The lone-pair cation I5+ in a hexagonal tungsten oxide-like framework: Synthesis, structure, and second-harmonic generating properties of Cs2I4O11

Article abstract of DOI:10.1002/anie.200460367

The layered iodate Cs₂I₄O₁₁ has been synthesized and characterized. Its hexagonal tungsten oxide-like framework consists of six-membered rings of corner-sharing IO₅ polyhedra with three lone pairs pointing inward and three lone pairs pointing outward (depicted for central ring). Capping of the layer by asymmetric IO₃ polyhedra on one side results in a noncentrosymmetric material with highly efficient second-harmonic generating properties.

Full text of DOI:10.1002/anie.200460367

Angewandte
Chemie
Layered Compounds
tion environments attributable to their lone pair. The five-
coordinate I5+ cation exhibits one short I O bond of
ꢀ
The Lone-Pair Cation I5+ in a Hexagonal Tungsten
Oxide-Like Framework: Synthesis, Structure, and
Second-Harmonic Generating Properties of
Cs2I4O11**
ꢀ
1.783(9) Š, three intermediate I O bonds of 2.027(15),
ꢀ
2.128(2), and 2.150(2) Š, one long I O bond of 2.441(16) Š,
and angles of 71.0(6)–171.2(2)8. The three-coordinate I5+
ꢀ
cation has a unique intermediate I O bond of 1.943(18) Š
and angles of 93.0(7)–134.8(8)8. These bond lengths and
angles are consistent with those of previously reported
iodates.[20–24] The IO5 and IO3 polyhedra share corners to
form the layered structure. In connectivity terms, the layer
Kang Min Ok and P. Shiv Halasyamani*
4= ꢀ
2+
can be described as a [3(IO = O= O = ) (IO= ) ]2ꢀ anion, with
3
2
2
1
3
Noncentrosymmetric (NCS) oxides exhibit a variety of
technologically important properties including ferroelectric-
ity, piezoelectricity, pyroelectricity, and second-order non-
linear optical behavior.[1–3] The rational design of new NCS
materials, however, remains a challenge, although a number
of strategies have been suggested.[4–9] We have focused on
synthesizing materials containing cations with lone pairs (e.g.,
Sb3+, Se4+, Te4+, etc.) in order to increase the incidence of
NCS in any new compound.[10,11] A structural topology that is
often observed in NCS is the hexagonal tungsten oxide
(HTO) framework. This structure has been reported for a
variety of cations, including V5+, Mo6+, W6+, and Sb5+.[12–18]
3
1
2
2
charge balance attained by two Cs+ cations.
One of the most novel aspects of the structure is the two-
dimensional IO5 layer (Figure 1). The HTO-like layer consists
of six-membered rings containing corner-sharing IO5 poly-
6
The framework consists of corner-sharing MO = octahedra
2
that are linked to form an array of three- and six-membered
rings. To date, structures have been observed in which other
cations, for example, Se4+, Te4+, P5+, and Sb3+, bridge, cap one
side, or cap both sides of the HTO layer. In instances of one-
sided capping, an NCS polar material is observed, often with
highly efficient second-harmonic generating (SHG) proper-
ties, that is, SHG > 400 ” SiO2.[13,17] In the reported materials,
the HTO framework has been restricted to octahedrally
coordinated d0 transition metals and the d10 Sb5+ cation. We
describe herein Cs2I4O11, the first material with a layered
HTO-like framework that contains a lone-pair cation, I5+. The
material also has I5+ cations capping the layer on one side,
which renders the structure NCS. Hence, SHG measurements
are also presented.
Crystals of Cs2I4O11 were grown hydrothermally by
combining Cs2CO3, Nb2O5, HIO3, and H2O in a Teflon-lined
autoclave at 2208C for 4 d. The product consisted of large
(maximum dimension 3 mm), colorless, faceted, hexagonal
crystals.
Cs2I4O11 exhibits a layered HTO-like framework consist-
ing of asymmetric IO5 and IO3 polyhedra.[19] Both the five-
and three-coordinate I5+ cations are in asymmetric coordina-
Figure 1. Ball-and-stick representation of the IO5 six-membered rings
in Cs2I4O11 with the lone-pairs shown schematically. Note how the lone
pairs on the IO5 polyhedra cancel, rendering the layer pseudocentro-
symmetric.
hedra in alternating orientation as one proceeds around the
ring. Cs2I4O11 is the first example of a lone-pair cation in a
HTO-like topology. In the rings themselves, three lone pairs
point inward and three outward. Thus, the lone pairs and
oxide anions alternate around the ring. The lone pairs have
two important structural consequences. First, the local dipole
moment on each IO5 polyhedron is in the direction of the lone
pair. If we sum all of the dipole moments with respect to the
IO5 group, the resultant moment is zero. In other words, there
is complete cancellation of the dipole moments with respect
to the IO5 polyhedra, that is, the IO5 layer is pseudocentro-
symmetric. This type of pseudocentrosymmetric layer was
observed previously in Rb2TeW3O12 and Cs2TeW3O12.[17] The
cancellation is relevant to the SHG efficiency. Second, the Cs+
cations are not coplanar with the IO5 ring, attributable to the
lone pairs. As shown in Figure 2, the Cs+ cations sit above and
below the rings. The environments of the Cs+ cations
influence any possible ion-exchange capabilities of the
material.[25] The IO5 layer is capped on one side by an
asymmetric three-coordinate I5+ cation, that is, an IO3 group
(see Figure 3). Alignment of the IO3 polyhedra in the [001]
direction results in an NCS and polar structure. The lone pairs
associated with the IO3 polyhedra also point along the [001]
[*] Dr. K. M. Ok, Prof. P. S. Halasyamani
Department of Chemistry
University of Houston
136 Fleming Building, Houston, TX 77204-5003 (USA)
Fax: (+1)713-743-2787
E-mail: psh@uh.edu
[**] We thank the Robert A. Welch Foundation for support. 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. P.S.H. is a Beckman
Young Investigator.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 5489 –5491
DOI: 10.1002/anie.200460367
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5489

