Cerium triiodate, Ce(IO₃)₃

The crystal structure of Ce(IO₃)₃ consists of one-dimensional chains of edge-sharing CeO₉ polyhedra which are crosslinked into two-dimensional layers through bridging IO₃⁻ groups. The layers are held together via long I...O contacts, resulting in an extended three-dimensional network. The I—O bond distances and O—I—O angles are normal, lying in the ranges 1.806 (4)–1.846 (4) Å and 89.9 (2)–100.9 (2)°, respectively. The three crystallographically independent iodate groups all show different coordination modes.

S1. Comment

Metal iodates are of considerable interest because some of these compounds exhibit piezoelectric and pyroelectric effects and they have potential applications in second-harmonic generation (Morosin et al., 1973). Ce(IO₃)₃ was previously synthesized by Abrahams et al. (1976), who predicted it to be isostructural with Gd(IO₃)₃ (Liminga et al., 1977) in space group P2₁/a, with a = 13.555 (20) Å, b = 8.565 (9) Å, c = 7.214 (12) Å, β = 99.68 (28)° and V = 826 (2) ų. Recently, Douglas et al. (2004) reinvestigated the rare earth iodates and reported the lattice constants of all anhydrous 4f-iodates except for those of Ce(IO₃)₃. They found that Ln(IO₃)₃ compounds (Ln is Pr, Nd, Sm, Eu, Gd, Tb, Ho or Er) crystallize in a Gd(IO₃)₃-type structure. In the course of our research on novel iodate nonlinear optical (NLO) materials, we have obtained single crystals of Ce(IO₃)₃. Our X-ray structural analysis indicated that the newly prepared Ce(IO₃)₃ is a new polymorph, with cell dimensions different from those given by Abrahams et al. We report its crystal structure here.

In the structure of Ce(IO₃)₃, each Ce³⁺ ion is coordinated to nine O atoms in a distorted monocapped square-antiprismatic geometry, as shown in Fig. 1. The Ce—O distances of 2.420 (4)–2.809 (5) Å (average 2.556 Å; Table 1) are very reasonable when compared with the ranges 2.427 (3)–2.803 (3) Å (average 2.536 Å) in Ce₂(SO₄)₃·5H₂O (Junk et al., 1999) and 2.496 (3)–2.519 (3) Å (average 2.511 Å) in Ce(HSO₄)₃ (Wickleder, 1998), all featuring nine-coordinate Ce.

The CeO₉ polyhedra share edges with each other to form zigzag chains parallel to the b axis (Fig. 2). There are two sets of Ce···Ce distances within the chains, of 4.2878 (7) and 4.5070 (7) Å. The longer Ceⁱ···Ceⁱⁱ contacts are associated with pairs of Ce atoms double-bridged through I3O₃⁻ groups (sharing the O9···O9ⁱⁱⁱ edge), while the shorter ones (Ce···Ceⁱ) are those involved in the two bridging I2O₃⁻ and two bridging I3O₃⁻ groups (sharing the O6···O6ⁱ edge) [symmetry codes: (i); (ii); (iii) Please provide missing symmetry codes]. These chains are cross-linked by I1O₃⁻ groups via µ₂-bridging O atoms, to result in a two-dimensional layer parallel to the (101) plane. Adjacent layers are further connected together through long I···O contacts [I2···O2 = 2.779 (5), I2···O5 = 2.390 (5), I3···O2 = 2.738 (5) and I3···O7 = 2.851 (5) Å], giving rise to an extended three-dimensional network.

