Europium(II) oxy­iodide

Yellow single crystals of Eu₄OI₆ were grown from fluxes in Ta ampoules and structurally characterized by X-ray diffraction. Eu₄OI₆ crystallizes in a hexagonal system and is isotypic with the corresponding chloride and bromide compounds. The O atom and one Eu atom lie on sites with 3m symmetry; the other Eu atom and the two unique I atoms are at sites with m symmetry. The structure is characterized by O-centered tetrahedra of divalent europium cations [Eu-O = 2.391 (15) and 2.416 (5) Å, and mean Eu-Eu = 3.94 Å] and hexagonal channels along [001] filled with iodide anions.

S1. Comment

Europium oxyhalides of general formula Eu₄OX₆ (X = Cl and Br) was first referred to by Tanguy et al. (1970), who indexed their X-ray patterns on the basis of the pattern of Ba₄OCl₆. The formation of single crystals has been observed as a by-product from oxygen-containing melts (Meyer & Schleid, 1987), and a systematic route based on reaction mixtures incorporating EuX₃ (X = Cl and Br), Eu₂O₃ and lithium metal succeeded in growing pale-yellow single crystals of good quality (Schleid & Meyer, 1987a). The synthesis and structure determination of the iodide phase Eu₄OI₆, however, have not yet been studied. The present contribution is part of a project targeted at synthesizing rare-earth cyanamides/carbodiimides, during which, by accident, we found a simple way to grow single crystals of Eu₄OX₆ (X = Cl, Br and I).

Eu₄OI₆ crystallizes in a hexagonal system in P6₃mc and is isotypic with Eu₄OCl₆ and Eu₄OBr₆. The most important structural feature of Eu₄OI₆ is the tetrahedral [Eu₄O]⁶⁺ unit (Fig. 1) containing divalent europium. The O atom resides in the center of the tetrahedron, the Eu—O distances being similar to one another [Eu1—O = 2.39 Å and Eu2—O = 2.42 Å]. The Eu—Eu distances are 3.89 (Eu1—Eu2) and 3.98 Å (Eu2—Eu2), and the tetrahedral angles are very close to their ideal values, namely 108.0 (3)° for Eu1—O—Eu2 and 110.9 (3)° for Eu2—O—Eu2. The average Eu—O (2.41 Å) and Eu—Eu (3.94 Å) distances in Eu₄OI₆ are only slightly larger than those in Eu₄OCl₆ (2.36 and 3.86 Å, respectively) and Eu₄OBr₆ (2.39 and 3.90 Å, respectively).

The Eu coordination is augmented by neighboring iodide anions, with interatomic distances ranging from 3.39 to 3.58 Å (Eu1, six bonds) and from 3.29 to 3.55 Å (Eu2, seven bonds). On the basis of the tabulated bond-valence parameter for the Eu^II—O [r₀ = 2.147 Å (Brese & O'Keeffe, 1991)] and Eu^II—I bonds [r₀ = 2.869 Å (generated from the crystal structure of EuI₂; Sanchez et al., 1985)], the empirical valences of atoms Eu1 and Eu2 sum to 1.69 and 2.11, respectively. Even including the three very long Eu1—O distances [3.96 Å] in the calculation, the Eu1 valence sum does not fully match the divalent state. We note that a similar effect is missing in Ba₄OCl₆, thereby indicating the possibility of weak metal–metal bonding in the rare-earth tetrahedron through residual electron density. In fact, the Eu^II—Eu^II distances in Eu₄OI₆ are almost identical with those in elemental Eu (3.99 Å; Sutton, 1965), and similar effects of metal–metal bonding in less than fully oxidized rare-earth metal compounds have been described in corresponding quantum-chemical investigations of extended solids (Landrum et al., 1999).