Communications
we only have b(I O) which is 140 ” 10ꢀ40 m4 Vꢀ1, and since the
ꢀ
ꢀ
IO5 layer is pseudocentrosymmetric the only b(I O) that
contribute to the SHG are those of the IO3 group. Using this
model, which also approximates the contribution attributable
ꢀ
to the lone pair to be 20% of b(I O), we calculate a hdefficalcd
of 13.0 pmVꢀ1. This value is in reasonable agreement with
hdeffiexptl of 9.6 pmVꢀ1.
In conclusion, we have reported the first example of a
lone-pair cation, I5+, in a HTO-like framework topology. The
material is NCS and exhibits a SHG efficiency of 300 ” SiO2.
We are in the process of examining other NCS properties of
Cs2I4O11, including ferroelectric, pyroelectric, and piezoelec-
tric behavior.
Figure 2. Ball-and-stick representation of one six-membered IO5 ring
with the lone pairs shown schematically. The Cs+ cations (gray
spheres) reside above and below the rings because of the lone pairs.
Experimental Section
Cs2I4O11 was obtained by hydrothermal reaction. Cs2CO3 (1.0 mmol,
99 + %, Aldrich), HIO3 (28.5 mmol, 99.99%, Aldrich), Nb2O5
(1.9 mmol, 99.99%, Aldrich), and 5.0 mL of water were loaded into
a
23 mL Teflon-lined autoclave and subsequently sealed. The
autoclave was gradually heated to 2208C, held for 4 d, and cooled
slowly to room temperature at a rate of 68Chꢀ1. The mother liquor
was decanted from the products, which were then washed with water
and ethanol. The product was obtained as a mixture of large colorless
hexagonal crystals in 85% yield based on cesium and white powder.
The white powder was confirmed to be Nb2O5 by powder X-ray
diffraction. The Cs2I4O11 crystals were extracted manually from the
bulk. Despite repeated attempts at different temperature, molar
ratios, and filling volumes, we were unable to grow single crystals of
Cs2I4O11 in the absence of Nb2O5. The role of Nb2O5 in the crystal
growth is unclear. IR (KBr, cmꢀ1): 829 (n , m), 816 (n , m), 798 (n
ꢀ
I
ꢀ
ꢀ
I
O
I
O
, m), 778 (n , m), 764 (n , s), 747 (n , m), 670 (n , m), 538 (d ,
ꢀ
I O
ꢀ
ꢀ
ꢀ
ꢀ
O
I
O
I
O
I
O
I
O
s), 458 (d , m), 435 (d , m), 422 (d O, m), 409 (d O, s).
ꢀ
ꢀ
ꢀ
I
ꢀ
I
I
O
I
O
Thermogravimetric measurements indicate that Cs2I4O11 is stable up
to 4208C. Two rapid weight-loss events that occur above this
temperature result in total volatilization around 8008C.
Received: April 19, 2004
Revised: July 8, 2004
Figure 3. Ball-and-stick representation of Cs2I4O11 in the ac plane. The
lone pairs on two rows of I5+ cations are shown schematically, along
with the direction of the dipole moment. Note how all the I5+ lone
pairs point in the same direction and render Cs2I4O11 noncentrosym-
metric.
Keywords: hydrothermal synthesis · iodine ·
.
layered compounds · nonlinear optics · oxides
direction and produce a dipole moment in that direction
(Figure 3).
[1] F. Jona, G. Shirane, Ferroelectric Crystals, Pergamon, Oxford,
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Since Cs2I4O11 is NCS, we performed powder SHG
measurements on the compound.[26] A detailed description
of the measurement was reported earlier.[27] Cs2I4O11 has an
SHG efficiency of approximately 300 ” SiO2. This efficiency is
comparable to other NCS iodates, including HIO3 (300 ”
SiO2),[26] LiIO3 (300 ” SiO2),[26] NdMoO2(IO3)4(OH) (350 ”
SiO2),[28] and AMoO3(IO3) (A = Rb or Cs, 400 ” SiO2).[29] By
sieving the polycrystalline powder into various particle sizes
(20–150 mm) and measuring the SHG as a function of particle
size, we were able to determine that Cs2I4O11 is non-phase-
matchable (Type 1). This information along with the SHG
efficiency allows us to estimate hdeffiexptl, which for Cs2I4O11 is
9.6 pmVꢀ1. We have previously published a structural model
that allows us to calculate hdefficalcd on the basis of metal–
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oxygen bond hyperpolarizabilities b(M O).[27] With Cs2I4O11,
ꢀ
5490
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Angew. Chem. Int. Ed. 2004, 43, 5489 –5491

