A non-twinned polymorph of CaTe₂O₅ from a hydro­thermally grown crystal

In contrast with the multiple twinning and/or domain form­ation found in the mica-like polymorphs of CaTe₂O₅, calcium penta­oxido­ditellurate(IV), that have been prepared by solid-state reactions and for which complete structure determinations have not been successful up to now, the crystal structure of a hydro­thermally grown phase was fully deter­mined from a non-twinned crystal. The structure is made up of alternating layers of Ca²⁺ cations and of ²_∞[Te₂O₅]²⁻ anions stacked along [100]. The lone-pair electrons E of the Te^IV atoms are stereochemically active and protrude into channels within the anionic layer. In comparison with analogous M^IITe₂O₅ structures (M = Mg, Mn, Ni or Cu) with ditellurate(IV) anions that are exclusively made up of corner-sharing TeO_x (x = 3–5) polyhedra resulting in flat ²_∞[Te₂O₅]²⁻ layers, the anionic layers in CaTe₂O₅ are undulating and are built of corner- and edge-sharing [TeO₄] polyhedra.

S1. Comment

The ditellurate(IV) CaTe₂O₅ has been the subject of several previous investigations and can be prepared in polycrystalline form by heating stoichiometric amounts of the binary oxides above 800 K (Trömel & Ziethen-Reichnach, 1970). Single crystals of this material were grown by slow cooling of the melt through the melting point (965 K; Redman et al., 1970) and they exhibit interesting ferroelectric properties over a wide temperature range (Sadovskaya et al.,1983, 1987). However, multiple twinning and/or formation of domain structures in the mica-like crystals have prevented a complete structure determination so far (Trömel & Ziethen-Reichnach, 1970; Redman et al., 1970; Gorbenko et al., 1983). More recent thermal analysis studies of CaTe₂O₅ (also prepared by solid-state reaction) revealed several phase transitions, with one room-temperature polymorph, denoted α-CaTe₂O₅, and three high-temperature polymorphs (β, γ and δ) (Mishra et al., 1998). The corresponding X-ray powder diffraction patterns of the different phases were indexed on the basis of monoclinic (α- and β-polymorphs), orthorhombic (γ-polymorph) and tetragonal (δ-polymorph) unit cells (Tripathi et al., 2001), but for none of these phases were the structures solved. We have likewise obtained a compound with the composition CaTe₂O₅ during hydrothermal treatment of phases in the system Ca–Te–O (see Experimental). Following the nomenclature of Mishra et al. (1998), this fifth CaTe₂O₅ polymorph is accordingly denoted ε-CaTe₂O₅.

The structure of ε-CaTe₂O₅ contains one Ca, two Te and five O atoms in the asymmetric unit. The main building units of the structure are two different [TeO₄] polyhedra and one [CaO₇] polyhedron. The [TeO₄] polyhedra are linked by corner- and edge-sharing to build undulating layers with an overall composition of ²_∞[Te₂O₅]^2- that propagate parallel to (100). Adjacent tellurate(IV) layers are interconnected by intermediate Ca²⁺ cations to establish a structure with alternating layers and a stacking sequence Ca²⁺ (at x ≈ 0)–²_∞[Te₂O₅]^2-–Ca²⁺ (at x ≈ 1) along [100] (Fig. 1).

The two Te atoms are in oxidation state +IV and they are each surrounded by three O atoms at short Te—O distances < 2 Å. A fourth O atom situated at longer distances of 2.178 (5) (Te2) and 2.450 (5) Å (Te1), respectively, complements their coordination spheres (Table 1). In general, the O coordination and coordination numbers of Te for various oxotellurates(IV) show great variation, with typical Te—O distances and coordination numbers in the ranges 1.8–2.35 Å and 3–5, respectively (Zemann, 1971). However, for saturation of the bond-valence sums, more remote O atoms up to Te—O distances of 2.60 Å should be considered as weakly bonding, giving rise to a [3+1] O coordination for Te1 and a [4] O coordination for Te2. The corresponding [TeO₄] polyhedra might be described as distorted trigonal bipyramids with one ligand occupied by the non-bonding lone-pair electrons E of the Te^IV atoms. As for the structures of other oxotellurate(IV) compounds (Zemann, 1971; Dolgikh, 1991), the stereochemical influence of the lone-pair electrons is obvious. They are situated opposite to each other on the interior of the undulating oxotellurate(IV) layers and point towards the voids (Fig. 1). Due to the spiral arrangement of the condensed [TeO₄] polyhedra, these voids form channels that run parallel to [011] (Fig. 2) and [011], with diameters of about 5 Å. Additional channels with a somewhat shorter diameter of about 4.4 Å extend parallel to [x ≈ 1/2, y, z ≈ 1/4] and [x ≈ 1/2, y, z ≈ 3/4] (Fig. 3).

