Gallium tris­(iodate), Ga(IO₃)₃

Ga(IO₃)₃ crystallizes in the space group P6₃, with the Ga atom at a site with imposed threefold symmetry. The crystal structure consists of slightly distorted GaO₆ octa­hedra that are bridged by I atoms of IO₃^- groups, giving rise to a three-dimensional polar network. The framework contains unoccupied hexa­gonal channels running parallel to the hexa­gonal [001] direction. The iodate groups have their stereochemically active non-bonded electron pairs pointing in the same direction along [001], which creates the polarity in the structure. The I-O bond distances and O-I-O angles are normal, being in the ranges 1.783 (3)-1.847 (2) Å and 94.68 (11)-99.61 (12)°, respectively.

S1. Comment

Metal iodates have attracted much attention because some of these compounds exhibit piezoelectric and pyroelectric effects, and they have potential applications in second harmonic generation (SHG) (Morosin et al., 1973). The continuted interest in these materials has led to the recent discovery of a series of new iodates, such as AMoO₃(IO₃) (A = K, Rb and Cs; Sykora et al., 2002), Hg(IO₃)₂ (Bentria, Benbertal, Bagieu-Beucher, Mosset & Zaccaro, 2003) and Bi(IO₃)₃ (Bentria, Benbertal, Bagieu-Beucher, Masse & Mosset, 2003), among which Hg(IO₃)₂ and AMoO₃(IO₃) (A = Rb and Cs) have been found to show large SHG responses. The title compound, Ga(IO₃)₃, was previously reported by Shklovskaya, who only presented the powder X-ray diffraction patterns of this compound. However, its crystal structure remains as yet undetermined. In the course of our investigation of novel iodate nonlinear optical (NLO) materials, we have obtained single crystals of Ga(IO₃)₃. Our present X-ray structural analysis indicates that Ga(IO₃)₃ crystallizes in a polar and chiral space group, P6₃, being a potential NLO material. In this paper, we report its crystal structure.

In the structure of Ga(IO₃)₃, each Ga³⁺ ion is coordinated to six O atoms in a slightly distorted octahedral geometry, the unique trans angle being 169.98 (10)° and the cis octahedral angles in the range 85.24 (10)–98.46 (10)° (Fig. 1 and Table 1). The Ga³⁺ ion shifts from the nominal center of its octahedron towards an octahedral face by about 0.016 Å, thus resulting in three short Ga—O bonds [3 × 1.960 (2) Å], each of which is trans to a long Ga—O bond [1.980 (2) Å]. A similar coordination environment has been found for Ga³⁺ in Na_1.98Ga_10.673O₁₇, with Ga—O distances of 3 × 1.785 Å and 3 × 1.812 Å (Michiue et al., 1991).

GaO₆ octahedra are arranged around 6₃ screw axes and bridged by the IO₃⁻ groups, leading to an overall three-dimensional framework, as shown in Fig. 2. Each IO₃⁻ group acts as a bidentate ligand bonded to two Ga³⁺ centers via two µ₂-O atoms, with the third O kept `dangling'. This form of ligation is common for IO<sub>3</sub><sup>−</sup> groups, as observed in VO<sub>2</sub>IO<sub>3</sub>·2H<sub>2</sub>O (Meschede & Mattes, 1976) and Ce(IO<sub>3</sub>)<sub>4</sub>·H<sub>2</sub>O (Ibers & Cromer, 1958). The I—O bond length for the `dangling' O atoms [1.783 (3) Å] is slightly shorter than those for the O atoms bound to the Ga³⁺ centers [1.828 (2) and 1.847 (2) Å] and both are very close to those generally found for iodate structures, for example 1.80 (1)–1.84 (1) Å in Hg(IO₃)₂ and 1.790 (8)–1.828 (8) Å in Bi(IO₃)₃ (Bentria, Benbertal, Bagieu-Beucher, Mosset & Zaccaro, 2003; Bentria, Benbertal, Bagieu-Beucher, Masse & Mosset, 2003). The O—I—O bond angles are normal, being in the range 94.68 (11)–99.61 (12)°. The pyramidal iodate groups are arranged in such a way that the `dangling' O atoms form an octahedral hole with a 4.490 (4) Å distance between trans O atoms. These holes are connected together to form one-dimensional open channels running parallel to the hexagonal [001] direction, and they are big enough to accommodate interstitial ions, such as Li⁺. However, our difference Fourier analyses did not reveal any significant electron-density peaks within these hexagonal tunnels. The refined structural model was further supported by bond valence sum (BVS) calculations (Brown & Altermatt, 1985), which produced reasonable values of 3.13 for Ga and 4.94 for I atoms.