When projected along [001] (Fig. 2), the crystal structure of Eu₄OI₆ exhibits one-dimensional hexagonal channels of approximate diameter 4.86 Å (atom-to-atom distance), which can be compared with the corresponding channel diameters in Eu₄OBr₆ (4.70 Å) and in Eu₄OCl₆ (4.62 Å). Analogous channels are also observed in related rare-earth oxyhalides, such as Sm₄OCl₆ [4.61 Å; calculated with the data given by Schleid & Meyer (1987b)] and Yb₄OCl₆ (4.37 Å; Schleid & Meyer, 1987c). It is not yet known whether these channels can provide enough empty space for the inclusion of other chemical species. To the best of our knowledge, hexagonal channels filled with only halide anions are exclusively found in the rare-earth oxyhalides. For related oxoselenate or phosphate compounds, similar hexagonal channels have been described in combination with electron `lone-pairs' on the Se and P atoms (Wontcheu & Schleid, 2003; Morris et al., 1994). In Tb₃O₂Cl[SeO₃]₂ (Wontcheu & Schleid, 2002), two chloride anions in diametrically opposed positions around the hexagon are involved in the channel formation.

S2. Experimental

Because of the extraordinary affinity of europium towards oxygen, all chemical manipulations were performed in a glovebox under dry argon, with oxygen and moisture levels below 1 p.p.m. The accidental synthesis of Eu₄OI₆ resulted from a 1:1 mixture of EuI₂ (Aldrich, 99.9%) and Li₂(NCN) [synthesized according to Liu (2002)], which was placed in a tantalum ampoule that was sealed with an arc welder and jacketed with quartz, both under argon. The sample was heated to 1073 K for two days and then slowly cooled (6 K min⁻¹) to room temperature. As a result of slight oxygen contamination (probably as LiOH) of the X-ray pure Li₂(NCN), yellow prisms (about 3 ×0.5 ×0.5 mm in size) of Eu₄OI₆ were obtained. The phase seemingly acts as a getter for traces of oxygen, in agreement with independent observations (Meyer & Schleid, 1987). Compound (I) can also be synthesized through deliberately introducing oxygen-containing educts (e.g. Eu₂O₃), and crystals of Eu₄OCl₆ and Eu₄OBr₆ can be grown using identical conditions. The halide/cyanamide melts seemingly catalyze the growth of single crystals, which are difficult to obtain otherwise. Because of their sensitivity to oxygen and moisture, the single crystals were mounted in glass capillaries inside a glovebox for the X-ray measurements.

S3. Refinement

The empirical SADABS method was used for absorption correction. The refinement was started from the structure of the isotypic chloride phase, with anisotropic displacement parameters for Eu and I atoms and an isotropic displacement parameter for O atoms. The absolute structure was determined on the basis of 336 Friedel pairs [Flack parameter = 0.07 (4)].

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SMART; data reduction: SAINT-Plus (Bruker, 1999); program(s) used to solve structure: SHELXTL (Sheldrick, 1998); program(s) used to refine structure: SHELXTL; molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The tetrahedral [Eu4O]^6+ unit in Eu4OI6, with bond distances given in Å; displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. Projection of the crystal structure of Eu4OI6 along [001].

tetraeuropium hexaiodine oxygen

Crystal data

Eu4OI6	Dx = 6.138 Mg m^−3
Mr = 1385.24	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63mc	Cell parameters from 9730 reflections
Hall symbol: P 6c -2c	θ = 2.3–28.3°
a = 10.404 (2) Å	µ = 28.82 mm^−1
c = 7.996 (3) Å	T = 293 K
V = 749.5 (3) Å^3	Prism, yellow
Z = 2	0.15 × 0.10 × 0.09 mm
F(000) = 1156

Data collection

Bruker Apex CCD diffractometer	727 independent reflections
Radiation source: fine-focus sealed tube	704 reflections with I > 2 σ(I)
Graphite monochromator	Rint = 0.056
ω scan	θmax = 28.3°, θmin = 2.3°
Absorption correction: empirical (using intensity measurements) SADABS, Sheldrick, 1996	h = −13→13
Tmin = 0.479, Tmax = 1.0	k = −13→13
9730 measured reflections	l = −10→10

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)] = 0.027	w = 1/[σ^2(Fo^2) + (0.0325P)^2 + 11.61P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.067	(Δ/σ)max < 0.001
S = 1.09	Δρmax = 0.86 e Å^−3
727 reflections	Δρmin = −1.73 e Å^−3
24 parameters	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
1 restraint	Absolute structure parameter: 0.07 (4)

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.