There are three crystallographically unique iodate groups, which adopt different coordination modes toward Ce³⁺ ions (Fig. 2). Each I1O₃⁻ group is a bidentate ligand bonded to two Ce³⁺ centres via two µ₂-O atoms, each I2O₃⁻ group functions as a tridentate ligand, chelating one Ce³⁺ and coordinated to a second Ce³⁺ ion through a µ₃-O atom, while each of the I3O₃⁻ groups acts as a tetradentate ligand that chelates one Ce³⁺ centre and simultaneously binds the other two via a µ₂-O and a µ₃-O atom, respectively. Despite the difference in the coordination schemes of the iodate groups, the I—O bond lengths of 1.806 (4)–1.846 (4) Å and the O—I—O angles of 89.9 (2)–100.9 (2)° show no particular distortions and are within the range observed previously for inorganic iodates (Douglas et al., 2004). Bond-valence sum (Brown & Altermatt, 1985) calculations give values of 3.14 for Ce and 4.74–4.93 for I atoms, in reasonable agreement with their expected formal valences.

The crystal structure of Ce(IO₃)₃ presented here is different from that previously reported by Abrahams et al. (1976). The latter contains a three-dimensional network consisting of irregular CeO₈ polyhedra bridged by bi- and tridentate IO₃⁻ ligands. It is the difference in the coordination modes of the iodate groups, as well as the variation in the Ce³⁺ coordination geometry, that is responsible for the structural versatility of Ce(IO₃)₃.

S2. Experimental

The title compound was synthesized using hydrothermal techniques. All reagents were of analytical grade. Ce(NO₃)₃·6H₂O (0.106 g, 0.244 mmol), I₂O₅ (0.287 g, 0.860 mmol) and H₂O (5 ml) were sealed in a 25 ml Teflon-lined autoclave. This was heated in an oven at 443 K for one week under autogenously generated pressure, then cooled slowly to room temperature. The product consisted of yellow block-like crystals with largest dimensions of 0.6 × 0.8 × 1.0 mm³, covered by a colourless liquid. The final pH of the reaction system was about 1.0. The crystals were isolated in about 84% yield (based on Ce) 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 a single phase of Ce(IO₃)₃ and no lines due to impurity phases were observed. In previous work (Douglas et al., 2004), Ln(IO₃)₃ compound (Ln is Pr, Nd, Sm, Eu, Gd, Tb, Ho or Er) were prepared by decomposition of the corresponding periodates under hydrothermal conditions. Here, the Ce analogue was synthesized directly from the hydrothermal reaction of Ce(NO₃)₃ with I₂O₅. I₂O₅ was found to play an important role in the crystal growth of Ce(IO₃)₃. The replacement of I₂O₅ by H₅IO₆ as an iodine source did not generate any crystalline products, instead forming lumps of amorphous gel.

S3. Refinement

Direct phase determination yielded the positions of the Ce and I atoms. The remaining O atoms were located in subsequent difference Fourier syntheses. All atoms were refined anisotropically. The highest residual electron-density peaks were located at 0.69 Å from the Ce atoms.

Computing details

Data collection: Rigaku/AFC Diffractometer Control Software (Rigaku, 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: SCHAKAL92 (Keller, 1992); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The coordination geometry about the Ce atom. Double-shaded circles denote Ce atoms and open circles denote O atoms. The monocapped square antiprism is indicated by thin lines.
	Fig. 2. The Ce(IO3)3 layer parallel to the (101) plane, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) x, 1 + y, z; (iii) 1 − x, 2 − y, 1 − z.]

Cerium triiodate

Crystal data

Ce(IO3)3	F(000) = 1156
Mr = 664.82	Dx = 5.431 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 25 reflections
a = 8.9188 (10) Å	θ = 16.8–22.5°
b = 5.9619 (11) Å	µ = 17.01 mm^−1
c = 15.4047 (12) Å	T = 290 K
β = 96.974 (8)°	Plate, yellow
V = 813.05 (19) Å^3	0.1 × 0.08 × 0.05 mm
Z = 4

Data collection

Rigaku AFC-7R diffractometer	3317 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.043
Graphite monochromator	θmax = 35.0°, θmin = 2.5°
2θ/ω scans	h = 0→14
Absorption correction: ψ scan (Kopfmann & Huber, 1968)	k = 0→9
Tmin = 0.201, Tmax = 0.420	l = −24→24
4133 measured reflections	3 standard reflections every 150 reflections
3582 independent reflections	intensity decay: −1.7%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.032	w = 1/[σ^2(Fo^2) + (0.0357P)^2 + 9.4569P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.087	(Δ/σ)max < 0.001
S = 1.17	Δρmax = 2.86 e Å^−3
3582 reflections	Δρmin = −2.67 e Å^−3
119 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0060 (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.

