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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 TeIV atoms are stereochemically active and protrude into channels within the anionic layer. In comparison with analogous MIITe2O5 structures (M = Mg, Mn, Ni or Cu) with ditellurate(IV) anions that are exclusively made up of corner-sharing TeOx (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.

Supporting information


Crystallographic Information File (CIF)
Contains datablocks I, global


Structure factor file (CIF format)
Contains datablock I

Comment top

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 TeIV 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 MIITe2O5 phases, where MII = 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 MIITe2O5 structures are similarly made up of layered 2[Te2O5]2- anions (except for the columnar arrangement of Te2O52- 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 TeOx (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 O2- 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 TeIV 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).

Experimental top

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.

Refinement top

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 top

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 top
[Figure 1] 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.
[Figure 2] Fig. 2. Channels in the 2[Te2O5]2- layer (polyhedral description) extending parallel to [011]. The channels parallel to [011] are similar.
[Figure 3] 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.
[Figure 4] Fig. 4. Construction of the 2[Te2O5]2- layers in other MIITe2O5 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) top
Crystal data top
CaTe2O5F(000) = 656
Mr = 375.28Dx = 4.616 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybcCell parameters from 1732 reflections
a = 9.382 (2) Åθ = 3.7–29.8°
b = 5.7095 (14) ŵ = 11.68 mm1
c = 11.132 (3) ÅT = 293 K
β = 115.109 (4)°Fragment, colourless
V = 540.0 (2) Å30.07 × 0.04 × 0.02 mm
Z = 4
Data collection top
1569 independent reflections
Radiation source: fine-focus sealed tube1352 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.039
ω scansθmax = 30.0°, θmin = 3.7°
Absorption correction: multi-scan
SADABS (Bruker, 2002)
h = 1312
Tmin = 0.495, Tmax = 0.800k = 88
5694 measured reflectionsl = 1415
Refinement top
Refinement on F20 restraints
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.036Secondary atom site location: difference Fourier map
wR(F2) = 0.079 w = 1/[σ2(Fo2) + (0.0394P)2 + 0.9268P]
where P = (Fo2 + 2Fc2)/3
S = 1.07(Δ/σ)max = 0.001
1569 reflectionsΔρmax = 1.96 e Å3
73 parametersΔρmin = 1.15 e Å3
Crystal data top
CaTe2O5V = 540.0 (2) Å3
Mr = 375.28Z = 4
Monoclinic, P21/cMo Kα radiation
a = 9.382 (2) ŵ = 11.68 mm1
b = 5.7095 (14) ÅT = 293 K
c = 11.132 (3) Å0.07 × 0.04 × 0.02 mm
β = 115.109 (4)°
Data collection top
1569 independent reflections
Absorption correction: multi-scan
SADABS (Bruker, 2002)
1352 reflections with I > 2σ(I)
Tmin = 0.495, Tmax = 0.800Rint = 0.039
5694 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.03673 parameters
wR(F2) = 0.0790 restraints
S = 1.07Δρmax = 1.96 e Å3
1569 reflectionsΔρmin = 1.15 e Å3
Special details top