PLAT731 Type_1 Test for consistency of Bond su's and Coordinate su's in CIF A large ratio of the reported and calculated bond s.u.'s is found. The use of a DFIX instruction might cause such a warning since calculated s.u.'s are based on reported variances only. Note_1: su's on the unitcell dimensions are taken into account in the calculation of expected su's. This may result in large differences between expected and reported su's when this contribution is not included in the reported su's, in particular for inaccurate unitcells. Note_2: Another source for the discrepancy between calculated and reported su's can be that the validation software has access only to the variances of the refined parameters as opposed to the full co-variance matrix used by e.g. SHELXL for the calculation of derived parameters with associated su's. Constrained/restrained refinement may cause largei co-variances.

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
Eu1	0.3333	0.6667	0.40454 (18)	0.0234 (3)
Eu2	0.20583 (3)	−0.20583 (3)	0.01196 (10)	0.01521 (16)
I1	0.13479 (5)	−0.13479 (5)	0.39107 (13)	0.0202 (2)
I2	0.46500 (5)	−0.46500 (5)	0.70720 (13)	0.0192 (2)
O	0.3333	0.6667	0.1055 (19)	0.007 (3)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Eu1	0.0251 (4)	0.0251 (4)	0.0200 (6)	0.0125 (2)	0.000	0.000
Eu2	0.0117 (2)	0.0117 (2)	0.0232 (3)	0.0067 (2)	−0.00086 (16)	0.00086 (16)
I1	0.0136 (3)	0.0136 (3)	0.0288 (5)	0.0032 (3)	0.00165 (19)	−0.00165 (19)
I2	0.0165 (3)	0.0165 (3)	0.0248 (5)	0.0083 (4)	0.00049 (19)	−0.00049 (19)

Geometric parameters (Å, º)

