Combining second-order Jahn-Teller distorted cations to create highly efficient SHG materials: Synthesis, characterization, and NLO properties of BaTeM2O9 (M = Mo6+ or W6+)

Article abstract of DOI:10.1021/ja035314b

Two new oxides, BaTeMo₂O₉ and BaTeW₂O₉, have been synthesized, by standard solid-state techniques, that have strong SHG intensities of 600 × SiO₂, on the order of LiNbO₃. Both materials contain cations susceptible to second-order Jahn-Teller (SOJT) distortions, resulting in asymmetric coordination environments. The SOJT distortion polarizes the M⁶⁺-O and Te⁴⁺-O bonds. Equally importantly, these polarizations constructively add, resulting in the large SHG responses. Powder SHG measurements on BaTeM₂O₉ (M = Mo⁶⁺ or W⁶⁺) indicated that both materials are phase-matchable and have a (d_eff)_exp of 28 and 22 pm/V, respectively. Using bond hyperpolarizability values (β's) of 130 × 10^-40 and 305 × 10^-40 m⁴/V for Te⁴⁺-O and Mo⁶⁺-O respectively, we calculate a deffcalc of 20pm/V for BaTeMo₂O₉. In addition, through the powder SHG measurements, we are able to give a more reasonable value for β(W⁶⁺-O), 230 × 10^-40 m⁴/V. This value is consistent with the smaller polarizability and magnitude of the intra-octahedral distortion of W⁶⁺ compared with Mo⁶⁺. Copyright

Full text of DOI:10.1021/ja035314b

Published on Web 06/10/2003
Combining Second-Order Jahn-Teller Distorted Cations to Create Highly
Efficient SHG Materials: Synthesis, Characterization, and NLO Properties of
2 9
BaTeM O
(M ) Mo6 or W )
+
6+
Hyun-Seup Ra, Kang Min Ok, and P. Shiv Halasyamani*
Department of Chemistry and the Center for Materials Chemistry, UniVersity of Houston, 136 Fleming Building,
Houston, Texas 77204-5003
Received March 25, 2003; E-mail: psh@uh.edu
Second-order nonlinear optical (NLO) materials, i.e., second-
harmonic generating (SHG) materials, are of current interest owing
to their uses in photonic technologies.1 A requirement for all SHG
-3
4
materials is a noncentrosymmetric (NCS) crystal structure. Re-
cently, a variety of strategies have been put forth to rationally design
5
-10
new NCS materials.
We have focused on creating new NCS
materials1
1-15
by synthesizing oxides containing cations susceptible
to second-order Jahn-Teller (SOJT) distortions.
16-22
SOJT effects
involve structural changes attributable to a nondegenerate ground-
state interacting with a low-lying excited state. The distortion occurs
if the energy gap between the highest occupied (HOMO) and lowest
unoccupied (LUMO) orbitals is small and if there is a symmetry-
allowed distortion permitting mixing of these two orbitals. The
0
distortion can occur in two different “families” of cations, d
4+
5+
6+
transition metals (Ti , Nb , W ) and cations with stereoactive
4+
3+
4+
lone-pairs (Se , Sb , Te ), and results in asymmetric coordination
environments. In this communication we report the synthesis,
characterization, and SHG properties of two new materials,
2 9
BaTeM O
(M ) Mo6 or W ), in which extremely strong SHG
+
6+
2
3
2 3
efficiencies are observed, ∼600 × SiO , on the order of LiNbO .
The strong SHG efficiency is attributable to the constructive
addition of the Te-O and M6+-O bond polarizations. We are also
able to give an estimate of â(W -O), by using our recently
6+
reported model.14
+
BaTeM
2
O
9
(M ) Mo6 or W6+) were synthesized by solid-state
techniques and their structures determined by standard crystal-
2
4
6+
lographic methods. Polycrystalline BaTeM
W
2
O
9
(M ) Mo or
) were synthesized by combining stoichiometric amounts of
CO , MoO (WO ), and TeO . The mixtures were heated in air
to 550 °C for 36h with two intermittent re-grindings. White
polycrystalline powders, subsequently shown to be BaTeMo
BaTeW ) were recovered. Single crystals of BaTeMo were
grown by pouring Ba CO (1.38 mmol) into the liquid mixture of
MoO (2.76 mmol) and TeO (6.90 mmol) at 570 °C in a platinum
crucible. The mixture was held at 570 °C for 12h and cooled slowly
6+
Figure 1. Ball-and-stick diagram of BaTeMo2O9. Note the asymmetric
coordination environments in both Te4 (green spheres) and Mo (blue
+
6+
Ba
2
3
3
3
2
spheres) cations.