In comparison with the crystal structures of analogous M^IITe₂O₅ phases, where M^II = Mg (Weil, 2005), and the isotypic β-Mn (Johnston & Harrison, 2002) [space group Pbcn], and denningite-type α-Mn (Miletich, 1993) [P4₂/nbc], Ni (Platte & Trömel, 1981) [Pnma] and Cu (Hanke et al., 1973) [P2₁/c], the structural set-up of the anionic layers in ε-CaTe₂O₅ is unique. When Te—O interactions of < 2.60 Å are considered as bonding, the other M^IITe₂O₅ structures are similarly made up of layered ²_∞[Te₂O₅]^2- anions (except for the columnar arrangement of Te₂O₅^2- anions in α-MnTe₂O₅, where the shortest Te—O distance between adjacent columns is > 2.9 Å), with the lone-pair electrons protruding into the empty space of the structures. However, all these layers are flat and constructed solely from corner-sharing TeO_x (x = 3–5) polyhedra (Fig. 4), in contrast with ε-CaTe₂O₅, with its undulating anionic layers composed of both corner- and edge-sharing [TeO₄] polyhedra (Fig. 3).

The Ca²⁺ cations form a distorted hexagonal layer parallel to (100) and they are bonded to seven O atoms from neighbouring anionic layers with Ca—O distances from 2.305 (4) to 2.682 (5) Å, which compare well with those observed for CaTeO₄ (Hottentot & Loopstra, 1979) or Ca₄Te₅O₁₄ (Weil, 2004). Moreover, the average Ca—O distance of 2.44 Å is in very good agreement with the value of 2.42 Å calculated from the sum of the ionic radii for O^2- and seven-coordinate Ca²⁺ given by Shannon (1976).

The O atoms exhibit different coordination environments, with coordination numbers of 4 for O3 (2 × Ca and 2 × Te as coordination partners), 3 for O1, O2 (both 1 × Te and 2 × Ca) and O4 (1 × Ca and 2 × Te), and 2 for O5 which acts exclusively as the bridging atom between two Te^IV centres. Results from bond-valence sum (BVS) calculations (Brown, 2002), using the parameters of Brese & O'Keeffe (1991), are in accordance with expected values: Ca = 2.08 (expected 2.00), Te1 = 4.15 (4.00), Te2 = 4.13 (4.00), O1 = 2.15 (2.00), O2 = 2.20 (2.00), O3 = 2.16 (2.00), O4 = 2.05 (2.00), O5 = 1.82 (2.00).

S2. Experimental

Single crystals of ε-CaTe₂O₅ were obtained in small amounts by hydrothermal treatment of α-CaTeO₃ as starting material. α-CaTeO₃ (Stöger et al., 2008) was prepared by the solid-state reaction of stoichiometric amounts of CaO and TeO₂ in evacuated fused silica ampoules at 973 K for 70 h. The reaction products obtained by this method contained small quantities (ca 3%) of the mixed-valent phase Ca₄Te₅O₁₄ (Weil, 2004). A 5 ml Teflon inlay was filled with 100 mg of the as-prepared polycrystalline product and 3 ml deionized water, placed in a steel autoclave and heated at 493 K for 7 d. The residue was washed with water, ethanol and acetone. It consisted mainly of the unchanged polycrystalline α-CaTeO₃ phase and a few colourless crystals with undefined habit of the title compound.

S3. Refinement

The highest remaining peak in the final difference Fourier map is located 0.75 Å from Te2 and the deepest hole is located 2.01 Å from O5.

