Single crystals of SnTe₃O₈ in the millimetre range grown by chemical vapor transport reactions

Tin(IV) trioxidotellurate(IV), SnTe₃O₈, is a member of the isotypic M^IVTe^IV₃O₈ (M = Ti, Zr, Hf, Sn) series crystallizing with eight formula units per unit cell in space group Ia. In comparison with the previous crystal structure model of SnTe₃O₈ based on powder X-ray diffraction data [Meunier & Galy (1971). Acta Cryst. B27, 602–608], the current model based on single-crystal X-ray data is improved in terms of precision and accuracy. Nearly regular [SnO₆] octa­hedra (Sn site symmetry ..) are situated in the voids of an oxidotellurate(IV) framework built up by corner-sharing [TeO₄] bis­phenoids (Te site symmetry 2..). A qu­anti­tative structural comparison revealed a very high degree of similarity for the structures with M = Ti, Zr, Sn in the M^IVTe₃O₈ series.

1. Chemical context

The crystal chemistry of oxidotellurates(IV) is dominated by the presence of the 5s² electron lone pair that, in the majority of cases, is stereochemically active, thus enabling one-sided coordination spheres around the Te^IV atom (Christy et al., 2016). This peculiar building block often results in compounds with non-centrosymmetric structures or structures with polar directions exhibiting inter­esting physical properties (Ra et al., 2003; Kim et al., 2014). In this context, the microwave dielectric properties of M^IVTe₃O₈ (M = Sn, Zr) ceramics were investigated some time ago (Subodh & Sebastian, 2008).

The crystal structure of the isotypic series M^IVTe₃O₈ was originally determined for M = Ti from a single crystal in space group Ia using photographic Weissenberg X-ray data, whereas for M = Sn, Zr and Hf, the crystal structures were refined from powder X-ray data (Meunier & Galy, 1971). In subsequent studies, crystal-structure refinements on the basis of single-crystal X-ray data were reported for the mineral winstanleyite with composition (Ti_0.96Fe_0.04)Te₃O₈ (Bindi & Cipriani, 2003), and for the synthetic compound ZrTe₃O₈ (Noguera et al., 2003; Lu et al., 2019). A powder X-ray study of the solid solution Sn_0.59Ti_0.41Te₃O₈ crystallizing in the M^IVTe₃O₈ structure type has also been reported (Ben Aribia et al., 2008).

Single-crystal growth of oxidotellurates(IV) can be accomplished through various crystallization methods including, for example, experiments under hydro­thermal conditions (Weil et al., 2018), cooling from the melt (Stöger et al., 2009), from salt melts as fluxing agents (Weil, 2019), or from chemical vapor transport reactions (Missen et al., 2020). The latter method (Binnewies et al., 2012) is particularly suitable for growing large crystals of high quality and was the method of choice for crystal growth of SnTe₃O₈ for which a more precise and accurate structure refinement appeared to be desirable.

2. Structural commentary

The asymmetric unit of SnTe₃O₈ comprises one Sn^IV atom, one Te^IV atom, and two oxide anions, residing on sites 8a (site symmetry ..), 24d (2..), 48e (1) and 16c (.3.), respectively. The tin atom is in an almost regular octa­hedral coordination by oxygen, with six equal Sn1—O1 distances, all trans angles equal to 180°, and cis angles ranging from 86.09 (4) to 93.91 (4)°. The Te1 site is coordinated by four O atoms in pairs of shorter (O1) and longer distances (O2) (Table 1). The resulting [TeO₄] coordination polyhedron is a distorted bis­phenoid. Considering the 5s² electron lone pair at the Te^IV atom, the corresponding [ΨTeO₄] polyhedron has a shape inter­mediate between a square pyramid and a trigonal bipyra­mid with the non-bonding electron pair occupying an equatorial position (Fig. 1). The geometry index τ₅ of the [ΨTeO₄] polyhedron is 0.471 (τ₅ = 0 for an ideal square pyramid and τ₅ = 1 for an ideal trigonal bipyramid; Addison et al., 1984). The position of the electron lone pair was calculated with the LPLoc software (Hamani et al., 2020), with resulting fractional coordinates of x = 0.28655, y = 0, z = 1/4. The radius of the electron lone pair was calculated to be 1.07 Å with a distance of 0.90 Å from the Te1 position. The coordination numbers of the oxide anions are two and three: O1 coord­inates to Sn1 and Te1 at the shorter of the two Te1—O distances whereas O2 coordinates to three Te1 atoms at the longer of the two Te1—O distances.

