Redetermination of the γ-form of tellurium dioxide

The crystal structure of γ-TeO₂ was redetermined on the basis of single-crystal X-ray diffraction data. The previous structure determination of this modification was based on laboratory powder X-ray diffraction data [Champarnaud-Mesjard et al. (2000). J. Phys. Chem. Solids, 61, 1499–1507]. The current redetermination revealed all atoms with anisotropic displacement parameters, accompanied with a much higher accuracy and precision in terms of bond lengths and angles, and the determination of the absolute structure. The crystal structure consists of TeO₄ bis­phenoids that combine through corner-sharing of all their oxygen atoms into a three-dimensional framework.

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Structure description

In a continuation of hydro­thermal phase formation studies to incorporate tetra­hedral XO₄ groups (X = S, Se) into framework structures of divalent metal oxotellurates(IV) (metal = Ca, Cd, Hg, Mg, Pb, Sr, Zn; Weil & Shirkhanlou, 2015, 2017a,b,c), the system Mn/Se/Te/O was investigated. In one of these experiments, single crystals of γ-TeO₂ were obtained serendipitously as a minor by-product.

Tellurium dioxide is polymorphic, with three reported crystalline forms at ambient pressure: the α-form (Lindqvist, 1968), the β-form (Beyer, 1967) and the γ-form (Champarnaud-Mesjard et al., 2000). Whereas the α- and β-forms can be found in nature as the rare minerals paratellurite and tellurite, respectively, the γ-form is synthetic and can usually be obtained as a polycrystalline material by recrystallizing TeO₂ glasses at low temperatures. This was also the procedure to prepare material for the previous structure determination of γ-TeO₂ that was based on laboratory X-ray diffraction data and refined using the Rietveld method (Champarnaud-Mesjard et al., 2000). The results of the current rerefinement using modern CCD data are reported here. The previous structure model is confirmed, however, with higher accuracy and precision, as exemplified by a comparison of the bond lengths and angles (Table 1). Moreover, the absolute structure of γ-TeO₂ was determined (Table 2).

Table 1
Comparison of bond lengths (Å) and angles (°) in the current and the previous refinement of γ-TeO₂

	Current refinement	Previous refinement ^a
Te1—O1	1.839 (3)	1.86 (2)
Te1—O2^i	1.906 (3)	1.94 (2)
Te1—O2^ii	2.048 (3)	2.02 (2)
Te1—O1^iii	2.241 (4)	2.20 (2)
O1—Te1—O2^i	100.36 (17)	99.2 (4)
O1—Te1—O2^ii	93.14 (17)	91.8 (5)
O1—Te1—O1^iii	91.69 (9)	91.9 (5)
O2^i—Te1—O2^ii	78.68 (9)	77.6 (5)
O2^i—Te1—O1^iii	75.60 (14)	76.1 (4)
O2^ii—Te1—O1^iii	154.28 (13)	153.6 (5)
Te1—O1—Te1^iv	131.6 (2)	133.1 (5)
Te1^v—O2—Te1^vi	125.18 (18)	125.1 (5)
Symmetry codes: (i) x, y, z − 1; (ii) −x + , −y, z − ; (iii) x + , −y + , −z; (iv) x − , −y + , −z; (v) x, y, z + 1; (vi) −x + , −y, z + . Notes: (a) Champarnaud-Mesjard et al. (2000); lattice parameters a = 4.898 (3), b = 8.576 (4), c = 4.351 (2) Å at room temperature.

Table 2
Experimental details

Crystal data
Chemical formula	TeO2
Mr	159.60
Crystal system, space group	Orthorhombic, P212121
Temperature (K)	296
a, b, c (Å)	4.8809 (2), 8.5668 (4), 4.3433 (2)
V (Å^3)	181.61 (1)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	15.91
Crystal size (mm)	0.18 × 0.01 × 0.01
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2015)
Tmin, Tmax	0.552, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	7370, 893, 816
Rint	0.056
(sin θ/λ)max (Å^−1)	0.844
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.025, 0.037, 1.02
No. of reflections	893
No. of parameters	28
Δρmax, Δρmin (e Å^−3)	1.81, −1.28
Absolute structure	Flack x determined using 296 quotients [(I^+)−(I^−)]/[(I^+)+(I^−)] (Parsons et al., 2013)
Absolute structure parameter	−0.03 (4)
Coordinates were taken from a previous refinement. Computer programs: APEX2 and SAINT (Bruker, 2015), SHELXL2014/7 (Sheldrick, 2015), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010).

