Structural modulation of molybdenyl iodate architectures by alkali metal cations in AMoO3(IO3) (A = K, RB, Cs): A facile route to new polar materials with large SHG responses

Article abstract of DOI:10.1021/ja012190z

Three new molybdenyl iodates, KMoO₃(IO₃) (1), RbMoO₃(IO₃) (2), and CsMoO₃(IO₃) (3), have been prepared through the hydrothermal reactions of MoO₃ with AIO₄ (A = K, Rb, or Cs) at 180°C. These compounds are isolated as nearly colorless, air-stable crystals. Single-crystal X-ray diffraction experiments reveal that 1 possesses a corrugated layered structure constructed from molybdenum oxide chains that are bridged by iodate anions. The puckering of the layers is caused by the alignment of bent molybdenyl (MoO₂²⁺) groups along one side of the molybdenum oxide chains. The K⁺ cations separate these layers from one another and serve to balance charge. In contrast, compounds 2 and 3, which are isostructural, form three-dimensional structures with small cavities filled with Rb⁺ or Cs⁺ cations. The differences between the structures of 1 and those of 2 and 3 are due to rotation of the molybdenyl units as translation occurs down the molybdenum oxide chains in order to accommodate the increased size of the Rb⁺ and Cs⁺ cations. This rotation allows for the iodate anions to bridge the molybdenum oxide chains in an additional dimension, creating a three-dimensional network structure. Furthermore, while 1 crystallizes in a centrosymmetric space group, 2 and 3 crystallize in polar space groups. Second-harmonic generation measurements on 2 and 3 show large responses of 400x α-quartz. Differential scanning calorimetry measurements demonstrate that 2 and 3 are thermally stable to 494 and 486°C, respectively. UV-vis diffuse reflectance spectra of these compounds show a high degree of transparency from 1 to 3 eV and a band gap of 3.1 eV.

Full text of DOI:10.1021/ja012190z

Published on Web 02/08/2002
Structural Modulation of Molybdenyl Iodate Architectures by
Alkali Metal Cations in AMoO3(IO3) (A ) K, Rb, Cs): A Facile
Route to New Polar Materials with Large SHG Responses
Richard E. Sykora,† Kang Min Ok,‡ P. Shiv Halasyamani,‡ and
Thomas E. Albrecht-Schmitt*,†
Contribution from the Departments of Chemistry, Auburn UniVersity, Auburn, Alabama 36849,
and UniVersity of Houston, Houston, Texas 77204-5003
Received September 18, 2001
Abstract: Three new molybdenyl iodates, KMoO3(IO3) (1), RbMoO3(IO3) (2), and CsMoO3(IO3) (3), have
been prepared through the hydrothermal reactions of MoO3 with AIO4 (A ) K, Rb, or Cs) at 180 °C. These
compounds are isolated as nearly colorless, air-stable crystals. Single-crystal X-ray diffraction experiments
reveal that 1 possesses a corrugated layered structure constructed from molybdenum oxide chains that
are bridged by iodate anions. The puckering of the layers is caused by the alignment of bent molybdenyl
(MoO22+) groups along one side of the molybdenum oxide chains. The K+ cations separate these layers
from one another and serve to balance charge. In contrast, compounds 2 and 3, which are isostructural,
form three-dimensional structures with small cavities filled with Rb+ or Cs+ cations. The differences between
the structures of 1 and those of 2 and 3 are due to rotation of the molybdenyl units as translation occurs
down the molybdenum oxide chains in order to accommodate the increased size of the Rb+ and Cs+ cations.
This rotation allows for the iodate anions to bridge the molybdenum oxide chains in an additional dimension,
creating a three-dimensional network structure. Furthermore, while 1 crystallizes in a centrosymmetric space
group, 2 and 3 crystallize in polar space groups. Second-harmonic generation measurements on 2 and 3
show large responses of 400× R-quartz. Differential scanning calorimetry measurements demonstrate that
2 and 3 are thermally stable to 494 and 486 °C, respectively. UV-vis diffuse reflectance spectra of these
compounds show a high degree of transparency from 1 to 3 eV and a band gap of 3.1 eV.
molecules.11-13 Each of these routes has yielded materials with
large second-harmonic-generation (SHG) responses, and com-
Introduction
The successful preparation of new nonlinear optical (NLO),
piezoelectric, and ferroelectric materials is dictated by the ability
to control structure at both the molecular and higher-dimensional
levels. To accomplish this goal, structural building units that
impart asymmetry must be incorporated into the architecture
of the compound to create a noncentrosymmetric (NCS)
structure.1-3 In this regard, several independent approaches have
been developed, including the supramolecular engineering of
inorganic-organic hybrids,4-6 the preparation of phases with
cooperative second-order Jahn-Teller distortions (SOJT),7-10
and the design of multilayered films of aligned polar organic
pounds such as bis(nicotinato)zinc,4 whose SHG response is
1000× that of R-quartz, clearly demonstrate the tremendous
possibilities for further advances in the preparation of new NLO
materials.