Table 1
Selected geometric parameters (Å, °)

Sn1—O1^i	2.0421 (11)	Te1—O2	2.1278 (3)
Te1—O1^ii	1.8800 (11)
O1^iii—Te1—O1^ii	102.42 (8)	O2^iv—Te1—O2	157.05 (6)
O1^iii—Te1—O2	86.60 (6)	Te1—O2—Te1^v	117.94 (2)
O1^ii—Te1—O2	79.05 (4)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) z, x, y.

Figure 1 The coordination environment around Te1. Displacement ellipsoids are drawn at the 90% probability level; the electron lone pair is given as a green sphere of arbitrary radius. [Symmetry codes: (v) –z + , x–1/2, y; (vii) −z + , −x + , −y + ; (viii) x, −y, −z + .]

In the crystal structure of SnTe₃O₈, the [SnO₆] octa­hedra are isolated from each other and arranged in rows running parallel to [100]. Each of the [TeO₄] bis­phenoids shares corners (O2) with other [TeO₄] bis­phenoids to form a three-dimensional oxidotellurate(IV) framework. The [SnO₆] octa­hedra are situated in the voids of this framework, thereby sharing each of the six corners with an individual [TeO₄] bis­phenoid. The crystal structure of SnTe₃O₈ is depicted in Fig. 2.

Figure 2 The crystal structure of SnTe3O8 in polyhedral representation, showing a projection along [00]. Displacement ellipsoids are as in Fig. 1; [TeO4] polyhedra are red, [SnO6] octa­hedra are blue.

The unit-cell parameter a from the previous powder X-ray study, 11.144 (3) Å, as well as inter­atomic distances of Sn1—O1 = 2.032 Å (6×), Te1—O1 = 1.850 Å (2×), Te1—O2 = 2.124 Å (2×), and angles O1—Te1—O1′ = 102.9°, and O2—Te1—O2′ = 156.8° (Meunier & Galy, 1971) agree with the present single-crystal study (Table 1), but with lower precision and accuracy. In comparison with the previous model based on powder X-ray data, the values of the bond-valence sums (Brown, 2002) using the parameters of Brese & O'Keeffe (1991) are much closer to the expected values of 4 for Sn and Te and 2 for O on basis of the current model [previous model: Sn1 4.28 valence units (v.u.), Te1 4.10 v.u., O1 2.09 v.u., O2 2.08 v.u.; current model: Sn1: 4.14 v.u., Te1 3.93 v.u., O1 1.99 v.u., O2 2.00 v.u.].

The relation of the isotypic crystal structures of M^IVTe₃O₈ compounds with that of the fluorite structure has been discussed previously for TiTe₃O₈ (Meunier & Galy, 1971; Wells, 1975). The unit-cell parameter a of cubic TiTe₃O₈ is ∼2a of cubic CaF₂, whereby the ordered distribution of the cationic sites leads to a doubling of the unit cell and also to a considerable distortion of the respective coordination environments. The original cubic coordination around the Ca^II cation in the fluorite structure is changed to an octa­hedral coordination of Sn^IV and a fourfold coordination of Te^IV in the superstructure of the M^IVTe₃O₈ compounds. Note that there are two additional O atoms at a distance of 3.2446 (19) Å around the M^IV site and two pairs of additional O atoms at a distance of 2.9076 (12) and 3.3957 (13) Å around the Te1 site in SnTe₃O₈, completing an eightfold coordination in each case. Correspondingly, each of the two O sites has a fourfold coordination in case the much longer distances are counted.

