula
fw
SrMo Se O
PbMo Se O
2 2 11
2
2
11
were used as received. SrMoO and BaMoO were synthesized by
4
4
613.42
732.99
P1 (No. 2)
standard solid-state reactions. A stoichiometric amount of SrCO (or
3
space group
a (Å)
P1
̅
(No. 2)
̅
BaCO ) and MoO was thoroughly ground and pressed into a pellet.
3
3
7.8759(6)
8.1509(5)
8.8076(6)
82.943(5)
65.326(5)
66.918(5)
472.09(6)
2
7.8560(6)
8.2307(6)
8.8381(6)
82.881(4)
64.843(4)
66.419(4)
473.34(6)
2
The pellet was heated to 600 °C for 24 h and cooled to room
temperature. The phase purities of SrMoO4 and BaMoO4 were
confirmed by powder X-ray diffraction (XRD).
b (Å)
c (Å)
Synthesis. Crystals of SrMo O (SeO ) and PbMo O (SeO )
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
2
5
3
2
2
5
3 2
were prepared by standard solid-state reactions. For single crystals of
−3
PbMo O (SeO ) , 0.335 g (1.50 × 10 mol) of PbO, 0.432 g (3.00 ×
2
5
3 2
−
3
−3
10
mol) of MoO , and 0.366 g (3.30 × 10 mol) of SeO were
3
2
thoroughly mixed with an agate mortar and pestle and pressed into a
pellet. The pellet was introduced into a fused-silica tube, which was
subsequently evacuated and sealed. The tube was gradually heated to
T (°C)
λ (Å)
200.0(2)
0.71073
4.315
200.0(2)
0.71073
5.143
4
00 °C for 36 h and cooled at a rate of 12 °C h−1 to room
ρcalcd (g cm−3)
μ (mm−1)
temperature. The product contained colorless block-shaped crystals
60% yield) with white polycrystalline PbMo O (SeO ) . The crystal
growth conditions such as the starting reagents and reaction
16.004
28.089
(
2
5
3 2
a
R(F)
0.0522
0.0566
2
b
R (F )
0.1155
0.1452
temperature for SrMo O (SeO ) are slightly different. A total of
w
o
2
5
3 2
.371 g (1.50 × 10− mol) of SrMoO and 0.333 g (3.00 × 10 mol)
3
−3
a
b
2
2
2 2
0
R(F) = ∑||F | − |F ||/∑|F |. R (F ) = [∑w(F − F ) /
4
o
c
o
w
o
o
c
2
2 1/2
of SeO were thoroughly mixed with an agate mortar and pestle and
∑w(F ) ] .
2
o
pressed into a pellet. The pellet was transferred to a fused-silica tube,
which was evacuated and sealed. The tube was heated to 500 °C for 24
Table 2. Selected Bond Distances (Å) for SrMo O (SeO )
3 2
−1
2
5
h and then cooled slowly at a rate of 6 °C h to 300 °C before being
quenched to room temperature. Colorless block-shaped crystals (31%
yield) of SrMo O (SeO ) were recovered with some unknown
and PbMo O (SeO )
2
5
3 2
2
5
3
2
SrMo O (SeO )
2
Mo(1)−O(1)
Mo(1)−O(2)
Mo(1)−O(3)
Mo(1)−O(4)
Mo(1)−O(5)
Mo(1)−O(5)
Mo(2)−O(3)
Mo(2)−O(6)
Mo(2)−O(7)
Mo(2)−O(8)
Mo(2)−O(9)
Mo(2)−O(9)
Se(1)−O(5)
Se(1)−O(8)
Se(1)−O(10)
Se(2)−O(4)
Se(2)−O(9)
Se(2)−O(11)
Sr(1)−O(2)
Sr(1)−O(4)
Sr(1)−O(7)
Sr(1)−O(8)
Sr(1)−O(10)
Sr(1)−O(10)
Sr(1)−O(11)
Sr(1)−O(11)
5
3
2
PbMo O (SeO )
5 3 2
2
Mo(1)−O(1)
Mo(1)−O(2)
Mo(1)−O(3)
Mo(1)−O(4)
Mo(1)−O(5)
Mo(1)−O(5)
Mo(2)−O(3)
Mo(2)−O(6)
Mo(2)−O(7)
Mo(2)−O(8)
Mo(2)−O(9)
Mo(2)−O(9)
Se(1)−O(5)
Se(1)−O(8)
Se(1)−O(10)
Se(2)−O(4)
Se(2)−O(9)
Se(2)−O(11)
Pb(1)−O(2)
Pb(1)−O(4)
Pb(1)−O(7)
Pb(1)−O(8)
Pb(1)−O(10)
Pb(1)−O(10)
Pb(1)−O(11)
Pb(1)−O(11)
polycrystalline materials. Pure polycrystalline samples of
AMo O (SeO ) (A = Sr and Ba) were obtained through similar
1.704(8)
1.714(8)
1.877(8)
2.091(8)
2.170(8)
2.326(7)
1.866(8)
1.710(8)
1.706(7)
2.060(7)
2.179(8)
2.278(8)
1.734(8)
1.737(7)
1.646(8)
1.710(8)
1.751(7)
1.649(8)
2.599(8)
2.692(8)
2.547(7)
2.665(8)
2.531(8)
2.586(8)
2.540(8)
2.676(8)
1.694(13)
1.722(12)
1.873(12)
2.089(12)