An alternative method for synthesizing acentric materials is
to incorporate pyramidal ligands with a nonbonding, but
stereochemically active, pair of electrons, such as selenite14-16
or iodate,17-26 into crystalline solids. These anions have a
(11) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118,
8034.
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Am. Chem. Soc. 1997, 119, 2205.
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N. R.; Dutta, P.; Marks, T. J. Chem. Mater. 2001, 12, 15.
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Chem. 1994, 33, 4370.
† Auburn University.
‡ University of Houston.
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Co.: Dordrecht, 1983.
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Inorg. Chem. 1994, 33, 6043. (c) Dussack, L. L.; Harrison, W. T. A.;
Jacobson, A. J. Mater. Res. Bull. 1996, 31, 249.
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9
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J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002 1951

A R T I C L E S
Sykora et al.
propensity for undergoing at least partial alignment in the solid
state to create NCS structures that are often polar. For instance,
the selenite anion has been used to prepare a series of early
transition metal compounds including AVSeO5 (A ) Rb+,
Cs+),14 A(VO)3(SeO3)2 (A ) NH4+, K+, Rb+, or Cs+),14,15 and
A2(MoO3)3SeO3 (A ) NH4+, Rb+, Cs+, or Tl+).16 In addition
to alignment of the selenite anions in the latter compounds, the
MoO6 polyhedra are actually highly perturbed from idealized
octahedral symmetry owing to a second-order Jahn-Teller
distortion (SOJT).27-32 This type of distortion is prevalent in
high-valent, d0 transition metals owing to the symmetry-allowed
mixing of a low-lying excited state with a nondegenerate
ground-state molecular orbital.33-35 This orbital mixing results
in a significant distortion of the geometry of the metal centers
along the C2, C3, or C4 axes that becomes amplified with
increasing charge on the metal centers because the gap between
the excited state and the ground state undergoes a simultaneous
decrease in size.33-35
In a similar fashion, the iodate anion has been utilized in
transition metal and lanthanide chemistry to prepare compounds
that may serve as optoelectronic materials. Again, alignment
of the pyramidal anions often occurs in these solids to create
NCS structures, but also the metals impart additional electronic
properties owing to the presence of unpaired electrons or charge-
transfer bands. Examples of compounds that combine these
features include Co(IO3)2,17 Cu(IO3)2,18,19 and Ln(IO3)3‚nH2O
((Ln ) Ce - Lu; n ) 0-6).20-26 Even simple alkali metal
iodates, such as LiIO3, have become standard materials exploited
for laser frequency-doubling applications.36 Typically these
compounds are air-stable, are transparent in regions of interest,
and have high laser damage thresholds.
Ba,38 or Pb40), as well as Ag4(UO2)4(IO3)2(IO4)2O2,39 which
contains the previously unknown tetraoxoiodate (IO43-) anion.
Unfortunately, the uranyl (UO22+) dication is easily placed upon
centers of inversion or higher symmetry owing to its linear
geometry, and this inhibits the formation of the acentric
structures that are so prevalent in iodate chemistry. In an effort
to rationally design asymmetry into these compounds, we have
replaced the uranyl dications with isoelectronic molybdenyl
(MoO22+) dications. Not only is Mo(VI) highly susceptible to
SOJT distortions (vide supra), but the bent OdModO bond
angle of approximately 104° prevents additional moieties from
obtaining an idealized geometry around the Mo(VI) center.
Furthermore, hydrothermal synthetic techniques have been
shown to be highly amenable to the preparation of new
molybdenum-containing compounds.41-45
Herein we report the hydrothermal synthesis of a series of
alkali metal molybdenyl iodates with the general formula
AMoO3(IO3) (A ) K+, Rb+, or Cs+). We will demonstrate that
the alkali metal cations have a dramatic influence on the
molybdenyl iodate architecture and in fact modulate the crystal
structures. Finally, SHG powder measurements reveal that these
compounds have large SHG responses that may allow for further
technological development.46
Experimental Section
Syntheses. MoO3 (99.95%, Alfa-Aesar), CsCl (99.999%, Alfa-
Aesar), Cs2CO3 (99%, Alfa-Aesar), Rb2CO3 (99%, Alfa-Aesar), H5-
IO6 (98%, Alfa-Aesar), and KIO4 (99.9%, Fisher) were used as received.
RbIO4 and CsIO4 were prepared from the reaction of Rb2CO3 or Cs2-
CO3 with H5IO6 as reported by de Waal et al.47 Distilled and Millipore-
filtered water with a resistance of 18.2 MΩ was used in all reactions.
Reactions were run in Parr 4749 23-mL autoclaves with PTFE liners
for 3 d at 180 °C and cooled at a rate of 9 °C/h to 23 °C. The resultant
mother liquors were decanted from crystalline products that were
subsequently washed with methanol. A Leco Tem-Press 27-mL
autoclave filled with 20 mL of water and counter-pressured with 2500
psi of argon was employed for reactions at 420 °C. These reactions
were run in sealed quartz or gold ampules. Extreme care should always
be taken when scoring and opening sealed tubes from hydrothermal
reactions since these are typically under pressure. SEM/EDX analyses
were performed using a JEOL 840/Link Isis instrument. K and Mo
percentages were calibrated against standards. Typical results are within
3% of actual ratios. IR spectra were collected on a Nicolet 5PC FT-IR
spectrometer from KBr pellets.
