Sr₂Cu₂TeO₆Br₂: honeycomb layers of copper(II) ions

Single crystals of distrontium(II) dicopper(II) tellurium(VI) hexa­oxide dibromide, Sr₂Cu₂TeO₆Br₂, were synthesized via solid state/gas phase reactions in sealed evacuated silica tubes. The building units are irregular SrO₄Br₃ polyhedra (a new type of coordination polyhedron around Sr^II), CuO₅ square pyramids and TeO₆ octa­hedra. The CuO₅ square pyramids and the TeO₆ octa­hedra together form layers in the bc plane bridged by Sr atoms. The Cu atoms are arranged to form puckered honeycomb layers. The Te atom lies on an inversion centre.

S1. Comment

The title compound is the first tellurate halide compound in the system AE–Cu^II–Te^VI–O–X (AE = alkaline earth and X = halide). In the family AE–Cu^II–Te^VI–O, very few compounds have been described before, viz. triclinic and tetragonal Ba₂CuTeO₆ (Koehl & Reinen, 1974; Iwanaga et al., 1999) and tetragonal Sr₂CuTeO₆ (Reinen & Weitzel, 1976).

An overview of the structure of the title compound is shown in Fig. 1; selected interatomic distances are given in Table 1. Atom Te1 has an octahedral TeO₆ coordination with Te—O bond distances in the range 1.915 (9)–1.943 (9) Å. Atom Cu1 is coordinated by five O atoms to form a CuO₅ square pyramid with Cu—O bond distances in the range 1.933 (9)–2.022 (10) Å in the square plane and 2.355 (10) Å to the pyramid apex. A more distant Br atom, sitting in the opposite apical position, completes an elongated CuO₅Br octahedron with a Cu—Br distance of 2.974 (2) Å. However, bond valence sum (BVS) calculations (Brown & Altermatt, 1985; Brese & O'Keeffe, 1991) suggest that the primary coordination sphere around Cu is less than 2.93 Å, so this distant Br atom is just at the border and is not considered as bonded.

The Cu—Te—O layers extend in the bc plane and are arranged such that each TeO₆ octahedron has common edges with four and common corners with two CuO₅ polyhedra. Furthermore, each CuO₅ square pyramid has a common edge with one and common corners with two more such pyramids (Fig. 2a). The Cu atoms form a distorted pseudo-hexagonal puckered honeycomb network, with Cu—Cu distances of 3.067 (3) and 3.615 (2) Å (Fig. 2b).

The Sr1 atom is coordinated by four O and three Br atoms to form an irregular SrO₄Br₃ polyhedron that, according to the best of our knowledge and a search of the ICSD (2005), actually constitutes a new coordination polyhedron for Sr^II (Fig. 3). The Sr—O distances are in the range 2.471 (9)–2.793 (10) Å and the Sr – Br distances are in the range 3.064 (2)–3.175 (2) Å. A fourth Br atom perches at a distance of 3.655 (2) Å, but does not contribute significantly to the BVS and is therefore not considered as bonded. It is common that BVSs give very low values for halide ions in oxohalides (0.3–0.4), thus indicating that such ions frequently have an unsaturated bond valence and take on the role of counter-ion rather than being integrated into the covalent/ionic network; an example is Te₆O₁₁Cl₂ (Giester, 1994). However, for the title compound, BVS calculations indicate that the Br ions are well integrated into the covalent/ionic network, as they have a value of ~0.9, which is near the expected value of 1 (see Table 2).

Each SrO₄Br₃ polyhedron is linked to four others by corner sharing (two via Br and two via O) and to two others via Br—Br edge sharing to form double layers that connect the Cu—Te—O layers (Fig. 1).

S2. Experimental

Single crystals of Sr₂Cu₂TeO₆Br₂ were synthesized by chemical transport reactions in sealed evacuated silica tubes. SrO (ABCR, 99.95%), CuBr₂ (Avocado Research Chemicals, +98%), CuO (Avocado Research Chemicals, +99%) and TeO₂ (ABCR, +99%) were used as starting materials and mixed in the molar ratio 1:1:3:1 in a mortar; the mixture was put into a silica tube (length ~5 cm), which was subsequently evacuated. The tube was heated for 72 h at 900 K in a muffle furnace. The product appeared as green single crystals, and black and light-yellow powders. Chemical analysis of the elements heavier than oxygen by use of a scanning electron microscope (JEOL820) equipped with an energy dispersive spectrometer (LINK AN10000) indicate that the black powder is unreacted CuO and the light-yellow powder has a composition of 0.8 at% Sr, 75.3 at% Cu, 22.1 at% Te and 1.9 at% Br. The light-yellow colour indicates that this unknown phase contains Cu^I and not Cu^II. There must be a redox reaction taking place during the reactions so that Te^IV is transformed to Te^VI to form the title compound and Cu^II is transformed to Cu^I to form the light-yellow powder. The synthesis products were non-hygroscopic.