2.135(13)
2.347(12)
1.878(12)
1.694(14)
1.718(12)
2.082(12)
2.174(13)
2.321(11)
1.759(11)
1.723(11)
1.647(12)
1.706(12)
1.725(13)
1.641(11)
2.675(11)
2.750(11)
2.604(12)
2.708(12)
2.552(13)
2.588(13)
2.633(12)
2.637(12)
2
5
3 2
−3
solid-state reactions. A total of 0.495 g (2.00 × 10 mol) of SrMoO4
−
3
(
0.595 g for BaMoO ), 0.288 g (2.00 × 10 mol) of MoO , and 0.488
4
3
−3
g (4.40 × 10 mol) of SeO were thoroughly mixed and introduced
into a fused-silica tube, and the tube was evacuated and sealed.
2
However, the tube was heated to 400 °C for 36 h. It should also be
noted that a 10% excess amount of SeO has been used initially in
2
order to synthesize pure polycrystalline phases of AMo O (SeO )
2
5
3 2
attributable to the easy sublimation of SeO at lower temperature. The
2
powder XRD patterns on the resultant polycrystalline products
showed that the materials were single phases and were in good
agreement with the generated patterns from the single-crystal data (see
the Supporting Information).
Single-Crystal XRD. The structures of SrMo O (SeO ) and
2
5
3 2
PbMo O (SeO ) were determined by standard crystallographic
2
5
3 2
3
methods. Colorless block crystals [0.023 × 0.027 × 0.072 mm for
3
SrMo O (SeO ) and 0.023 × 0.040 × 0.067 mm for
2
5
3 2
PbMo O (SeO ) ] were used for single-crystal data analyses. All of
2
5
3 2
the data were collected using a Bruker SMART BREEZE
diffractometer equipped with a 1K CCD area detector using
graphite-monochromated Mo Kα radiation at 200 K. A hemisphere
of data was collected using a narrow-frame method with a scan width
−1
of 0.30° in ω and an exposure time of 5 s frame . The first 50 frames
were remeasured at the end of the data collection to monitor the
instrument and crystal stability. The maximum correction applied to
the intensities was <1%. The data were integrated using the SAINT
34
program, with the intensities corrected for Lorentz factor, polar-
ization, air absorption, and absorption attributable to variation of the
path length through the detector faceplate. A semiempirical absorption
correction was made on the hemisphere of data with the SADABS
step size of 0.02° and a step time of 0.2 s. The experimental powder
XRD pattern is in good agreement with the calculated data from the
single-crystal model. Because the crystal structure of BaMo O (SeO )
3 2
35
program. The data were solved and refined using SHELXS-97 and
3
6,37
SHELXL-97, respectively.
All calculations were performed using
2
5
38
27
the WinGX-98 crystallographic software package. Crystallographic
data and selected bond distances for the reported material are given in
Tables 1 and 2.
has been published, the diffraction pattern of BaMo O (SeO ) was
2 5 3 2
39
analyzed using the Rietveld method with the GSAS program. The 2θ
range was 10−100° with a step size of 0.02° and a step time of 1 s. The
final diffraction plot with the difference between the observed and
calculated profiles of BaMo O (SeO ) has been deposited as
Powder XRD. Powder XRD was used to confirm the phase purity
for the synthesized materials. The powder XRD pattern was collected
on a Bruker D8-Advance diffractometer using Cu Kα radiation at room
temperature with 40 kV and 40 mA. The polycrystalline samples were
mounted on sample holders and scanned in the 2θ range 5−70° with a
2
5
3 2
Supporting Information.