In our laboratory, we have focused on the structural and
chemical influences of alkali metals, alkaline-earth metals, and
transition metals in the hydrothermal preparation of new uranyl
iodate compounds.37-40 These studies have resulted in a series
of low-dimensional compounds such as A2[(UO2)3(IO3)4O2] (A
) K,38 Rb,40 or Tl40) and AE[(UO2)2(IO3)2O2](H2O) (AE ) Sr,40
(21) Liminga, R.; Abrahams, S. C.; Bernstein, J. L. J. Chem. Phys. 1975, 62,
755.
(22) Nassau, K.; Shiever, J. W.; Prescott, B. E. J. Solid State Chem. 1975, 14,
122.
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16, 173.
(24) Abrahams, S. C.; Bernstein, J. L.; Nassau, K. J. Solid State Chem. 1977,
22, 243.
(25) Liminga, R.; Abrahams, S. C.; Bernstein, J. L. J. Chem. Phys. 1977, 67,
1015.
KMoO3(IO3) (1). MoO3 (119 mg, 0.83 mmol) and KIO4 (381 mg,
1.66 mmol) were loaded in a 23-mL PTFE-lined autoclave. Water (1
mL) was then added to the solids. The product mixture included large
clusters of colorless plates and large colorless cubes resting in a colorless
liquid. The colorless plates (very pale yellow in bulk) of 1 were easily
isolated to afford a yield of 268 mg (90% based on Mo), whereas the
colorless cubes were established as KIO3, based on a unit cell
determination. EDX analysis for KMoO3(IO3) provided a K:Mo:I ratio
of 1:1:1. IR (KBr, cm-1): 931 (ModOsym) (s), 916 (ModOasym) (s),
882 (s), 808 (s), 581 (s), 452 (s), 414 (s).
(26) Gupta, P. K. S.; Ammon, H. L.; Abrahams, S. C. Acta Crystallogr. 1989,
C45, 175.
(27) Opik, U.; Pryce, M. H. L. Proc. R. Soc. London 1937, A161, 220.
(28) Wheeler, R. A.; Whangbo, M. H.; Hughbanks, T.; Hoffman, R.; Burdett,
J. K.; Albright, T. A. J. Am. Chem. Soc. 1986, 108, 2222.
(29) Pearson, R. G. J. Mol. Struct. 1983, 103, 25.
(30) Kang, S. K.; Tang, H.; Albright, T. A. J. Am. Chem. Soc. 1993, 115, 1971.
(31) Cohen, R. E. Nature 1992, 358, 136.
(32) Burdett, J. K. Molecular Shapes; Wiley-Interscience: New York, 1980.
(33) Kunz, M.; Brown, I. D. J. Solid State Chem. 1995, 115, 395.
(34) Goodenough, J. B.; Longo, J. M. Crystallographic and magnetic properties
of perovskite and perovskite-related compounds. In Landolt-Bornstein;
Hellwege, K. H., Hellwege, A. M., Eds.; Springer-Verlag: Berlin, 1970;
Vol. 4, pp 126-314.
(35) Brown, I. D. Acta Crystallogr. 1977, B33, 1305.
(36) Rosker, M. J.; Marcy, H. O. New nonlinear materials for high-power
frequency conversion: from the near-ultraviolet to the long-wave infrared.
In NoVel Optical Materials and Applications; Khoo, I.-C., Simoni, F.,
Umeton, C., Eds.; John Wiley & Sons: New York, 1997; pp 175-204.
(37) Bean, A. C.; Peper, S. M.; Albrecht-Schmitt, T. E. Chem. Mater. 2001,
13, 1266.
(41) Finn, R. C.; Zubieta, J. Inorg. Chem. 2001, 40, 2466.
(42) Hagrman, P. J.; LaDuca, R. L., Jr.; Koo, H.-J.; Rarig, R., Jr.; Haushalter,
R. C.; Whangbo, M.-H.; Zubieta, J. Inorg. Chem. 2000, 39, 4311.
(43) Zapf, P. J.; Hammond, R. P.; Haushalter, R. C.; Zubieta, J. Chem. Mater.
1998, 10, 1366.
(44) Zapf, P. J.; LaDuca, R. L., Jr.; Rarig, R. S., Jr.; Johnson, K. M., III; Zubieta,
J. Inorg. Chem. 1998, 37, 3411.
(38) Bean, A. C.; Ruf, M.; Albrecht-Schmitt, T. E. Inorg. Chem. 2001, 40, 3959.
(39) Bean, A. C.; Campana, C. F.; Kwon, O.; Albrecht-Schmitt, T. E. J. Am.
Chem. Soc. 2001, 123, 8806.
(45) Zapf, P. J.; Haushalter, R. C.; Zubieta, J. Chem. Mater. 1997, 9, 2019.