Computing details

Data collection: SMART (Bruker, 2002); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The crystal structure of ε-CaTe2O5 in a projection along [001]. [TeO4] polyhedra are shaded, Ca atoms are displayed as large filled spheres, Te atoms as small shaded spheres and O atoms as open spheres.
	Fig. 2. Channels in the ^2∞[Te2O5]^2- layer (polyhedral description) extending parallel to [011]. The channels parallel to [011] are similar.
	Fig. 3. Part of the ^2∞[Te2O5]^2- layer of ε-CaTe2O5 in a projection along [010], showing the channels along [x ≈ 1/2, y, z ≈ 1/4] and [x ≈ 1/2, y, z ≈ 3/4]. Displacement ellipsoids are drawn at the 90% probability level. Short Te—O bonds of < 2.20 Å are drawn with solid bonds, whereas the longer Te—O bond [2.450 (5) Å] is drawn with open bonds.
	Fig. 4. Construction of the ^2∞[Te2O5]^2- layers in other M^IITe2O5 structures, where M = Mg, Mn, Cu. The Te atoms are displayed as shaded spheres and the O atoms as open spheres. Short Te—O bonds of < 2.20 Å are drawn with solid bonds and longer Te—O bonds are drawn with open bonds. Crystal data for M = Mg, Ni and Cu were taken from Weil (2005), Platte & Trömel (1981) and Hanke et al. (1973), respectively.

Calcium pentaoxidoditellurate(IV)

Crystal data

CaTe2O5	F(000) = 656
Mr = 375.28	Dx = 4.616 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 1732 reflections
a = 9.382 (2) Å	θ = 3.7–29.8°
b = 5.7095 (14) Å	µ = 11.68 mm^−1
c = 11.132 (3) Å	T = 293 K
β = 115.109 (4)°	Fragment, colourless
V = 540.0 (2) Å^3	0.07 × 0.04 × 0.02 mm
Z = 4

Data collection

Bruker APEX CCD diffractometer	1569 independent reflections
Radiation source: fine-focus sealed tube	1352 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.039
ω scans	θmax = 30.0°, θmin = 3.7°
Absorption correction: multi-scan SADABS (Bruker, 2002)	h = −13→12
Tmin = 0.495, Tmax = 0.800	k = −8→8
5694 measured reflections	l = −14→15

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)] = 0.036	Secondary atom site location: difference Fourier map
wR(F^2) = 0.079	w = 1/[σ^2(Fo^2) + (0.0394P)^2 + 0.9268P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max = 0.001
1569 reflections	Δρmax = 1.96 e Å^−3
73 parameters	Δρmin = −1.15 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
Ca	0.01913 (15)	0.7969 (2)	0.63901 (12)	0.0138 (2)
Te1	0.26400 (5)	0.32196 (7)	0.72052 (4)	0.01340 (12)
Te2	0.34909 (5)	0.64469 (7)	0.49039 (4)	0.01257 (12)
O1	0.1455 (5)	0.4895 (8)	0.7882 (4)	0.0150 (9)
O2	0.1183 (6)	0.1442 (8)	0.5884 (4)	0.0199 (10)
O3	0.2264 (6)	0.5903 (8)	0.5982 (5)	0.0185 (9)
O4	0.2030 (5)	0.0176 (8)	0.8467 (4)	0.0176 (9)
O5	0.4774 (6)	0.3938 (9)	0.5903 (5)	0.0182 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca	0.0167 (6)	0.0150 (6)	0.0105 (5)	0.0028 (5)	0.0064 (5)	0.0016 (4)
Te1	0.0113 (2)	0.0169 (2)	0.0105 (2)	0.00192 (14)	0.00315 (15)	−0.00057 (13)
Te2	0.0117 (2)	0.0147 (2)	0.00964 (19)	−0.00146 (14)	0.00295 (14)	−0.00065 (13)
O1	0.017 (2)	0.017 (2)	0.015 (2)	0.0026 (17)	0.0105 (18)	0.0043 (17)
O2	0.027 (3)	0.017 (2)	0.010 (2)	−0.0062 (19)	0.0022 (19)	−0.0026 (16)
O3	0.018 (2)	0.023 (2)	0.018 (2)	0.0068 (19)	0.0105 (19)	0.0060 (18)
O4	0.016 (2)	0.021 (2)	0.011 (2)	0.0002 (18)	0.0011 (17)	0.0042 (17)
O5	0.017 (2)	0.025 (2)	0.015 (2)	0.0067 (19)	0.0090 (18)	0.0105 (18)