IR Spectroscopy. IR spectra were recorded on a Varian 1000
Fourier transform IR spectrometer in the 400−4000 cm− range, with
1
5
394
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Inorganic Chemistry
Article
−
1
the sample embedded in a KBr matrix. IR (SrMo O (SeO ) , cm ):
environments with six O atoms attributable to the SOJT
distortions. Especially, the two unique Mo6+ cations distort
2
5
3 2
9
6
9
50 (m), 936 (m), 922 (m), 901 (s), 862 (s), 817 (m), 726 (s, br),
−1
20 (s, br), 571 (m), 535 (m), 473 (m). IR (PbMo O (SeO ) , cm ):
2
5
3
2
along the local C [111] direction, toward a face, of their
3
44 (m), 931 (m), 912 (m), 892 (s), 840 (m), 826 (m), 806 (m), 720
respective octahedra (see Figure 2a). The out-of-center
(
s, br), 646 (m, sh), 617 (s), 560 (m), 518 (m), 463 (m).
Thermogravimetric Analysis. Thermogravimetric analysis was
performed on a Setaram LABSYS TG-DTA/DSC thermogravimetric
analyzer. The polycrystalline samples were contained within alumina
−1
crucibles and heated at a rate of 10 °C min from room temperature
to 800 °C under flowing argon.
Scanning Electron Microscopy/Energy-Dispersive Analysis
by X-ray (SEM/EDAX). SEM/EDAX has been performed using a
Hitachi S-3400N/Horiba Energy EX-250 instrument. EDAX for
SrMo O (SeO ) and PbMo O (SeO ) exhibits Sr:Mo:Se and
2
5
3
2
2
5
3 2
Pb:Mo:Se ratios of approximately 1:2:2.
Second-Order NLO Measurements. Powder SHG measure-
ments on polycrystalline BaMo O (SeO ) were performed on a
2
5
3 2
40
modified Kurtz-NLO system using 1064 nm radiation. A DAWA Q-
switched Nd:YAG laser, operating at 20 Hz, was used for the
measurements. Because SHG efficiency has been shown to depend
strongly on the particle size; polycrystalline samples were ground and
sieved (Newark Wire Cloth Co.) into distinct particle size ranges (20−
4
5, 45−63, 63−75, 75−90, 90−125, and >125 μm). To make relevant
comparisons with known SHG materials, crystalline α-SiO2 and
LiNbO were also ground and sieved into the same particle size ranges.
3
Powders with particle size 45−63 μm were used for comparing SHG
intensities. All of the powder samples with different particle sizes were
placed in separate capillary tubes. No index-matching fluid was used in
any of the experiments. The SHG light, i.e., 532 nm green light, was
collected in reflection and detected by a photomultiplier tube
(
Hamamatsu). To detect only the SHG light, a 532 nm narrow-pass
interference filter was attached to the tube. A digital oscilloscope
Tektronix TDS1032) was used to view the SHG signal. A detailed
description of the equipment and methodology used has been
(
41
published.
RESULTS AND DISCUSSION
■
Structures. SrMo O (SeO ) and PbMo O (SeO ) are
2
5
3 2
2
5
3 2
(No. 2).
isostructual and crystallize in the CS space group P1
The materials are new quaternary mixed-metal selenites
̅
exhibiting chain structures with distorted MoO octahedra
6
linked by asymmetric SeO polyhedra (see Figure 1). There are
3
6+
6+
two unique Mo cations within an asymmetric unit. Both Mo
cations are in highly distorted octahedral coordination
Figure 2. ORTEP (50% probability ellipsoids) representations in
SrMo O (SeO ) showing (a) the MoO octahedra with C out-of-
2
5
3
2
6
3
center distortions, (b) the asymmetric SeO polyhedra, and (c) the
3
SrO polyhedra.