(46) Sykora, R. E.; Albrecht-Schmitt, T. E. Provisional Patent No. 60/324,136,
2001.
(40) Bean, A. C.; Albrecht-Schmitt, T. E. J. Solid State Chem. 2001, 161, 416.
(47) de Waal, D.; Range, K.-J. Z. Naturforsch. 1996, 51b, 1365.
9
1952 J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002

Structural Modulation of Molybdenyl Iodates
A R T I C L E S
Table 1. Crystallographic Data for KMoO3(IO3) (1), RbMoO3(IO3)
(2), and CsMoO3(IO3) (3)
RbMoO3(IO3) (2). MoO3 (103 mg, 0.72 mmol) and RbIO4 (397
mg, 1.44 mmol) were loaded in a 23-mL PTFE-lined autoclave. Water
(1 mL) was then added to the solids. The product mixture consisted of
an almost clear and colorless liquid over clusters of colorless plates,
along with large colorless cubes. Manual separation of the colorless
plates (very pale yellow in bulk) of 2 resulted in the isolation of 261
mg (90% yield based on Mo), while the colorless cubes were determined
to be RbIO3. EDX analysis for RbMoO3(IO3) provided a Rb:Mo:I ratio
of 1:1:1. IR (KBr, cm-1): 935 (ModOsym) (m), 882 (s), 861 (s), 812
(s), 789 (s), 766 (m), 740 (s, sh), 721 (s), 670 (s, br), 448 (w), 420
(m).
CsMoO3(IO3) (3). MoO3 (91 mg, 0.63 mmol) and CsIO4 (409 mg,
1.26 mmol) were loaded in a 23-mL PTFE-lined autoclave. Water (3
mL) was then added to the solids. The product mixture consisted of an
almost clear and colorless liquid over clusters of very pale yellow rods,
along with large colorless cubes. Manual separation of the very pale
yellow rods of 3 produced 237 mg (83% yield based on Mo), while
the colorless cubes were determined to be CsIO3. EDX analysis for
CsMoO3(IO3) provided a Cs:Mo:I ratio of 1:1:1. IR (KBr, cm-1): 938
(ModOsym) (m), 878 (s), 857 (s), 814 (s), 797 (s), 768 (s), 743 (s), 709
(s), 668 (s, br), 441(w), 418 (m).
formula
formula mass (amu) 357.94
KMoO3(IO3)
RbMoO3(IO3)
404.31
CsMoO3(IO3)
451.75
space group
a (Å)
b (Å)
Pbca (No. 61) Pna21 (No. 33) Pna21 (No. 33)
7.7991(3)
7.3372(3)
19.2695(8)
1102.67(8)
8
7.7277(6)
10.2137(8)
7.3442(6)
579.67(8)
4
7.8633(5)
10.3920(6)
7.3869(5)
603.62(7)
4
c (Å)
V (Å3)
Z
T (°C)
λ (Å)
-80
-80
-80
0.71073
4.312
0.71073
4.633
0.71073
4.971
F
calcd (g cm-3
)
µ(Mo KR) (cm-1
)
86.88
0.0216
158.88
0.0194
131.90
0.0209
2
R(F) for Fo
>
2
2σ(Fo )a
2
Rw(Fo )b
0.0525
0.0421
0.0517
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|. b Rw(Fo ) ) [∑[w(Fo2 - Fc2)2]/∑wFo ]
.
2
4 1/2
nation (XPREP), structure solution (XS), and refinement (XL).51 The
final refinement included anisotropic displacement parameters for all
atoms. Some crystallographic details are listed in Table 1; additional
details can be found in the Supporting Information.
Thermal Analysis. Thermal data for compounds 1-3 were collected
using a TA Instruments model 2920 differential scanning calorimeter
(DSC). Samples (∼15 mg) were encapsulated in aluminum pans and
heated at 10 °C/min from 25 to 600 °C under a nitrogen atmosphere.
Secord-Order NLO Measurements. Powder SHG measurements
were performed on a modified Kurtz-NLO system using a 1064-nm
light source.48 A detailed description of the apparatus has been
published.10 Polycrystalline RbMoO3(IO3) and CsMoO3(IO3) were
ground separately and sieved into distinct particle size ranges, <20,
20-45, 45-63, 63-75, 75-90, and 90-125 µm. To make relevant
comparisons with known SHG materials, crystalline SiO2 and LiNbO3
were also ground and sieved into the same particle size range. All of
the powders were placed in separate capillary tubes. No index-matching
fluid was used in any of the experiments. I2ω/I2ω(SiO2) is taken for a
particle size range of 45-63 µm.