2
O
9
1
.728(9)-2.233(7) Å (1.73(2) - 2.22(2) Å), whereas the Te-O
(
O
2 9
2 9
O
6
+
4+
bonds range from 1.867(8)-2.024(19) Å. Both the M and Te
cations are in asymmetric coordination environments attributable
to SOJT distortions. These distortions polarize the MO and TeO
polyhedra. Interestingly for the two materials the intra-octahedral
2
3
3
2
6
3
-
1
to 450 °C at 6 °C h before being quenched to room temperature.
6
+
Colorless block-shaped crystals were manually extracted. Single
distortions are not the same. In BaTeMo
2 9
O , the two unique Mo
crystals of BaTeW
CO (2.32 mmol), WO
60 °C in a platinum crucible. The mixture was held at 760 °C for
O
2 9
were grown by heating a mixture of Ba
2
-
cations distort along the local C [111] direction, toward a face, of
their respective octahedra. This out-of-center distortion produces
three ‘short’ (1.728(9)-1.847(9) Å) and three ‘long’ (2.036(8)-2.233-
3
3
3
(4.64 mmol), and TeO (3.48 mmol) at
2
7
2
-1
6+
4h and cooled slowly to 700 °C at 1 °C h before being quenched
(7) Å) Mo -O bonds (see Figure 2). In BaTeW
2
O
9
, the two unique
[110] direction, toward
[111] direction, resulting in two ‘short’
(1.73(2) Å and 1.77(2) Å), two ‘normal’ (1.90(2) Å and 1.99(2)
6+
to room temperature. Colorless block- shaped crystals were also
manually extracted. The quality of BaTeMo crystals was
substantially better than BaTeW
W
cations distort either along the local C
an edge, or the local C
2
O
2 9
3
O .
2 9
6+
Both materials crystallize as clear, colorless faceted blocks. The
compounds have two-dimensional layered structures consisting of
Å), and two ‘long’ (2.22(2) Å x 2) W -O bonds, and for the
C
3
-W6 distortion three ‘short’ (1.73(2)-1.85(2) Å) and three ‘long’
+
MO
6
(M ) Mo6 or W ) octahedra linked to asymmetric TeO
+
6+
3
(2.09(2)-2.14(2) Å) W -O bonds. For Te , an asymmetric
6+
4+
2+
polyhedra. The anionic layers are separated by Ba cations (see
Figure 1). The bond distances for Mo6 -O (W6+-O) range from
coordination environment is observed owing to the stereoactive
+
6+
6+
lone-pair. Bond valence calculations for the M (M ) Mo or
7764
9
J. AM. CHEM. SOC. 2003, 125, 7764-7765
10.1021/ja035314b CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
In summary, we have demonstrated that highly efficient SHG
materials can be designed by synthesizing oxides containing cations
2 9
susceptible to SOJT distortions. In BaTeM O the polarizations
attributable to the M6 -O and Te -O bonds constructively add,
resulting in the large SHG responses. On the basis of the powder
SHG measurements, we have also determined a more reasonable
+
4+
value for â(W6 -O), 230 × 10
+
-40
m /V.
4
Acknowledgment. We thank the Welch Foundation, NSF-
Career (DMR-0092054), and the ACS-PRF Program for support.
P.S.H. is a Beckman Young Investigator.
Supporting Information Available: Two X-ray crystallographic
files in CIF format and a calculated and observed X-ray diffraction
pattern for BaTeMo O . This material is available free of charge via
2 9
the Internet at http://pubs.acs.org.
Figure 2. ORTEP (50% probability ellipsoids) in BaTeMo2O9 showing
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(
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(
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1
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(
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4
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(
9) Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511-522.
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(
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2
3
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2 3
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(
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(
14) Goodey, J.; Broussard, J.; Halasyamani, P. S. Chem. Mater. 2002, 14,
but more importantly to the constructiVe addition of these polariza-
tions. Additional powder SHG measurements indicated both materi-
als are type-1 phase-matchable.23 From the SHG efficiency and
3174-3180.