Bi(IO₃)₃ is related to Ga(IO₃)₃ in stoichiometry but differs in the structure. In the former compound, each Bi³⁺ ion is coordinated by nine O atoms, forming a highly distorted monocapped square antiprism. The BiO₉ polydedra are edge-connected to form zigzag chains extending along the b axis; these chains are linked through IO₃⁻ groups forming layers parallel to (101); and the layers are held together via long I···O contacts, resulting in a three-dimensional framework. It is the variation in the coordination environments around the metal cations that is mainly responsible for a structure change from Ga(IO₃)₃ to Bi(IO₃)₃.

It is clear from Fig. 2 that the iodate groups in Ga(IO₃)₃ are all aligned along the polar c axis, which creates the polarity in the structure, and the compound may be a potential NLO material. To confirm this, SHG measurements were performed using a modified Kurtz-NLO system with a 1064 nm light source (Kurtz & Perry, 1968) on powder of the ground crystals. Generation of green light of 532 nm was clearly observed, further supporting the assignment of this structure in a non-centrosymmetric setting.

S2. Experimental

The title compound was synthesized under mild hydrothermal conditions. The starting materials are all of analytical-grade purity. Ga₂O₃ (0.046 g, 0.245 mmol), H₅IO₆ (0.334 g, 1.465 mmol), HNO₃ (0.5 ml) and H₂O (2 ml) were sealed in an 15 ml Teflon-lined autoclave, heated at 443 K for one week, and then cooled slowly to room temperature. The product consisted of colorless, hexagonal, column-like crystals, the largest having dimensions of 0.6 × 0.6 × 1.0 mm, in colorless mother liquor. The final pH of the reaction system was about 1.0. The crystals in about 64% yield (based on Ga) were isolated by washing the reaction product with deionized water and anhydrous ethanol followed by drying with anhydrous acetone. Powder X-ray diffraction analysis revealed that the product is the single phase of Ga(IO₃)₃, no lines due to impurity phases being observed. The synthesis of Ga(IO₃)₃ involved a complex redox process in which the I⁷⁺ ion in IO₄⁻ was reduced by water to I⁵⁺ in IO₃⁻. This type of reaction has already been noted in the preparation of many other metal iodates (Hector et al., 2002). HNO₃ was found to enhance the solvation processes and crystal growth of the target material. A separate set of experiments was conducted, in which a mixture of Ga₂O₃, H₅IO₆ and water (1:6:454) was heated hydrothermally at 443 K for 7 d. The reaction resulted in a fine white powder of unreacted Ga₂O₃, no crystals of Ga(IO₃)₃ being obtained.

Computing details

Data collection: Rigaku/AFC Diffractometer Control Software (Rigaku Corporation, 1994); cell refinement: Rigaku/AFC Diffractometer Control Software; data reduction: Rigaku/AFC Diffractometer Control Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The local coordination geometries of Ga and I in Ga(IO3)3, where displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. The crystal structure of Ga(IO3)3, projected along the hexagonal [001] direction, where single shaded circles and open circles are I and O atoms, respectively, and octahedral GaO6 units are represent by polyhedra.