Results and Discussion
Syntheses. The reactions of MoO3 with KIO4, RbIO4, or
CsIO4 under mild hydrothermal conditions results in the
formation of KMoO3(IO3) (1), RbMoO3(IO3) (2), or CsMoO3-
(IO3) (3), respectively. These compounds are produced as
moderate-sized, nearly colorless crystals. While AIO3 (A ) K+,
Rb+, Cs+) are also isolated as byproducts from these reactions,
an excess of AIO4 improves both the yield and crystal quality
of 1-3. In addition, we have found that these reactions are
cation selective in that the reaction of MoO3 with KIO4 and
CsCl only produces 3. We have been unable to prepare 1-3
by using I2O5 or HIO3 as the source of iodate. This is a
consequence of pH dependence on product formation since the
change of I2O5 to AIO4 (A ) K+, Rb+, Cs+) starting materials
results in a dramatic increase in pH. The pH of the reaction
described for the synthesis of 3 is 3.9. When CsCl and I2O5 are
UV-Vis Diffuse Reflectance Spectra. Diffuse reflectance spectra
for 2 and 3 were measured from 1800 to 200 nm using a Shimadzu
UV3100 spectrophotometer equipped with an integrating sphere at-
tachment, with BaSO4 being used as the standard. The Kubelka-Monk
function was used to convert diffuse reflectance data to absorbance
spectra.49
substituted for CsIO4, the pH is lowered to 0.3. The observed
reduction of heptavalent iodine in the metaperiodate, IO4
-
,
Crystallographic Studies. Single crystals of 1, 2, and 3 with the
dimensions 0.032 mm × 0.140 mm × 0.140 mm, 0.040 mm × 0.080
mm × 0.080 mm, and 0.046 mm × 0.046 mm × 0.196 mm,
respectively, were mounted on glass fibers and aligned on a Bruker
SMART APEX CCD X-ray diffractometer. For each crystal, intensity
measurements were performed using graphite-monochromated Mo KR
radiation from a sealed tube and a monocapillary collimator. SMART
was used for preliminary determination of the cell constants and data
collection control. The intensities of reflections of a sphere were
collected by a combination of three sets of exposures (frames). Each
set had a different φ angle for the crystal, and each exposure covered
a range of 0.3° in ω. A total of 1800 frames were collected with an
exposure time per frame of 30 s.
The determination of integral intensities and global cell refinement
were performed with the Bruker SAINT (v 6.02) software package using
a narrow-frame integration algorithm. A semiempirical absorption
correction was applied, based on the intensities of symmetry-related
reflections measured at different angular settings using SADABS.50 The
program suite SHELXTL (v 5.1) was used for space group determi-
starting materials to pentavalent iodine in IO3- is a ubiquitous
reaction in our hydrothermal chemistry that we have also noted
in the syntheses of uranyl iodate compounds.37-40,52 We propose
that water is the reducing agent in these reactions.
Explorations of crystal growth conditions have shown that
the crystal sizes of 2 and 3 are related to the amount of water
employed in the reactions with crystals of larger size growing
from larger volumes of water. For instance, crystals of 3 several
millimeters in length can be grown by reacting MoO3 (1.26
mmol) with CsIO4 (2.53 mmol) in 10 mL of water at 180 °C
for 3 days. Furthermore, the synthesis of 3 can easily be scaled
up to produce gram quantities of these crystals.
Structures. The structures of compounds 1-3 are all
constructed from nearly identical six-coordinate cis-MoO4(IO3)2
fundamental building units, as shown in Figure 1. These units
corner-share two trans oxygen atoms with neighboring poly-
hedra to form an infinite one-dimensional molybdenum oxide
chain. These chains are then bridged by the iodate anions. The
key difference between the structure of 1 and those of 2 and 3
(48) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798.
(49) Wendlandt, W. W.; Hecht, H. G. Reflectance Spectroscopy; Interscience
Publishers: New York, 1966.
(50) SADABS, Program for absorption correction using SMART CCD based
on the method of Blessing: Blessing, R. H. Acta Crystallogr. 1995,
A51, 33.
(51) Sheldrick, G. M. SHELXTL PC, Version 5.0, An Integrated System for
Solving, Refining, and Displaying Crystal Structures from Diffraction Data;
Siemens Analytical X-ray Instruments, Inc.: Madison, WI, 1994.
9
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A R T I C L E S
Sykora et al.
Figure 1. View of the six-coordinate cis-MoO4(IO3)2 structural building
units in KMoO3(IO3) (1), RbMoO3(IO3) (2), and CsMoO3(IO3) (3). The
Mo(VI) centers show orthorhombic distortions along a C2 axis of an
idealized octahedron owing to a second-order Jahn-Teller distortion; 50%
probability ellipsoids are shown for 1.
Figure 3. Illustration of the corrugated layered structure of KMoO3(IO3)
(1) created by the alignment of the molybdenyl (MoO22+) units. The K+
2
cations serve to separate the [MoO3(IO3)]- layers from one another and
∞
to balance charge.
Figure 2. Depiction of the differences between one-dimensional
molybdenum oxide chains built from cis-MoO4(IO3)2 units in the structure
of KMoO3(IO3) (1) versus those observed for RbMoO3(IO3) (2) and
CsMoO3(IO3) (3). In 2 and 3, a 21 rotation occurs down the polar c-axis,
whereas the molybdenyl (MoO22+) units align on one side of the chain in
KMoO3(IO3).