Eu1—O	2.391 (15)	Eu2—I2^ix	3.5458 (11)
Eu1—I2^i	3.3891 (16)	Eu2—I2^x	3.5458 (11)
Eu1—I2^ii	3.3891 (16)	Eu2—Eu1^iv	3.8900 (16)
Eu1—I2^iii	3.3891 (16)	Eu2—Eu2^xi	3.9796 (14)
Eu1—I1^iii	3.5794 (12)	Eu2—Eu2^xii	3.9796 (14)
Eu1—I1^ii	3.5794 (12)	I1—Eu2^xiii	3.4032 (9)
Eu1—I1^i	3.5794 (12)	I1—Eu2^xiv	3.4032 (9)
Eu1—Eu2^iii	3.8900 (16)	I1—Eu1^iv	3.5794 (12)
Eu1—Eu2^i	3.8900 (16)	I2—Eu2^xv	3.3757 (12)
Eu1—Eu2^ii	3.8900 (16)	I2—Eu2^xvi	3.3757 (12)
Eu2—O^iv	2.416 (5)	I2—Eu1^iv	3.3891 (16)
Eu2—I1	3.2905 (15)	I2—Eu2^xvii	3.5458 (11)
Eu2—I2^v	3.3757 (12)	I2—Eu2^xviii	3.5458 (11)
Eu2—I2^vi	3.3757 (12)	O—Eu2^iii	2.416 (5)
Eu2—I1^vii	3.4032 (9)	O—Eu2^i	2.416 (5)
Eu2—I1^viii	3.4032 (9)	O—Eu2^ii	2.416 (5)
O—Eu1—I2^i	135.57 (3)	I1—Eu2—I2^ix	69.34 (2)
O—Eu1—I2^ii	135.57 (3)	I2^v—Eu2—I2^ix	144.44 (3)
I2^i—Eu1—I2^ii	74.64 (4)	I2^vi—Eu2—I2^ix	74.51 (2)
O—Eu1—I2^iii	135.57 (3)	I1^vii—Eu2—I2^ix	140.62 (2)
I2^i—Eu1—I2^iii	74.64 (4)	I1^viii—Eu2—I2^ix	71.08 (2)
I2^ii—Eu1—I2^iii	74.64 (4)	O^iv—Eu2—I2^x	74.63 (13)
O—Eu1—I1^iii	88.28 (3)	I1—Eu2—I2^x	69.34 (2)
I2^i—Eu1—I1^iii	70.829 (18)	I2^v—Eu2—I2^x	74.51 (2)
I2^ii—Eu1—I1^iii	70.829 (18)	I2^vi—Eu2—I2^x	144.44 (3)
I2^iii—Eu1—I1^iii	136.16 (5)	I1^vii—Eu2—I2^x	71.08 (2)
O—Eu1—I1^ii	88.28 (3)	I1^viii—Eu2—I2^x	140.62 (2)
I2^i—Eu1—I1^ii	70.829 (18)	I2^ix—Eu2—I2^x	125.14 (4)
I2^ii—Eu1—I1^ii	136.16 (5)	O^iv—Eu2—Eu1^iv	35.8 (3)
I2^iii—Eu1—I1^ii	70.829 (18)	I1—Eu2—Eu1^iv	59.10 (3)
I1^iii—Eu1—I1^ii	119.910 (3)	I2^v—Eu2—Eu1^iv	112.83 (3)
O—Eu1—I1^i	88.28 (3)	I2^vi—Eu2—Eu1^iv	112.83 (3)
I2^i—Eu1—I1^i	136.16 (5)	I1^vii—Eu2—Eu1^iv	131.46 (2)
I2^ii—Eu1—I1^i	70.829 (18)	I1^viii—Eu2—Eu1^iv	131.46 (2)
I2^iii—Eu1—I1^i	70.829 (18)	I2^ix—Eu2—Eu1^iv	64.19 (2)
I1^iii—Eu1—I1^i	119.910 (3)	I2^x—Eu2—Eu1^iv	64.19 (2)
I1^ii—Eu1—I1^i	119.910 (3)	O^iv—Eu2—Eu2^xi	34.56 (16)
O—Eu1—Eu2^iii	36.203 (19)	I1—Eu2—Eu2^xi	109.689 (18)
I2^i—Eu1—Eu2^iii	111.68 (2)	I2^v—Eu2—Eu2^xi	91.102 (16)
I2^ii—Eu1—Eu2^iii	111.68 (2)	I2^vi—Eu2—Eu2^xi	53.882 (17)
I2^iii—Eu1—Eu2^iii	171.77 (3)	I1^vii—Eu2—Eu2^xi	160.71 (2)
I1^iii—Eu1—Eu2^iii	52.07 (2)	I1^viii—Eu2—Eu2^xi	109.012 (16)
I1^ii—Eu1—Eu2^iii	105.72 (3)	I2^ix—Eu2—Eu2^xi	55.864 (13)
I1^i—Eu1—Eu2^iii	105.72 (3)	I2^x—Eu2—Eu2^xi	109.051 (14)