S3. Refinement

The maximum residual electron density is at the fractional coordinates (0.7971, 0.0106, 0.7975) and the minimum is at (0.3424, 0.0672, 0.3253). The peak and hole are 0.88 and 1.68 Å, respectively, from atom Sr1.

Computing details

Data collection: EXPOSE in IPDS (Stoe & Cie, 1997); cell refinement: CELL in IPDS; data reduction: INTEGRATE in IPDS; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. An overview of the structure of Sr2Cu2TeO6Br2. CuO5 square pyramids are grey; TeO6 octahedra are white.
	Fig. 2. (a) The local coordination around a TeO6 octahedron in a Cu—Te—O layer. CuO5 square pyramids are grey, The Cu, Te and O atoms are dark grey, light grey and white, respectively. (b) The pseudo-hexagonal Cu honeycomb network in the Cu—Te—O layers. Two different Cu—Cu distances are present, viz. 3.615 (2) Å (thick lines) and 3.067 (3) Å (thiner lines). The symmetry codes are as in Table 1.
	Fig. 3. The irregular SrO4Br3 polyhedron. There is also a long Sr—Br distance marked with a dashed line. The symmetry codes are as in Table 1.

distrontium dicopper(II) tellurium(VI) hexaoxide dibromide

Crystal data

Sr2Cu2TeO6Br2	F(000) = 608
Mr = 685.74	Dx = 4.990 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 1006 reflections
a = 9.422 (3) Å	θ = 1.9–28.2°
b = 5.1788 (17) Å	µ = 28.10 mm^−1
c = 9.388 (3) Å	T = 291 K
β = 94.92 (3)°	Plate, green
V = 456.4 (3) Å^3	0.07 × 0.06 × 0.02 mm
Z = 2

Data collection

Stoe IPDS diffractometer	1099 independent reflections
Radiation source: fine-focus sealed tube	629 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.113
Detector resolution: 6.7 pixels mm^-1	θmax = 28.0°, θmin = 4.3°
ϕ scans	h = −12→12
Absorption correction: numerical [X-RED (Stoe & Cie, 2001) and X-SHAPE (Stoe & Cie, 1999)]	k = −6→6
Tmin = 0.045, Tmax = 0.135	l = −12→12
4112 measured reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.050	Secondary atom site location: difference Fourier map
wR(F^2) = 0.126	w = 1/[σ^2(Fo^2) + (0.0659P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.88	(Δ/σ)max < 0.001
1099 reflections	Δρmax = 3.79 e Å^−3
61 parameters	Δρmin = −1.45 e Å^−3

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
Te1	0.0000	0.0000	0.5000	0.0145 (3)
Sr1	−0.27752 (15)	0.3978 (3)	0.31832 (16)	0.0224 (3)
Br1	−0.38759 (17)	0.8884 (3)	0.16320 (18)	0.0271 (4)
Cu1	0.08010 (18)	0.4855 (4)	0.36478 (15)	0.0163 (4)
O1	0.0354 (11)	0.6641 (17)	0.1857 (10)	0.0148 (19)
O2	0.1142 (10)	0.2992 (19)	0.5465 (9)	0.016 (2)
O3	0.1624 (11)	−0.1847 (18)	0.4423 (10)	0.017 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Te1	0.0202 (6)	0.0120 (6)	0.0112 (5)	−0.0012 (5)	0.0009 (4)	−0.0001 (5)
Sr1	0.0230 (7)	0.0189 (7)	0.0243 (7)	0.0014 (6)	−0.0030 (5)	−0.0015 (6)
Br1	0.0248 (8)	0.0237 (8)	0.0331 (8)	−0.0006 (7)	0.0033 (6)	0.0024 (7)
Cu1	0.0241 (9)	0.0129 (8)	0.0117 (7)	0.0002 (8)	0.0005 (6)	0.0003 (7)
O1	0.025 (5)	0.010 (5)	0.010 (4)	−0.003 (4)	0.006 (4)	0.002 (3)
O2	0.025 (5)	0.013 (4)	0.009 (4)	−0.004 (4)	−0.005 (4)	−0.005 (3)
O3	0.024 (5)	0.009 (5)	0.019 (5)	0.005 (4)	0.006 (4)	0.003 (4)