(the latter two being isostructural) is that as translation occurs
2+
down the molybdenum oxide chains, the molybdenyl (MoO2
)
units align on one side of the chain in 1, whereas in 2 and 3 the
cis-MoO4(IO3)2 units are subject to a 21 rotation down the polar
c-axis, as illustrated in Figure 2. In 1, the alignment of the
molybdenyl units creates a corrugated layered structure, as
shown in Figure 3. In contrast, the rotation of the cis-MoO4-
(IO3)2 units in 2 and 3 connects alternating molybdenum centers
on all sides of the chains, creating a three-dimensional solid,
as depicted in Figure 4. In compounds 1-3, the alkali metal
cations serve to fill the void spaces and to balance charge. In 1,
the K+ cations reside in eight-coordinate square antiprismatic
environments, while in 2 and 3, the Rb+ and Cs+ cations possess
nine-coordinate distorted geometries. These cations, however,
clearly play an additional role in that the increase in ionic radii
from K+ (1.65 Å) to Rb+ (1.75 Å) or Cs+ (1.90 Å) is the source
of the substantial structural transformation observed between 1
and 2 or 3.53 Furthermore, it has been noted by Bergman et al.
Figure 4. Three-dimensional structures of RbMoO3(IO3) (2) and CsMoO3-
(IO3) (3) created by the bridging of molybdenum oxide chains by iodate
anions. The small cavities in this structure are occupied by the Rb+ or Cs+
cations. As these compounds are isostructural, only part of the structure of
CsMoO3(IO3) is shown.
that large cations such as Cs+ and Ba2+ have a tendency to
adopt acentric coordination environments, aiding in the forma-
tion of NCS compounds.54
The Mo(VI) centers in the cis-MoO4(IO3)2 polyhedra are
highly distorted from idealized octahedral symmetry, as shown
in Figure 1. Based on an average of the bond lengths in the
structures of 1-3, these hexavalent cations display a 2 + 2 + 2
bonding scheme, with two long bonds of 2.243(4) Å, two
(53) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(54) Bergman, J. G., Jr.; Boyd, G. D.; Ashkin, A.; Kurtz, S. K. J. Appl. Phys.
1969, 70, 2860.
(52) Gorski, A.; Milczarek, M. Pol. J. Chem. 1984, 58, 635.
9
1954 J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002

Structural Modulation of Molybdenyl Iodates
A R T I C L E S
Table 2. Selected Bond Distances (Å) and Angles (Deg) for
KMoO3(IO3) (1)
Distances
Mo(1)-O(2)
Mo(1)-O(1)
Mo(1)-O(3)
Mo(1)-O(5)
I(1)-O(4)
1.710(2)
2.020(2)
2.186(2)
1.779(2)
1.845(2)
1.719(2)
1.897(2)
2.236(2)
1.820(2)
Mo(1)-O(3)′
Mo(1)-O(6)′
I(1)-O(5)
I(1)-O(6)
Angles
O(1)-Mo(1)-O(2)
O(2)-Mo(1)-O(3)′
O(2)-Mo(1)-O(3)
O(1)-Mo(1)-O(5)
O(3)′-Mo(1)-O(5)
O(1)-Mo(1)-O(6)′
O(3)′-Mo(1)-O(6)′
O(5)-Mo(1)-O(6)′
103.4(1)
97.3(1)
93.6(1)
O(1)-Mo(1)-O(3)′
O(1)-Mo(1)-O(3)
O(3)′-Mo(1)-O(3)
O(2)-Mo(1)-O(5)
O(3)-Mo(1)-O(5)
O(2)-Mo(1)-O(6)′
O(3)-Mo(1)-O(6)′
103.1(1)
97.0(1)
154.2(1)
168.0(1)
83.56(9)
93.7(1)
88.6(1)
80.95(9)
161.7(1)
81.00(9)
74.31(9)
75.00(9)
Table 3. Selected Bond Distances (Å) and Angles (Deg) for
RbMoO3(IO3) (2)
Distances
Mo(1)-O(1)′
Mo(1)-O(3)′
Mo(1)-O(5)
I(1)-O(4)
1.712(4)
1.894(4)
2.216(4)
1.811(3)
1.815(3)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(6)′
I(1)-O(5)
1.727(4)
1.987(4)
2.285(3)
1.832(3)
Figure 5. View down the b-axis showing the alignment of the stere-
ochemically active lone pairs of electrons on the iodate anions that gives
rise to part of the polarity in the structures of RbMoO3(IO3) (2) and CsMoO3-
(IO3) (3). These anions are aligned along the polar c-axis. Rb+ or Cs+ cations
have been omitted for clarity.