8
distortions result in three short [1.704(8) to 1.877(8) Å for
SrMo O (SeO ) and 1.694(13)−1.878(12) Å for
2
5
3 2
PbMo O (SeO ) ] and three long [2.060(7)−2.326(7) Å for
2
5
3 2
SrMo O (SeO ) and 2.082(12)−2.347(12) Å for
2
5
3 2
PbMo O (SeO ) ] Mo−O bond distances. The two unique
2
5
3 2
4+
Se cations are in distorted trigonal-pyramidal environments,
bonded to three O atoms (see Figure 2b). The Se−O bond
lengths range from 1.646(8) to 1.751(7) Å for
SrMo O (SeO ) and from 1.641(11) to 1.759(11) Å for
2
5
3 2
4
+
PbMo O (SeO ) . The Se cations are in asymmetric
2
5
3 2
coordination environments attributable to the stereoactive
lone pair. Finally, each unique Sr2+ or Pb cation exhibits an
eight-coordinate square-antiprismatic environment, with Sr−O
or Pb−O contact distances ranging from 2.531(8) to 2.692(8)
2+
Å and from 2.552(13) to 2.750(11) Å for SrMo O (SeO ) and
Figure 1. Ball-and-stick representation of SrMo O (SeO ) in the ab
2
5
3 2
2
5
3 2
PbMo O (SeO ) , respectively.
plane. The distorted MoO octahedra and asymmetric SeO polyhedra
2
5
3 2
6
3
link to form chains along the [010] direction (blue, Mo; green, Se;
yellow, Sr; red, O).
As can be seen in Figure 3a, the backbone of the structure
may be described as zigzag chains of edge- and corner-shared
5
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Inorganic Chemistry
Article
Figure 3. Ball-and-stick models of SrMo O (SeO ) representing (a) zigzag chains of edge- and corner-shared MoO octahedra running along the
2
5
3
2
6
[
010] direction, (b) SeO polyhedra, and (c) the “linking” of the chains by SeO and SrO groups. (d) Note that a pseudo-two-dimensional topology
3
3
8
is observed in the ac plane attributable to the Sr−O contacts (blue, Mo; green, Se; yellow, Sr; red, O).
MoO octahedra. Two Mo(1)O octahedra share their edges
indicated by the thermogravimetric analysis diagram,
PbMo O (SeO ) is stable up to 400 °C. Above the
6
6
through O(5) and form a Mo(1) O dimer. Also, two
2
10
2
5
3 2
Mo(2)O6 octahedra share their edges through O(9) and
form another Mo(2) O dimer. Then the Mo(1) O and
temperature, decompositions occur attributable to the sub-
limation of SeO . The thermal stabilities of SrMo O (SeO )
3 2
2
10
2
10
2
2
5
Mo(2) O dimers share their corners through O(3) and form
and PbMo O (SeO ) were also confirmed by powder XRD. As
2
10
2 5 3 2
infinite zigzag chains along the [010] direction. Then, the
can be seen in the Supporting Information, no substantial
changes in the peak position and intensity are observed in the
XRD patterns up to 400 °C. However, the XRD patterns
obtained at higher temperatures revealed that the materials
chains are capped by the SeO groups along the [100] and
3
[
−100] directions (see Figure 3b). Interestingly, all SeO3
groups serve as intrachain linkers. Finally, interchain Sr−O
(
dashed lines in Figure 3c,d). The interchain contacts might give
SrMo O (SeO ) and PbMo O (SeO ) a pseudo-two-dimen-
or Pb−O) contacts are observed in the ac plane (see the
decomposed to the mixture of SrMoO (PDF#08-0482) and
4
MoO3 (PDF#05-0508) or PbMoO4 (PDF#44-1486) and
MoO (PDF#05-0508).
2
5
3 2
2
5
3 2
3
sional topology. In connectivity terms, the structures of
SrMo O (SeO ) and PbMo O (SeO ) may be described as
Second-Order NLO Measurements. Although the
27
structure of NCS BaMo O (SeO ) has been published, the
2
5
3 2
2
5
3 2
2
5
3 2
−
1.333
anionic chains of {[Mo(1) O4/1O4/2O2/3
]
[Mo-
[Se(2)-
NLO properties of the material have not been studied yet.
Thus, we investigated its NLO properties. Powder SHG
measurements, using 1064 nm radiation, indicated that
BaMo O (SeO ) has a SHG efficiency of approximately 80
2
−
1.333
+0.333
(
2) O4/1O4/2
O
]
[Se(1)O1/1O1/2O1/3
]
2
2/3
0.333 −2
+
O O O ]
} . The charge balance is maintained by
1/1 1/2 1/3
2
+
2+
incorporation of the Sr or Pb cations. Bond-valence-sum
2
5
3 2
4
2,43
6+
4+
2+
2+
calculations
for the Mo , Se , and Sr (or Pb ) result in
times that of α-SiO . By sieving BaMo O (SeO ) powder into
2 2 5 3 2
values in the ranges 5.92−6.05, 3.98−4.17, and 2.18 (or 1.93),
various particle sizes, ranging from 20 to 150 μm, and
measuring the SHG as a function of the particle size, we
were able to determine the type 1 phase-matching capabilities
of the material. As seen in Figure 4, BaMo O (SeO ) is phase-
respectively.