(15) Goodey, J.; Ok, K. M.; Broussard, J.; Hofmann, C.; Escobedo, F. V.;
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(
16) Opik, U.; Pryce, M. H. L. Proc. R. Soc. (London) 1957, A238, 425-447.
phase-matching behavior, we are able to estimate 〈deff
NLO susceptibility, for each material. For BaTeMo (BaTeW
exp is 28 (22) pm/V. Since 〈deff â(M-O), the bond
〉
exp, the bulk
(
17) Bader, R. F. W. Mol. Phys. 1960, 3, 137-151.
2
O
9
2 9
O ),
(18) Bader, R. F. W. Can. J. Chem. 1962, 40, 1164-1175.
(
(
(
19) Pearson, R. G. J. Am. Chem. Soc. 1969, 91, 4947-4955.
〈
d
eff
〉
exp
〉 ∝
20) Pearson, R. G. J. Mol. Struct. (THEOCHEM) 1983, 103, 25-34.
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J. K.; Albright, T. A. J. Am. Chem. Soc. 1986, 108, 2222-2236.
hyperpolarizability, it should be possible to estimate â for a given
n+
M
-O bond once the crystal structure, the type-1 phase-matching
(
22) Kunz, M.; Brown, I. D. J. Solid State Chem. 1995, 115, 395-406.
behavior, and 〈deff
〉
exp are known. We have developed such a model
n+
(23) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798-3813.
(
and recently published a table of â’s for a variety of M -O
24) For BaTeMo
2
O
9
(BaTeW
2 9
O
) colorless faceted crystals, 0.03 mm × 0.05
14
bonds. The model also works in “reverse”, that is â for the various
mm × 0.08 mm (0.20 mm × 0.30 mm × 0.40 mm), were used. Single-
crystal data were collected on a Siemens SMART diffractometer equipped
with a 1K CCD area detector using graphite monochromated Mo KR
radiation at 293 K. The data were integrated using the Siemens SAINT
[SAINT, Version 4.05; Siemens Analytical X-ray Systems, Inc.: Madison,
WI, 1995] program, with the intensities corrected for Lorentz, polarization,
air absorption, and absorption attributable to the variation in the path length
through the detector faceplate. ψ-Scan absorption corrections were applied.
The structures were solved by direct methods using SHELXS-97 [Sheld-
rick, G. M. SHELXS-97: A program for automatic solution of crystal
structures; University of Goettingen; Goettingen, Germany, 1997.] and
refined using SHELXL-97 [Sheldrick, G. M. SHELXS-97: A program
for crystal structure refinement; University of Goettingen: Goettingen,
n+
M
-O bonds can be input into the relevant equations, and 〈deff〉
calc
4
+
-40
may be obtained. For BaTeMo
2 9
O
, using â(Te -O) ) 130 × 10
4
6+
-40 4
m /V and â(Mo -O) ) 305 × 10
m /V results in 〈deff
〉
calc
)
2
0 pm/V, which is in reasonable agreement with 〈deff
〉
exp ) 28 pm/
V.
For BaTeW O
130 × 10
2 9
a different situation occurs. A â(W6+-O) of 570
m /V has been reported. This value seems
-
40
4
28
(
erroneously large, given that third-row transition metals are less
polarizable than second-row transition metals and that the magnitude
Germany, 1997.]. Crystal data for BaTeMo
P2 , a ) 5.5407(5) (5.490(2)) Å, b ) 7.4661(7) (7.446(3)) Å, c ) 8.8448-
9) (8.887(3)) Å, â ) 90.841(2)° (90.370(6)°), V ) 365.80(6) (363.3(2))
2 9 2 9
O (BaTeW O ): monoclinic,
6
+
6+
1
of the out-of-center distortion for W is smaller than Mo . Since
(
, the W6+-O and Te4+-O bond polarizations
3
in BaTeW
2
O
9
Å , Z ) 2 (2), R(F) ) 0.0313 (0.0601), GOF ) 1.078 (1.082).
(25) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244-247.
exp and â(Te4 -O) to estimate
+
constructively add, we may use 〈deff
〉
(
(
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27) Balraj, V.; Vidyasagar, K. Inorg. Chem. 1999, 38, 1394-1400.
â(W6 -O). In doing so, we arrive at a value of â(W -O) of 230
+
6+
×
10- m /V. This value is consistent with the smaller polariz-
40
4
(28) Wiegel, M.; Emond, M. H. J.; de Bruin, T. H. M.; Blasse, G. Chem. Mater.
1994, 6, 973-976.
6+
ability and magnitude of the intra-octahedral distortion of W
6+
compared with Mo
.
JA035314B
J. AM. CHEM. SOC.
9
VOL. 125, NO. 26, 2003 7765