A qu­anti­tative structural comparison of the M^IVTe₃O₈ structures where single crystal data are available (M = Ti, Zr, Sn) was undertaken with the program compstru (de la Flor et al., 2016) available at the Bilbao Crystallographic Server (Aroyo et al., 2006). Table 2 lists the degree of lattice distortion (S), the maximum distance between the atomic positions of paired atoms (|u|), the arithmetic mean of all distances, and the measure of similarity (Δ) relative to SnTe₃O₈ as the reference structure. All these values show a very high similarity between the crystal structures in the isotypic M^IVTe₃O₈ series.

Table 2
Atom pairs and their absolute distances |u| (Å) in the isotypic series MTe₃O₈ with SnTe₃O₈ as the reference structure, as well as degree of lattice distortion (S), arithmetic mean of the distances (d_av, Å) and measure of similarity (Δ)

	TiTe3O8 ^a	(Ti0.96Fe0.04)Te3O8 ^b	ZrTe3O8^c	ZrTe3O8 ^d
M^IV1	0	0	0	0
Te1	0.0475	0.0360	0.0065	0.0059
O1	0.1061	0.0834	0.0713	0.0694
O2	0.1374	0.0968	0.0543	0.0446
S	0.0107	0.0102	0.0076	0.0092
dav	0.0878	0.0668	0.0453	0.0436
Δ	0.011	0.008	0.006	0.006
Notes: (a) a = 10.956 (3) Å; Meunier & Galy (1971); (b) a = 10.965 (1) Å; Bindi & Cipriani (2003); (c) a = 11.308 (1) Å; Noguera et al. (2003); (d) a = 11.340 (4) Å; Lu et al. (2019).

3. Synthesis and crystallization

Reagent-grade chemicals were used without further purification. SnO₂ (71 mg, 0.47 mmol) and TeO₂ (225 mg, 1.40 mmol) were thoroughly mixed in the molar ratio 1:3 and placed in a silica tube to which 50 mg of TeCl₄ were added as the transport agent. The silica ampoule was then evacuated and torch-sealed, placed in a two-zone furnace using a temperature gradient 973 K (source) → 873 K (sink) for three days. Cubic, canary-yellow crystals had formed in the millimetre size range in the colder sink region as the only product (Fig. 3). Powder X-ray diffraction of the remaining material in the source region revealed SnTe₃O₈ as the main phase and SnO₂ as a side phase. For the single-crystal diffraction study, a fragment was broken from a larger crystal.

Figure 3 Photograph of Sn3TeO8 single crystals grown by chemical vapor transport reactions.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. Atomic coordinates and the labelling scheme were adapted from isotypic TiTe₃O₈ (Meunier & Galy, 1971).

Table 3
Experimental details

Crystal data
Chemical formula	SnTe3O8
Mr	629.49
Crystal system, space group	Cubic, Ia
Temperature (K)	296
a (Å)	11.1574 (4)
V (Å^3)	1388.96 (15)
Z	8
Radiation type	Mo Kα
μ (mm^−1)	16.04
Crystal size (mm)	0.06 × 0.06 × 0.01
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.452, 0.748
No. of measured, independent and observed [I > 2σ(I)] reflections	14087, 735, 697
Rint	0.048
(sin θ/λ)max (Å^−1)	0.907
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.014, 0.030, 1.07
No. of reflections	735
No. of parameters	21
Δρmax, Δρmin (e Å^−3)	1.27, −0.86
Computer programs: APEX3 and SAINT (Bruker, 2018), SHELXL (Sheldrick, 2015), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010).