IR Spectroscopy. The IR spectra of SrMo O (SeO ) and
2
5
3 2
PbMo O (SeO ) revealed overlapped bands of Mo−O, Se−O,
2
5
3 2
2
5
3 2
−1
Pb−O, and Sr−O vibrations below 1000 cm . The Mo−O and
Se−O vibrations are only partially resolved as previously
reported.
matchable. Once the SHG efficiency and the phase-matching
capability of a material are known, the bulk SHG efficiency,
2
4,27
44
Also, no vibrational bands are observed between
⟨d ⟩ , can be estimated. For BaMo O (SeO ) , ⟨d ⟩ is
eff exp
2
5
3 2
eff exp
−1
−1
4
000 and 1000 cm , which is consistent with similar materials.
approximately 10.3 pm V .
The IR spectra for the reported materials have been deposited
as Supporting Information.
Thermal Analysis. The thermal behaviors of the reported
materials were investigated using thermogravimetric analysis
and powder XRD measured at different temperatures. As
Structure−Property Relationships. The observed SHG
response can be understood by analyzing the polarization of the
asymmetric polyhedra. Specifically, macroscopic NCS is
normally observed when locally polar asymmetric coordination
polyhedra add constructively. Thus, it is crucial to determine
5
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Inorganic Chemistry
Article
In order to better understand the asymmetric coordination
environment and the macroscopic net polarization, we did
calculate the local dipole moments for Mo6+ and Se in
AMo O (SeO ) (A = Sr, Pb, and Ba). This approach has been
4+
2
5
3 2
described earlier with respect to metal oxyfluoride octahe-
4
5,46
dra.
polyhedra in the reported materials range about from 8.00 to
.73 D, respectively. The values are consistent with the recently
We found that the local dipole moments for the SeO3
8
47
reported dipole moments for SeO polyhedra. Also, the local
3
dipole moments for the four MoO octahedra exhibit values
6
ranging from 4.56 to 5.98 D. Especially with BaMo O (SeO ) ,
2
5
3 2
the local dipole moments for SeO and MoO were calculated
3
6
to be 8.73 and 4.56, respectively. A complete calculation of
dipole moments for the constituted polyhedra is listed in Table
Figure 4. Phase-matching curve (type 1) for BaMo O (SeO ) . The
curve is to guide the eye and is not a fit to the data.
2
5
3 2
3
.
the “net” direction of the polarizations, in which the origin and
magnitude of the SHG properties can be obtained. In
BaMo O (SeO ) , both kinds of SOJT-distortive cations,
Table 3. Calculation of Dipole Moments for SeO and MoO
Polyhedra
3
6
2
5
3 2
4+
Mo6 and Se may significantly contribute toward the SHG
+
compound
species
dipole moment (D)
6
+
efficiency. First, the Mo cations distort either along the [011]
or [0−11] directions, resulting in two short, two normal, and
two long Mo−O bonds. Once taken as a whole, a net moment
is observed pointing in the [001] direction (see Figure 5).
SrMo O (SeO )
3 2
Se(1)O3
Se(2)O3
Mo(1)O6
Mo(2)O6
Se(1)O3
Se(2)O3
Mo(1)O6
Mo(2)O6
Se(1)O3
Mo(1)O6
8.17
8.00
5.33
4.99
8.13
8.49
5.98
5.36
8.73
4.56
2
5
PbMo O (SeO )
2
5
3
2
BaMo O (SeO )
2
5
3
2
CS Nonpolar versus NCS Polar Structures. Although
SrMo O (SeO ) , PbMo O (SeO ) , and BaMo O (SeO ) are
2
5
3 2
2
5
3 2
2
5
3 2
stoichiometrically equivalent, an interesting change in the
crystallographic polarity occurs from the CS nonpolar structure
2
+
for the materials containing relatively smaller cations, Sr or
2+
Pb , to the NCS polar structure for that with the larger one,
2+
Ba . While SrMo O (SeO ) or PbMo O (SeO ) exhibits a
2
5
3 2
2
5
3 2
CS pseudo-two-dimensional-layered structure, Ba-
Mo O (SeO ) shows a NCS three-dimensional framework.