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

	x	y	z	Uiso*/Ueq
Ce	0.53898 (3)	0.26355 (5)	0.400306 (18)	0.00871 (7)
I1	0.61370 (4)	0.59066 (6)	0.19809 (2)	0.00962 (8)
I2	0.83581 (4)	0.63715 (5)	0.47725 (2)	0.00892 (8)
I3	0.27932 (4)	0.81153 (5)	0.34913 (2)	0.00880 (8)
O1	0.5856 (5)	0.3179 (7)	0.2445 (3)	0.0131 (7)
O2	0.4496 (6)	0.5911 (9)	0.1164 (3)	0.0201 (9)
O3	0.7608 (5)	0.4993 (8)	0.1325 (3)	0.0152 (7)
O4	0.7846 (5)	0.4705 (7)	0.3798 (3)	0.0140 (7)
O5	0.9098 (5)	0.4145 (8)	0.5528 (3)	0.0173 (8)
O6	0.6433 (5)	0.6017 (8)	0.5077 (3)	0.0169 (8)
O7	0.3556 (5)	1.0468 (8)	0.2936 (3)	0.0164 (8)
O8	0.4216 (6)	0.6061 (8)	0.3297 (3)	0.0180 (8)
O9	0.3641 (5)	0.9118 (7)	0.4559 (3)	0.0140 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ce	0.00745 (12)	0.01044 (13)	0.00870 (12)	0.00144 (8)	0.00284 (8)	0.00038 (8)
I1	0.00861 (14)	0.01044 (14)	0.01024 (14)	−0.00164 (9)	0.00279 (10)	−0.00045 (9)
I2	0.00709 (13)	0.00899 (13)	0.01094 (14)	−0.00048 (9)	0.00213 (10)	−0.00020 (9)
I3	0.00771 (14)	0.00941 (13)	0.00934 (14)	−0.00033 (9)	0.00124 (10)	0.00023 (9)
O1	0.0173 (19)	0.0115 (16)	0.0108 (16)	−0.0009 (14)	0.0036 (13)	0.0018 (12)
O2	0.0147 (19)	0.024 (2)	0.019 (2)	−0.0040 (16)	−0.0066 (16)	0.0070 (17)
O3	0.0145 (18)	0.0153 (18)	0.0175 (18)	−0.0031 (14)	0.0085 (15)	−0.0022 (14)
O4	0.0123 (17)	0.0149 (17)	0.0153 (17)	−0.0024 (14)	0.0038 (14)	−0.0046 (14)
O5	0.0144 (19)	0.0168 (19)	0.021 (2)	0.0010 (15)	0.0028 (15)	0.0099 (15)
O6	0.0119 (17)	0.0187 (19)	0.021 (2)	−0.0006 (14)	0.0080 (15)	−0.0043 (16)
O7	0.019 (2)	0.0174 (19)	0.0130 (17)	−0.0065 (15)	0.0001 (14)	0.0039 (14)
O8	0.019 (2)	0.0170 (19)	0.019 (2)	0.0116 (16)	0.0069 (16)	−0.0003 (15)
O9	0.0172 (19)	0.0149 (17)	0.0089 (15)	−0.0008 (14)	−0.0021 (13)	−0.0020 (13)

Geometric parameters (Å, º)