In the crystal structure, the tellurium atom is surrounded by four oxygen atoms in the shape of a bis­phenoid, with a Te—O bond lengths range of 1.839 (3) – 2.241 (4) Å. Each TeO₄ polyhedron is linked to four symmetry-related TeO₄ polyhedra by sharing corners, which leads to the formation of a three-dimensional framework structure. Characteristic for the crystal chemistry of tellurium(IV) oxides or oxotellurates(IV) (Christy et al., 2016), the 5s² electron lone pair situated at the Te^IV atom is stereochemically active and points towards the open space of this arrangement (Fig. 1).

Figure 1 Projection of the crystal structure of γ-TeO2 along [101]. TeO4 groups are given as polyhedra (Te atoms red, O atoms colourless) with anisotropic displacement ellipsoids drawn at the 74% probability level.

Synthesis and crystallization

100 mg TeO₂, 350 mg MnSeO₄·H₂O and 70 mg KOH were mixed and placed in a 5 ml capacity Teflon container that was subsequently filled with 2 ml water. The container was closed with a Teflon lid and placed in a steel autoclave for ten days at 483 K under autogenous pressure. After cooling down to room temperature, the solid reaction product was filtered off and washed with water and ethanol. The obtained material consisted of a dark-brown to black powder as the main product besides few light-brown plate-like crystals and very few colourless needles. Powder X-ray diffraction of the dark-brown material revealed Mn₂TeO₆ (Hund, 1971), and single-crystal X-ray diffraction showed the plate-like crystals to be spiro­ffite-type Mn₂Te₃O₈ (Cooper & Hawthorne, 1996); the colourless needles correspond to the title compound.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Coordinates and atom labels were taken from the previous refinement (Champarnaud-Mesjard et al., 2000). The maximum and minimum electron density peaks are located 0.68 and 0.73 Å, respectively, from atom Te1.

Structural data

Computing details

Data collection: APEX2 (Bruker, 2015); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: coordinates from previous refinement; program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

(I)

Crystal data

O2Te	Dx = 5.837 Mg m^−3
Mr = 159.60	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P212121	Cell parameters from 2511 reflections
a = 4.8809 (2) Å	θ = 4.8–32.2°
b = 8.5668 (4) Å	µ = 15.91 mm^−1
c = 4.3433 (2) Å	T = 296 K
V = 181.61 (1) Å^3	Needle, colourless
Z = 4	0.18 × 0.01 × 0.01 mm
F(000) = 272

Data collection

Bruker APEXII CCD diffractometer	816 reflections with I > 2σ(I)
ω–scans'	Rint = 0.056
Absorption correction: multi-scan (SADABS; Bruker, 2015)	θmax = 36.9°, θmin = 4.8°
Tmin = 0.552, Tmax = 0.747	h = −8→8
7370 measured reflections	k = −14→14
893 independent reflections	l = −7→7

Refinement

Refinement on F^2	Primary atom site location: isomorphous structure methods
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.014P)^2 + 0.0171P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.025	(Δ/σ)max < 0.001
wR(F^2) = 0.037	Δρmax = 1.81 e Å^−3
S = 1.02	Δρmin = −1.28 e Å^−3
893 reflections	Absolute structure: Flack x determined using 296 quotients [(I^+)-(I^-)]/[(I^+)+(I^-)] (Parsons et al., 2013)
28 parameters	Absolute structure parameter: −0.03 (4)
0 restraints

Special details

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

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

	x	y	z	Uiso*/Ueq
Te1	0.96976 (6)	0.10122 (4)	0.13698 (7)	0.01056 (7)
O1	0.7704 (7)	0.2822 (4)	0.1778 (9)	0.0147 (8)
O2	0.8602 (7)	0.0379 (4)	0.7347 (8)	0.0140 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Te1	0.01128 (12)	0.00983 (11)	0.01058 (11)	0.00182 (11)	−0.00163 (11)	−0.00096 (13)
O1	0.0143 (16)	0.0114 (17)	0.018 (2)	0.0056 (12)	−0.0012 (14)	0.0006 (15)
O2	0.0144 (18)	0.0200 (19)	0.0075 (15)	−0.0067 (14)	0.0006 (13)	−0.0036 (14)

Geometric parameters (Å, º)

Te1—O1	1.839 (3)	O1—Te1^iv	2.241 (3)
Te1—O2^i	1.906 (3)	O2—Te1^v	1.906 (3)
Te1—O2^ii	2.048 (3)	O2—Te1^vi	2.048 (3)
Te1—O1^iii	2.241 (4)
O1—Te1—O2^i	100.36 (17)	O2^i—Te1—O1^iii	75.60 (14)
O1—Te1—O2^ii	93.14 (17)	O2^ii—Te1—O1^iii	154.28 (13)
O2^i—Te1—O2^ii	78.68 (9)	Te1—O1—Te1^iv	131.6 (2)
O1—Te1—O1^iii	91.69 (9)	Te1^v—O2—Te1^vi	125.18 (18)

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