Geometric parameters (Å, º)

Te1—O2	1.915 (9)	Sr1—Cu1^i	3.395 (2)
Te1—O3	1.922 (9)	Sr1—Cu1^ii	3.425 (2)
Te1—O1^i	1.943 (9)	Sr1—Br1^vii	3.655 (2)
Te1—Cu1	2.9439 (19)	Cu1—O1	1.933 (9)
Te1—Cu1^ii	3.0734 (19)	Cu1—O2	1.962 (9)
Te1—Sr1	3.6367 (17)	Cu1—O3^viii	1.988 (10)
Sr1—O2^ii	2.471 (9)	Cu1—O1^i	2.022 (10)
Sr1—O1^i	2.586 (10)	Cu1—O2^ii	2.355 (10)
Sr1—O3^iii	2.651 (10)	Cu1—Br1^i	2.974 (2)
Sr1—O3^iv	2.793 (10)	Cu1—Cu1^ii	3.067 (3)
Sr1—Br1	3.064 (2)	Cu1—Te1^viii	3.0734 (19)
Sr1—Br1^v	3.146 (2)	Cu1—Sr1^iv	3.395 (2)
Sr1—Br1^vi	3.175 (2)	Cu1—Sr1^ii	3.425 (2)
Sr1—Cu1	3.391 (2)	Cu1—Cu1^iv	3.615 (2)
O2^iii—Te1—O3	88.8 (4)	O3^iv—Sr1—Br1^vi	120.5 (2)
O2—Te1—O3	91.2 (4)	Br1—Sr1—Br1^vi	75.01 (6)
O2—Te1—O1^ix	95.7 (4)	Br1^v—Sr1—Br1^vi	73.90 (6)
O3—Te1—O1^ix	87.5 (4)	O2^ii—Sr1—Br1^vii	64.2 (2)
O2^iii—Te1—O1^i	95.7 (4)	O1^i—Sr1—Br1^vii	118.2 (2)
O2—Te1—O1^i	84.3 (4)	O3^iii—Sr1—Br1^vii	60.1 (2)
O3—Te1—O1^i	92.5 (4)	O3^iv—Sr1—Br1^vii	169.5 (2)
O2^ii—Sr1—O1^i	78.1 (3)	Br1—Sr1—Br1^vii	93.42 (5)
O2^ii—Sr1—O3^iii	68.7 (3)	Br1^v—Sr1—Br1^vii	123.83 (7)
O1^i—Sr1—O3^iii	61.3 (3)	Br1^vi—Sr1—Br1^vii	66.27 (6)
O2^ii—Sr1—O3^iv	106.0 (3)	O1—Cu1—O2	176.8 (4)
O1^i—Sr1—O3^iv	60.4 (3)	O1—Cu1—O3^viii	87.5 (4)
O3^iii—Sr1—O3^iv	121.2 (3)	O2—Cu1—O3^viii	94.3 (4)
O2^ii—Sr1—Br1	83.6 (2)	O1—Cu1—O1^i	96.7 (2)
O1^i—Sr1—Br1	130.1 (2)	O2—Cu1—O1^i	81.0 (4)
O3^iii—Sr1—Br1	147.7 (2)	O3^viii—Cu1—O1^i	168.0 (4)
O3^iv—Sr1—Br1	81.5 (2)	O1—Cu1—O2^ii	87.9 (4)
O2^ii—Sr1—Br1^v	159.4 (2)	O2—Cu1—O2^ii	90.0 (4)
O1^i—Sr1—Br1^v	81.8 (2)	O3^viii—Cu1—O2^ii	75.8 (4)
O3^iii—Sr1—Br1^v	97.9 (2)	O1^i—Cu1—O2^ii	93.1 (4)
O3^iv—Sr1—Br1^v	66.6 (2)	Cu1—O1—Cu1^iv	132.1 (5)
Br1—Sr1—Br1^v	112.99 (7)	Cu1—O2—Cu1^ii	90.0 (4)
O2^ii—Sr1—Br1^vi	124.0 (2)	Cu1^ii—Cu1—Cu1^i	108.59 (9)
O1^i—Sr1—Br1^vi	151.1 (2)	Cu1^ii—Cu1—Cu1^iv	104.39 (9)
O3^iii—Sr1—Br1^vi	106.5 (2)	Cu1^i—Cu1—Cu1^iv	91.49 (7)

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