Figure 5. Ball-and-stick representation of BaMo O (SeO ) (blue,
Mo; green, Se; red, O). A moment is observed toward the [001]
2
5
3
2
2
5
3 2
2
+
2+
With SrMo O (SeO ) or PbMo O (SeO ) , the Sr or Pb
2 5 3 2 2 5 3 2
direction attributable to the alignment of the C out-of-center distorted
2
cation is in an eight-coordinate square-antiprismatic environ-
MoO octahedra. Also, a larger net moment attributed to the sum of
2+
6
ment. However, the Ba cation in BaMo O (SeO ) possesses
2
5
3 2
the aligned asymmetric SeO3 groups is observed in the [00−1]
direction. Once taken as a whole, a net moment is observed along the
a 10-coordinate geometry. The different coordination environ-
2+
ments are consistent with the ionic radii of the cations (Sr ,
[
00−1] direction.
2
+
2+
48
1
.26 Å; Pb , 1.29 Å; Ba , 1.42 Å). Relatively smaller cations,
2
+
2+
Sr or Pb , interact with oxide ligands on four MoO
6
Meanwhile, each SeO3 unit also has a dipole moment
attributable to the different charge distributions on Se and O
atoms. As seen in Figure 5, the lone pairs on the asymmetric
octahedra and four SeO polyhedra (see Figure 6a). As seen
3
in Figure 6a, in order to maintain the crowded eight-coordinate
2
+
2+
square-antiprismatic environment around the Sr or Pb
4+
Se cations approximately point toward the [011] and [0−11]
cation, the MoO6 octahedra and SeO3 polyhedra are
coordinated in an antiparallel manner. By doing so, any
unfavorable repulsion of polyhedra can be minimized. In other
directions. Because the local moment for SeO points in the
3
opposite direction of the lone pair, a net moment in the [00−1]
direction attributable to the alignment of lone pairs on Se4+
cations is observed (see Figure 5). At this point, one can notice
that the moments for MoO octahedra and SeO polyhedra
2+
2+
words, the Sr or Pb cation serves as an inversion center,
which renders the material CS nonpolar. In BaMo O (SeO ) ,
2
5
3 2
a 10-fold coordination environment is observed, attributable to
6
3
2+
point in opposite directions. As we will discuss later, the local
dipole moment calculations indicate that the moment for SeO3
the large ionic size of Ba (see Figure 6b). The larger cation,
2+
Ba , can interact with oxide ligands on eight MoO octahedra
6
is larger than that of the MoO octahedra. Thus, taking the
and two SeO groups. More specifically, four pairs of corner-
6
3
moments as a whole, a net moment is observed along the [00−
sharing MoO6 octahedra; i.e., Mo O groups completely
2
11
1
] direction. This net moment that is mainly arising from the
encompass the Ba2+ cation. Then the only way to assert the 10-
2+
alignment of asymmetric SeO groups is responsible for the
coordinate environment in the Ba cation with two more SeO3
3
observed moderate SHG response, 80 times that of α-SiO2.
groups is to connect Se4+ cations to O(4) and O(6) [the
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Inorganic Chemistry
Article
crystallographic NCS. Here the size of the A-site cations is a
major factor in determining the space group centricities.
CONCLUSIONS
We have successfully synthesized stoichiometrically equivalent
mixed-metal selenite materials, AMo O (SeO ) (A = Sr, Pb,
■
2
5
3 2
and Ba), by standard solid-state reactions. Crystallographic data
indicate that SrMo O (SeO ) and PbMo O (SeO ) are CS
2
5
3 2
2
5
3 2
nonpolar with pseudo-two-dimensional structures, whereas
BaMo O (SeO ) is NCS polar with a three-dimensional
2
5
3 2
framework structure. Detailed structural analyses suggest that
the cation size and coordination number play critical roles to
determine the symmetry and framework of the materials.
Powder SHG measurements on BaMo O (SeO ) using 1064
2
5
3 2
nm radiation indicate that the material is phase-matchable (type
) with a SHG efficiency of approximately 80 times that of α-
1
SiO . We are in the process of synthesizing other new NCS
2
mixed-metal oxide materials and will be reporting on them
shortly.
ASSOCIATED CONTENT
Supporting Information
■
*
S
X-ray crystallographic file in CIF format, calculated and
observed XRD patterns, thermal analysis diagrams, and IR
spectra for AMo O (SeO ) . This material is available free of
2
5
3 2
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: kmok@cau.ac.kr. Phone: +82-2-820-5197 Fax: +82-2-
25-4736.
■
*
8
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
■
(
NRF) funded by Ministry of Education, Science &
Technology (Grant 2010-0002480).
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■
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