I(1)-O(6)
Angles
O(1)′-Mo(1)-O(2)
O(2)-Mo(1)-O(3)′
O(2)-Mo(1)-O(3)
O(1)′-Mo(1)-O(5)
O(3)′-Mo(1)-O(5)
O(1)′-Mo(1)-O(6)′
O(3)′-Mo(1)-O(6)′
O(5)-Mo(1)-O(6)′
104.0(2)
98.0(2)
93.6(2)
165.0(2)
87.0(1)
86.6(2)
83.9(2)
79.4(1)
O(1)′-Mo(1)-O(3)′
O(1)′-Mo(1)-O(3)
O(3)′-Mo(1)-O(3)
O(2)-Mo(1)-O(5)
O(3)-Mo(1)-O(5)
O(2)-Mo(1)-O(6)′
O(3)-Mo(1)-O(6)′
96.8(2)
94.8(2)
161.14(3)
89.8(2)
78.2(1)
168.9(2)
82.0(2)
intermediate bonds of 1.945(4) Å, and two short bonds of
1.716(4) Å. The two short distances result from the ModO
double bonds that define the molybdenyl (MoO22+) unit, the
intermediate distances are the bonds to the bridging oxo groups,
and the two longest bonds are those with the oxygen atoms
from the bridging iodate ligands. This bonding pattern is readily
explained by an orthorhombic, second-order Jahn-Teller distor-
tion along a C2 axis of the MoO6 octahedron.33,34 As previously
discussed, the Mo(VI) centers are readily susceptible to such
distortions, because while the ground state (HOMO) has
octahedral symmetry, there is a low-lying excited state (LUMO)
close enough in energy to allow for mixing. This results in a
lowering of symmetry of the Mo(VI) center.33,34 Furthermore,
because the HOMO-LUMO gap decreases in size with increas-
ing charge on the metal center, metal ions capable of obtaining
high oxidation states, such as Mo(VI), tend to show particularly
pronounced distortions.3,33,34 This type of distortion has also
been noted in other Mo(VI) compounds, including Mo(VI)
selenites.14-16 Bond valence sum calculations help to further
illustrate the high valency of the Mo centers, with values of
6.01, 5.95, and 6.04 being determined for 1, 2, and 3,
respectively.55,56
structure show that the iodate groups do indeed display net
alignment along the polar c-axis, as illustrated in Figure 5.
Selected bond distances and angles for 1, 2, and 3 are given in
Tables 2, 3, and 4, respectively.
Nonlinear Optical Properties. Given the mm2 crystal class
of 2 and 3, we thought it was important to provide independent
verification of their acentric space group and to measure the
SHG responses of these compounds. SHG measurements were
performed using a modified Kurtz-NLO system with a 1064-
nm light source on powders ground from crystals of 2 and 3.
Generation of SHG light of 532 nm was carefully measured,
and these studies indicated large responses for 2 and 3 of 400×
R-quartz. The materials are also phase-matchable, as shown in
Figures 6 and 7. Calculations using methods described earlier10,57
give an average NLO susceptibility, d or deff , of 23 pm/V.
2ω
ijk
The large responses of 2 and 3 are due not only to the polarity
of the space group and the alignment of the lone pairs on the
iodate anions, but also to the large degree of polarization of
the Mo-O bonds.
Thermal Behavior. To evaluate the thermal behavior of 1-3,
DSC thermograms were measured from 25 to 600 °C under a
flowing nitrogen atmosphere. It is well established that iodate
compounds decompose through thermal disproportionation to
yield metaperiodates or periodates, oxygen, and iodine.52
Compounds 2 and 3 show simple thermograms, with the onset
of endothermic decomposition occurring at 494 and 486 °C,
The final important feature in the structures of 1-3 is the
orientation of the C3V iodate anions or, more specifically, the
alignment of the stereochemically active lone pair of electrons
on the iodine atoms. Compound 1 crystallizes in the centrosym-
metric space group Pbca, whose origin is at 1h. This precludes
any possibility of net alignment of either the iodate anions or
the SOJT distortion of the Mo(VI) centers. Compounds 2 and
3, however, crystallize in the polar space group Pna21, which
is in the crystal class mm2. Therefore, the polarity in the
structures should be along the c-axis. Examinations of the crystal
(55) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244.
(56) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. 1991, B47, 192.
(57) Ok, K. M.; Bhuvanesh, N. S. P.; Halasyamani, P. S. J. Solid State Chem.
2001, 161, 57.
9
J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002 1955

A R T I C L E S
Sykora et al.
Figure 8. UV-vis diffuse reflectance spectrum of CsMoO3(IO3) (3)
showing the absorption edge at 3.1 eV. Instrument artifact is denoted by /.
additional broadened endotherm centered around 404 °C is also
observed. We became suspicious that this transformation
actually corresponds to a phase transition of 1 to the same
structure type as that of 2 and 3. Therefore, we investigated
several hydrothermal reactions of MoO3 with KIO4 at 420 °C
for 3 days. This reaction temperature is higher than the observed
endotherm at 404 °C and should therefore allow for the
observation of any transformations of 1. Under these conditions,
only 1 and elemental iodine were isolated as solids. The
observation of iodine as a product of these reactions can be
ascribed to the high temperatures that cause thermal dispropor-
tionation of iodate. Unfortunately, there is only a narrow window
of approximately 50 °C between the endotherm at 404 °C and
thermal disproportionation at 453 °C, preventing higher tem-
perature reactions from revealing any possible phase transfor-
mations.
Figure 6. Phase matching curve (particle size) vs SHG intensity for
RbMoO3(IO3) (2).