O—Eu1—Eu2^i	36.203 (19)	Eu1^iv—Eu2—Eu2^xi	59.235 (15)
I2^i—Eu1—Eu2^i	171.77 (3)	O^iv—Eu2—Eu2^xii	34.56 (16)
I2^ii—Eu1—Eu2^i	111.68 (2)	I1—Eu2—Eu2^xii	109.689 (18)
I2^iii—Eu1—Eu2^i	111.68 (2)	I2^v—Eu2—Eu2^xii	53.882 (17)
I1^iii—Eu1—Eu2^i	105.72 (3)	I2^vi—Eu2—Eu2^xii	91.102 (16)
I1^ii—Eu1—Eu2^i	105.72 (3)	I1^vii—Eu2—Eu2^xii	109.012 (16)
I1^i—Eu1—Eu2^i	52.07 (2)	I1^viii—Eu2—Eu2^xii	160.71 (2)
Eu2^iii—Eu1—Eu2^i	61.53 (3)	I2^ix—Eu2—Eu2^xii	109.051 (14)
O—Eu1—Eu2^ii	36.203 (19)	I2^x—Eu2—Eu2^xii	55.864 (13)
I2^i—Eu1—Eu2^ii	111.68 (2)	Eu1^iv—Eu2—Eu2^xii	59.235 (15)
I2^ii—Eu1—Eu2^ii	171.77 (3)	Eu2^xi—Eu2—Eu2^xii	60.0
I2^iii—Eu1—Eu2^ii	111.68 (2)	Eu2—I1—Eu2^xiii	109.11 (2)
I1^iii—Eu1—Eu2^ii	105.72 (3)	Eu2—I1—Eu2^xiv	109.11 (2)
I1^ii—Eu1—Eu2^ii	52.07 (2)	Eu2^xiii—I1—Eu2^xiv	141.41 (4)
I1^i—Eu1—Eu2^ii	105.72 (3)	Eu2—I1—Eu1^iv	68.83 (3)
Eu2^iii—Eu1—Eu2^ii	61.53 (3)	Eu2^xiii—I1—Eu1^iv	99.216 (19)
Eu2^i—Eu1—Eu2^ii	61.53 (3)	Eu2^xiv—I1—Eu1^iv	99.216 (19)
O^iv—Eu2—I1	94.9 (3)	Eu2^xv—I2—Eu2^xvi	72.24 (3)
O^iv—Eu2—I2^v	84.9 (3)	Eu2^xv—I2—Eu1^iv	105.17 (3)
I1—Eu2—I2^v	142.48 (2)	Eu2^xvi—I2—Eu1^iv	105.17 (3)
O^iv—Eu2—I2^vi	84.9 (3)	Eu2^xv—I2—Eu2^xvii	154.51 (3)
I1—Eu2—I2^vi	142.48 (2)	Eu2^xvi—I2—Eu2^xvii	103.95 (2)
I2^v—Eu2—I2^vi	74.99 (4)	Eu1^iv—I2—Eu2^xvii	100.15 (3)
O^iv—Eu2—I1^vii	141.71 (3)	Eu2^xv—I2—Eu2^xviii	103.95 (2)
I1—Eu2—I1^vii	88.65 (3)	Eu2^xvi—I2—Eu2^xviii	154.51 (3)
I2^v—Eu2—I1^vii	70.13 (2)	Eu1^iv—I2—Eu2^xviii	100.15 (3)
I2^vi—Eu2—I1^vii	114.36 (3)	Eu2^xvii—I2—Eu2^xviii	68.27 (3)
O^iv—Eu2—I1^viii	141.71 (3)	Eu1—O—Eu2^iii	108.0 (3)
I1—Eu2—I1^viii	88.65 (3)	Eu1—O—Eu2^i	108.0 (3)
I2^v—Eu2—I1^viii	114.36 (3)	Eu2^iii—O—Eu2^i	110.9 (3)
I2^vi—Eu2—I1^viii	70.13 (2)	Eu1—O—Eu2^ii	108.0 (3)
I1^vii—Eu2—I1^viii	76.35 (3)	Eu2^iii—O—Eu2^ii	110.9 (3)
O^iv—Eu2—I2^ix	74.63 (13)	Eu2^i—O—Eu2^ii	110.9 (3)

Symmetry codes: (i) −y, x−y, z; (ii) −x+y+1, −x+1, z; (iii) x, y+1, z; (iv) x, y−1, z; (v) −x+y+1, −x, z−1; (vi) −y, x−y−1, z−1; (vii) y, −x+y, z−1/2; (viii) x−y, x, z−1/2; (ix) y+1, −x+y+1, z−1/2; (x) x−y−1, x−1, z−1/2; (xi) −x+y+1, −x, z; (xii) −y, x−y−1, z; (xiii) x−y, x, z+1/2; (xiv) y, −x+y, z+1/2; (xv) −x+y+1, −x, z+1; (xvi) −y, x−y−1, z+1; (xvii) x−y, x−1, z+1/2; (xviii) y+1, −x+y, z+1/2.