Geometric parameters (Å, º)

Ca—O1^i	2.305 (4)	Te1—O3	1.980 (5)
Ca—O2^ii	2.326 (5)	Te1—O4	2.450 (5)
Ca—O2^iii	2.358 (5)	Te1—Ca^iv	3.6122 (15)
Ca—O1	2.360 (5)	Te1—Ca^v	3.6498 (15)
Ca—O3	2.476 (5)	Te1—Ca^ii	3.8095 (15)
Ca—O4^iii	2.554 (5)	Te2—O4^vi	1.854 (4)
Ca—O4^i	2.682 (5)	Te2—O5	1.898 (4)
Ca—Te1	3.4183 (14)	Te2—O3	2.009 (5)
Ca—Te1^i	3.6122 (15)	Te2—O5^vii	2.178 (5)
Ca—Te1^iii	3.6498 (15)	Te2—Te2^vii	3.2063 (10)
Te1—O2	1.832 (4)	Te2—Ca^viii	3.8366 (15)
Te1—O1	1.852 (4)
O1^i—Ca—O2^ii	99.70 (18)	O1—Te1—O4	78.84 (17)
O1^i—Ca—O2^iii	94.25 (18)	O3—Te1—O4	157.89 (18)
O2^ii—Ca—O2^iii	73.20 (18)	O4^vi—Te2—O5	100.4 (2)
O1^i—Ca—O1	108.59 (11)	O4^vi—Te2—O3	92.1 (2)
O2^ii—Ca—O1	138.95 (17)	O5—Te2—O3	85.15 (19)
O2^iii—Ca—O1	131.45 (17)	O4^vi—Te2—O5^vii	90.1 (2)
O1^i—Ca—O3	170.92 (16)	O5—Te2—O5^vii	76.5 (2)
O2^ii—Ca—O3	89.15 (17)	O3—Te2—O5^vii	161.59 (18)
O2^iii—Ca—O3	86.34 (18)	Te1—O1—Ca^iv	120.3 (2)
O1—Ca—O3	64.91 (15)	Te1—O1—Ca	107.86 (19)
O1^i—Ca—O4^iii	75.06 (16)	Ca^iv—O1—Ca	113.00 (18)
O2^ii—Ca—O4^iii	139.72 (16)	Te1—O2—Ca^ii	132.4 (2)
O2^iii—Ca—O4^iii	67.54 (15)	Te1—O2—Ca^v	120.6 (2)
O1—Ca—O4^iii	77.61 (16)	Ca^ii—O2—Ca^v	106.80 (18)
O3—Ca—O4^iii	96.89 (16)	Te1—O3—Te2	123.4 (2)
O1^i—Ca—O4^i	67.01 (15)	Te1—O3—Ca	99.62 (19)
O2^ii—Ca—O4^i	93.71 (16)	Te2—O3—Ca	136.9 (2)
O2^iii—Ca—O4^i	155.45 (17)	Te2^ix—O4—Te1	125.6 (2)
O1—Ca—O4^i	71.77 (15)	Te2^ix—O4—Ca^v	120.2 (2)
O3—Ca—O4^i	114.73 (16)	Te1—O4—Ca^v	93.64 (15)
O4^iii—Ca—O4^i	118.88 (9)	Te2^ix—O4—Ca^iv	123.7 (2)
O2—Te1—O1	103.9 (2)	Te1—O4—Ca^iv	89.37 (14)
O2—Te1—O3	91.2 (2)	Ca^v—O4—Ca^iv	95.96 (15)
O1—Te1—O3	85.27 (19)	Te2—O5—Te2^vii	103.5 (2)
O2—Te1—O4	77.95 (18)

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