Supporting information

Computing details

Data collection: APEX3 (Bruker, 2018); cell refinement: SAINT (Bruker, 2018); data reduction: SAINT (Bruker, 2018); program(s) used to solve structure: coordinates from previous refinement; program(s) used to refine structure: SHELXL (Sheldrick, 2015); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Tin(IV) trioxidotellurate(IV)

Crystal data

SnTe3O8	Mo Kα radiation, λ = 0.71073 Å
Mr = 629.49	Cell parameters from 5266 reflections
Cubic, Ia3	θ = 3.7–38.9°
a = 11.1574 (4) Å	µ = 16.04 mm^−1
V = 1388.96 (15) Å^3	T = 296 K
Z = 8	Plate, light yellow
F(000) = 2160	0.06 × 0.06 × 0.01 mm
Dx = 6.021 Mg m^−3

Data collection

Bruker APEXII CCD diffractometer	697 reflections with I > 2σ(I)
ω– and φ–scans	Rint = 0.048
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θmax = 40.2°, θmin = 3.7°
Tmin = 0.452, Tmax = 0.748	h = −18→20
14087 measured reflections	k = −20→20
735 independent reflections	l = −19→20

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0127P)^2 + 1.9293P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.014	(Δ/σ)max = 0.001
wR(F^2) = 0.030	Δρmax = 1.27 e Å^−3
S = 1.07	Δρmin = −0.85 e Å^−3
735 reflections	Extinction correction: SHELXL-2017/1 (Sheldrick 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
21 parameters	Extinction coefficient: 0.00046 (3)

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
Sn1	0.000000	0.000000	0.000000	0.00501 (4)
Te1	0.20584 (2)	0.000000	0.250000	0.00804 (4)
O1	0.43242 (10)	0.13738 (10)	0.39972 (11)	0.0129 (2)
O2	0.16789 (10)	0.16789 (10)	0.16789 (10)	0.0078 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.00501 (4)	0.00501 (4)	0.00501 (4)	−0.00031 (3)	−0.00031 (3)	−0.00031 (3)
Te1	0.00518 (5)	0.01273 (6)	0.00620 (5)	0.000	0.000	−0.00229 (4)
O1	0.0095 (4)	0.0117 (4)	0.0174 (5)	0.0019 (3)	0.0018 (4)	0.0094 (4)
O2	0.0078 (3)	0.0078 (3)	0.0078 (3)	0.0020 (3)	0.0020 (3)	0.0020 (3)

Geometric parameters (Å, º)

Sn1—O1^i	2.0421 (11)	Sn1—O1^vi	2.0421 (11)
Sn1—O1^ii	2.0421 (11)	Te1—O1^v	1.8800 (11)
Sn1—O1^iii	2.0421 (11)	Te1—O1^vii	1.8800 (11)
Sn1—O1^iv	2.0421 (11)	Te1—O2^viii	2.1278 (3)
Sn1—O1^v	2.0421 (11)	Te1—O2	2.1278 (3)
O1^i—Sn1—O1^ii	86.09 (4)	O1^iv—Sn1—O1^vi	93.91 (4)
O1^i—Sn1—O1^iii	93.91 (4)	O1^v—Sn1—O1^vi	86.09 (4)
O1^ii—Sn1—O1^iii	180.00 (9)	O1^v—Te1—O1^vii	102.42 (8)
O1^i—Sn1—O1^iv	86.09 (4)	O1^v—Te1—O2^viii	79.05 (4)
O1^ii—Sn1—O1^iv	93.91 (4)	O1^vii—Te1—O2^viii	86.60 (6)
O1^iii—Sn1—O1^iv	86.09 (4)	O1^v—Te1—O2	86.60 (6)
O1^i—Sn1—O1^v	93.91 (4)	O1^vii—Te1—O2	79.05 (4)
O1^ii—Sn1—O1^v	86.09 (4)	O2^viii—Te1—O2	157.05 (6)
O1^iii—Sn1—O1^v	93.91 (4)	Te1^ix—O1—Sn1^x	134.17 (6)
O1^iv—Sn1—O1^v	180.00 (9)	Te1—O2—Te1^xi	117.94 (2)
O1^i—Sn1—O1^vi	180.00 (9)	Te1—O2—Te1^xii	117.94 (2)
O1^ii—Sn1—O1^vi	93.91 (4)	Te1^xi—O2—Te1^xii	117.94 (2)
O1^iii—Sn1—O1^vi	86.09 (4)

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