UV-Vis Diffuse Reflectance Spectra. The transparency of
potential NLO materials in regions of interest is of critical
concern since minimal absorbance of the source and SHG light
reduces laser damage to the crystal and improves efficiency.
Furthermore, the electronic spectra of these compounds provide
fundamental information on the nature of conduction in these
extended structures. UV-vis diffuse reflectance spectra were
collected from crystalline samples that were ground into fine
powders. The spectrum of 3 from 1800 to 200 nm is shown (in
electronvolts) in Figure 8. The spectrum of 2 is nearly identical
to that of 3 and is not shown for the sake of brevity. These
spectra show that 2 and 3 are essentially transparent to
approximately 3 eV. Plots of absorbance squared versus energy
and the square root of absorbance versus energy at the absorption
edge (3-3.4 eV) both show nearly linear dependence, with R2
values of 0.9745 and 0.9911, respectively. Therefore, it is
unclear whether these compounds have direct or indirect band
gaps. Extrapolation of absorbance squared versus energy to
absorbance ) 0 provides an approximate band gap of 3.10
eV.59,60
Figure 7. Phase matching curve (particle size) vs SHG intensity for
CsMoO3(IO3) (3).
Table 4. Selected Bond Distances (Å) and Angles (Deg) for
CsMoO3(IO3) (3)
Distances
Mo(1)-O(1)′
Mo(1)-O(3)′
Mo(1)-O(5)
I(1)-O(4)
1.710(4)
1.856(4)
2.207(4)
1.798(4)
1.808(4)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(6)′
I(1)-O(5)
1.715(4)
2.015(4)
2.330(4)
1.835(4)
I(1)-O(6)
Angles
O(1)′-Mo(1)-O(2)
O(2)-Mo(1)-O(3)′
O(2)-Mo(1)-O(3)
O(1)′-Mo(1)-O(5)
O(3)′-Mo(1)-O(5)
O(1)′-Mo(1)-O(6)′
O(3)′-Mo(1)-O(6)′
O(5)-Mo(1)-O(6)′
103.0(2)
98.5(2)
93.5(2)
163.9(2)
87.6(2)
85.5(2)
83.8(2)
79.7(1)
O(1)′-Mo(1)-O(3)′
O(1)′-Mo(1)-O(3)
O(3)′-Mo(1)-O(3)
O(2)-Mo(1)-O(5)
O(3)-Mo(1)-O(5)
O(2)-Mo(1)-O(6)′
O(3)-Mo(1)-O(6)′
97.2(2)
92.8(2)
162.20(3)
91.4(2)
79.0(2)
170.7(2)
82.3(2)
respectively. This decomposition is in the same temperature
range as those of other iodate compounds that we and others
have investigated.20,22 These peaks are linked to exotherms at
501 and 489 °C for 2 and 3, respectively. These exotherms
correspond to reactions with the aluminum pans, resulting in
the formation of MAl(MoO4)2 (M ) Rb+, Cs+), as established
by single-crystal X-ray diffraction.58
Conclusions
In this study we have demonstrated that new polar materials
with large SHG responses can be created by combining
(58) Klevtsova, R. F.; Klevtsova, P. V. Kristallographia 1970, 15, 953.
(59) Pankove, J. I. Optical Processes in Semiconductors; Prentice Hall, Inc.:
Engelwood Cliffs, NJ, 1971.
(60) Feger, C. R.; Kolis, J. W.; Gorny, K.; Pennington, C. J. Solid State Chem.
1999, 143, 254.
The thermal behavior of 1 is similar to that of 2 and 3 in that
thermal disproportionation occurs at 453 °C. However, an
9
1956 J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002

Structural Modulation of Molybdenyl Iodates
A R T I C L E S
transition metals with highly asymmetric coordination environ-
ments, such as Mo(VI) in the form of molybdenyl, with anions
containing a stereochemically active lone pair of electrons. These
high-valent metals have the additional benefit of polarizing
M-O bonds, which aids in obtaining a substantial SHG
response. These compounds are thermally robust and show good
transparency from 1 to 3 eV. In future studies, we will address
the use of additional countercations in these hydrothermal
syntheses so as to ascertain their role in the formation of these
new inorganic architectures.
mental Facilities supported by the National Science Foundation
under Award No. DMR-9632667 and the Texas Center for
Superconductivity at the University of Houston. This work was
also supported by the NSF-Career Program through DMR-
0092054, and an acknowledgment is made to the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, for partial support of this research. The
authors also thank Mutlu Kartin (Clemson University) for
assistance in obtaining the UV-vis diffuse reflectance spectra.
Acknowledgment. T.E.A.-S. acknowledges NASA (ASGC)
and the Department of Energy, Heavy Elements Program (Grant
No. DE-FG02-01ER15187), for partial support of this work.
P.S.H. acknowledges the Robert A. Welch Foundation for
support. This work used the MRSEC/TCSUH Shared Experi-
Supporting Information Available: X-ray crystallographic
files for 1-3 (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA012190Z
9
J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002 1957