Ce—O6^i	2.420 (4)	I1—O1	1.806 (4)
Ce—O3^ii	2.480 (4)	I1—O2	1.811 (4)
Ce—O8	2.485 (4)	I1—O3	1.833 (4)
Ce—O9^i	2.506 (4)	I2—O4	1.812 (4)
Ce—O1	2.507 (4)	I2—O5	1.835 (4)
Ce—O7^iii	2.529 (5)	I2—O6	1.846 (4)
Ce—O4	2.567 (4)	I3—O8	1.814 (4)
Ce—O6	2.700 (5)	I3—O7	1.818 (4)
Ce—O9^iii	2.809 (5)	I3—O9	1.826 (4)
O6^i—Ce—O3^ii	151.90 (16)	O8—Ce—O9^iii	121.82 (15)
O6^i—Ce—O8	72.93 (16)	O9^i—Ce—O9^iii	64.14 (16)
O3^ii—Ce—O8	135.12 (14)	O1—Ce—O9^iii	123.47 (13)
O6^i—Ce—O9^i	78.75 (16)	O7^iii—Ce—O9^iii	58.50 (13)
O3^ii—Ce—O9^i	74.66 (15)	O4—Ce—O9^iii	155.54 (14)
O8—Ce—O9^i	144.25 (15)	O6—Ce—O9^iii	122.24 (13)
O6^i—Ce—O1	136.01 (16)	O1—I1—O2	97.9 (2)
O3^ii—Ce—O1	71.30 (14)	O1—I1—O3	95.4 (2)
O8—Ce—O1	65.44 (14)	O2—I1—O3	100.9 (2)
O9^i—Ce—O1	145.16 (15)	O4—I2—O5	99.4 (2)
O6^i—Ce—O7^iii	97.06 (16)	O4—I2—O6	89.9 (2)
O3^ii—Ce—O7^iii	88.15 (16)	O5—I2—O6	91.7 (2)
O8—Ce—O7^iii	86.01 (16)	O4—I2—O5^iv	85.33 (18)
O9^i—Ce—O7^iii	119.20 (14)	O5—I2—O5^iv	75.7 (2)
O1—Ce—O7^iii	67.23 (14)	O6—I2—O5^iv	165.60 (18)
O6^i—Ce—O4	123.83 (15)	O8—I3—O7	97.7 (2)
O3^ii—Ce—O4	68.50 (15)	O8—I3—O9	98.5 (2)
O8—Ce—O4	82.62 (16)	O7—I3—O9	91.9 (2)
O9^i—Ce—O4	95.82 (15)	I1—O1—Ce	122.9 (2)
O1—Ce—O4	64.94 (14)	I1—O3—Ce^v	120.3 (2)
O7^iii—Ce—O4	131.28 (14)	I2—O4—Ce	107.13 (19)
O6^i—Ce—O6	66.39 (17)	I2—O5—I2^iv	104.3 (2)
O3^ii—Ce—O6	113.34 (15)	I2—O6—Ce^i	145.3 (2)
O8—Ce—O6	75.52 (16)	I2—O6—Ce	101.06 (18)
O9^i—Ce—O6	73.38 (15)	Ce^i—O6—Ce	113.61 (17)
O1—Ce—O6	113.89 (14)	I3—O7—Ce^vi	109.74 (19)
O7^iii—Ce—O6	158.00 (16)	I3—O8—Ce	138.4 (2)
O4—Ce—O6	58.73 (13)	I3—O9—Ce^i	136.1 (2)
O6^i—Ce—O9^iii	68.39 (15)	I3—O9—Ce^vi	98.86 (18)
O3^ii—Ce—O9^iii	91.62 (14)	Ce^i—O9—Ce^vi	115.86 (16)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x+3/2, y−1/2, −z+1/2; (iii) x, y−1, z; (iv) −x+2, −y+1, −z+1; (v) −x+3/2, y+1/2, −z+1/2; (vi) x, y+1, z.