Gallium tris(iodate)

Crystal data

Ga(IO3)3	Dx = 5.216 Mg m^−3
Mr = 594.42	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63	Cell parameters from 25 reflections
Hall symbol: P 6c	θ = 20.1–22.2°
a = 9.0924 (5) Å	µ = 15.90 mm^−1
c = 5.2862 (8) Å	T = 290 K
V = 378.47 (6) Å^3	Prism, colorless
Z = 2	0.2 × 0.1 × 0.1 mm
F(000) = 524

Data collection

Rigaku AFC-7R diffractometer	902 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.033
Graphite monochromator	θmax = 32.5°, θmin = 2.6°
2θ–ω scans	h = 0→13
Absorption correction: ψ scan (Kopfman & Huber, 1968)	k = −13→11
Tmin = 0.150, Tmax = 0.210	l = −7→7
2720 measured reflections	3 standard reflections every 150 reflections
913 independent reflections	intensity decay: −0.9%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.013P)^2 + 0.3153P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.016	(Δ/σ)max = 0.001
wR(F^2) = 0.037	Δρmax = 0.63 e Å^−3
S = 1.27	Δρmin = −1.03 e Å^−3
913 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
41 parameters	Extinction coefficient: 0.0907 (19)
1 restraint	Absolute structure: Flack (1983), 412 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.01 (2)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Direct phase determination yielded the positions of the Ga and I atoms. The remaining O atoms were located from the subsequent difference Fourier synthesis. All atoms were refined anisotropically. Refinement with the present model resulted in the small residual of R1 = 0.0158, while refinement of the inverted structure led to poor convergence with R1 = 0.0267.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ga1	0.6667	0.3333	0.39431 (10)	0.00503 (11)
I1	0.68630 (2)	0.662936 (19)	0.75374 (7)	0.00592 (7)
O1	0.5772 (3)	0.4499 (3)	0.6077 (5)	0.0094 (4)
O2	0.8307 (3)	0.7799 (3)	0.5055 (6)	0.0154 (5)
O3	0.5221 (3)	0.7137 (3)	0.6667 (4)	0.0077 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ga1	0.00420 (14)	0.00420 (14)	0.0067 (2)	0.00210 (7)	0.000	0.000
I1	0.00570 (9)	0.00658 (9)	0.00658 (9)	0.00388 (6)	−0.00030 (12)	−0.00149 (7)
O1	0.0094 (10)	0.0057 (9)	0.0130 (10)	0.0037 (8)	0.0017 (9)	−0.0022 (8)
O2	0.0126 (11)	0.0134 (11)	0.0186 (12)	0.0052 (9)	0.0104 (9)	0.0032 (10)
O3	0.0073 (10)	0.0115 (11)	0.0079 (8)	0.0074 (9)	0.0006 (7)	0.0006 (7)

Geometric parameters (Å, º)

Ga1—O3^i	1.960 (2)	Ga1—O1	1.980 (2)
Ga1—O3^ii	1.960 (2)	I1—O2	1.783 (3)
Ga1—O3^iii	1.960 (2)	I1—O3	1.828 (2)
Ga1—O1^iv	1.980 (2)	I1—O1	1.847 (2)
Ga1—O1^v	1.980 (2)
O3^i—Ga1—O3^ii	86.28 (10)	O3^i—Ga1—O1	169.98 (10)
O3^i—Ga1—O3^iii	86.28 (10)	O3^ii—Ga1—O1	98.46 (10)
O3^ii—Ga1—O3^iii	86.28 (10)	O3^iii—Ga1—O1	85.24 (10)
O3^i—Ga1—O1^iv	98.46 (10)	O1^iv—Ga1—O1	90.75 (11)
O3^ii—Ga1—O1^iv	85.24 (10)	O1^v—Ga1—O1	90.75 (11)
O3^iii—Ga1—O1^iv	169.98 (10)	O2—I1—O3	96.61 (12)
O3^i—Ga1—O1^v	85.24 (10)	O2—I1—O1	99.61 (12)
O3^ii—Ga1—O1^v	169.98 (10)	O3—I1—O1	94.68 (11)
O3^iii—Ga1—O1^v	98.46 (10)	I1—O1—Ga1	130.58 (13)
O1^iv—Ga1—O1^v	90.75 (11)	I1—O3—Ga1^vi	121.80 (12)

Symmetry codes: (i) y, −x+y, z−1/2; (ii) x−y+1, x, z−1/2; (iii) −x+1, −y+1, z−1/2; (iv) −x+y+1, −x+1, z; (v) −y+1, x−y, z; (vi) −x+1, −y+1, z+1/2.