Angewandte
Chemie
[10] K. M. Ok, N. S. P. Bhuvanesh, P. S. Halasyamani, Inorg. Chem.
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[19] Crystal data for Cs2I4O11: colorless hexagonal prismatic crystal
(0.02 ” 0.04 ” 0.10 mm), Mr = 949.42, hexagonal P63 (no. 173),
a = b = 7.348(3), c = 13.871(6) Š, a = b = 90, g = 1208, V=
648.5(5) Š3, Z = 2, 1calcd = 4.862 gcmꢀ3, m = 151.85 cmꢀ1. Data
collection: Siemens SMART CCD area detector, MoKa radiation
(l = 0.71073 Š), T= 293 K. A total of 3858 unique reflections
were measured, of which 997 with I > 2.0s(I) were used for the
structure solution. Corrections were made for Lorentzian and
polarization effects, air absorption, and absorption attributable
to the variation in the path length through the detector faceplate.
Psi scans were used for the absorption correction on the
hemisphere of data. The data were solved and refined by using
SHELXS-97 and SHELXL-97, respectively. Final R/Rw on j F2 j
were 0.0453/0.1017, GOF= 1.090 for 36 parameters. The final
Fourier difference map revealed minimum and maximum peaks
of ꢀ1.650 and + 1.736 eŠꢀ3. All calculations were performed by
using the WinGX-98 crystallographic software package. Even
though large crystals (up to 3 mm) were sometimes obtained, a
small crystal as described above was used for the crystal
structure determination to minimize absorption. Further details
of the crystal structure investigation may be obtained from the
Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leo-
poldshafen, Germany (fax: (+ 49) 7247-808-666; e-mail: crys-
data@fiz-karlsruhe.de) on quoting the depository number CSD-
413942. G. M. Sheldrick, SHELXS-97—A program for auto-
matic solution and crystal structures, University of Gꢁttingen,
Gꢁttingen, Germany, 1997; G. M. Sheldrick, SHELXL-97—A
program for crystal structure refinement, University of Gꢁttin-
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logr. 1999, 32, 837.
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[22] B. W. Lucas, Acta Crystallogr. Sect. C 1984, 40, 1989.
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[24] K. Stahl, M. Szafranski, Acta Chem. Scand. 1992, 46, 1146.
[25] Attempts to exchange the Cs+ ions were unsuccessful. Since
Cs2I4O11 is water soluble, about 100 mg of the material was
stirred in 5 mL of a 0.5m solution of MNO3 (M = Na+, K+, Rb+)
in ethanol/water (4/1). Experiments at 608C for 3 d resulted in
incomplete reactions; therefore, the reactions were performed at
908C for 4 d in sealed fused-silica tubes to prevent evaporation
of ethanol. Under these conditions Cs2I4O11 reacted with the
alkali metal nitrates to produce NaIO3 (Pbnm),[23] KIO3 (P1),[22]
and RbIO3 (R3m)[20] in phase-pure form. The phase purity of
these materials was confirmed by powder X-ray diffraction
analysis, and the refined unit cell for each material matched the
literature values (see Supporting Information).
[26] S. K. Kurtz, T. T. Perry, J. Appl. Phys. 1968, 39, 3798.
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