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 (Å2) top
Ca0.01913 (15)0.7969 (2)0.63901 (12)0.0138 (2)
Te10.26400 (5)0.32196 (7)0.72052 (4)0.01340 (12)
Te20.34909 (5)0.64469 (7)0.49039 (4)0.01257 (12)
O10.1455 (5)0.4895 (8)0.7882 (4)0.0150 (9)
O20.1183 (6)0.1442 (8)0.5884 (4)0.0199 (10)
O30.2264 (6)0.5903 (8)0.5982 (5)0.0185 (9)
O40.2030 (5)0.0176 (8)0.8467 (4)0.0176 (9)
O50.4774 (6)0.3938 (9)0.5903 (5)0.0182 (9)
Atomic displacement parameters (Å2) top
Ca0.0167 (6)0.0150 (6)0.0105 (5)0.0028 (5)0.0064 (5)0.0016 (4)
Te10.0113 (2)0.0169 (2)0.0105 (2)0.00192 (14)0.00315 (15)0.00057 (13)
Te20.0117 (2)0.0147 (2)0.00964 (19)0.00146 (14)0.00295 (14)0.00065 (13)
O10.017 (2)0.017 (2)0.015 (2)0.0026 (17)0.0105 (18)0.0043 (17)
O20.027 (3)0.017 (2)0.010 (2)0.0062 (19)0.0022 (19)0.0026 (16)
O30.018 (2)0.023 (2)0.018 (2)0.0068 (19)0.0105 (19)0.0060 (18)
O40.016 (2)0.021 (2)0.011 (2)0.0002 (18)0.0011 (17)0.0042 (17)
O50.017 (2)0.025 (2)0.015 (2)0.0067 (19)0.0090 (18)0.0105 (18)
Geometric parameters (Å, º) top
Ca—O1i2.305 (4)Te1—O31.980 (5)
Ca—O2ii2.326 (5)Te1—O42.450 (5)
Ca—O2iii2.358 (5)Te1—Caiv3.6122 (15)
Ca—O12.360 (5)Te1—Cav3.6498 (15)
Ca—O32.476 (5)Te1—Caii3.8095 (15)
Ca—O4iii2.554 (5)Te2—O4vi1.854 (4)
Ca—O4i2.682 (5)Te2—O51.898 (4)
Ca—Te13.4183 (14)Te2—O32.009 (5)
Ca—Te1i3.6122 (15)Te2—O5vii2.178 (5)
Ca—Te1iii3.6498 (15)Te2—Te2vii3.2063 (10)
Te1—O21.832 (4)Te2—Caviii3.8366 (15)
Te1—O11.852 (4)
O1i—Ca—O2ii99.70 (18)O1—Te1—O478.84 (17)
O1i—Ca—O2iii94.25 (18)O3—Te1—O4157.89 (18)
O2ii—Ca—O2iii73.20 (18)O4vi—Te2—O5100.4 (2)
O1i—Ca—O1108.59 (11)O4vi—Te2—O392.1 (2)
O2ii—Ca—O1138.95 (17)O5—Te2—O385.15 (19)
O2iii—Ca—O1131.45 (17)O4vi—Te2—O5vii90.1 (2)
O1i—Ca—O3170.92 (16)O5—Te2—O5vii76.5 (2)
O2ii—Ca—O389.15 (17)O3—Te2—O5vii161.59 (18)
O2iii—Ca—O386.34 (18)Te1—O1—Caiv120.3 (2)
O1—Ca—O364.91 (15)Te1—O1—Ca107.86 (19)
O1i—Ca—O4iii75.06 (16)Caiv—O1—Ca113.00 (18)
O2ii—Ca—O4iii139.72 (16)Te1—O2—Caii132.4 (2)
O2iii—Ca—O4iii67.54 (15)Te1—O2—Cav120.6 (2)
O1—Ca—O4iii77.61 (16)Caii—O2—Cav106.80 (18)
O3—Ca—O4iii96.89 (16)Te1—O3—Te2123.4 (2)
O1i—Ca—O4i67.01 (15)Te1—O3—Ca99.62 (19)
O2ii—Ca—O4i93.71 (16)Te2—O3—Ca136.9 (2)
O2iii—Ca—O4i155.45 (17)Te2ix—O4—Te1125.6 (2)
O1—Ca—O4i71.77 (15)Te2ix—O4—Cav120.2 (2)
O3—Ca—O4i114.73 (16)Te1—O4—Cav93.64 (15)
O4iii—Ca—O4i118.88 (9)Te2ix—O4—Caiv123.7 (2)
O2—Te1—O1103.9 (2)Te1—O4—Caiv89.37 (14)
O2—Te1—O391.2 (2)Cav—O4—Caiv95.96 (15)
O1—Te1—O385.27 (19)Te2—O5—Te2vii103.5 (2)
O2—Te1—O477.95 (18)
Symmetry codes: (i) x, y+1/2, z+3/2; (ii) x, y+1, z+1; (iii) x, y+1, z; (iv) x, y1/2, z+3/2; (v) x, y1, z; (vi) x, y+1/2, z1/2; (vii) x+1, y+1, z+1; (viii) x, y+3/2, z1/2; (ix) x, y+1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaCaTe2O5
Crystal system, space groupMonoclinic, P21/c
Temperature (K)293
a, b, c (Å)9.382 (2), 5.7095 (14), 11.132 (3)
β (°) 115.109 (4)
V3)540.0 (2)
Radiation typeMo Kα
µ (mm1)11.68
Crystal size (mm)0.07 × 0.04 × 0.02
Data collection
DiffractometerBruker APEX CCD
Absorption correctionMulti-scan
SADABS (Bruker, 2002)
Tmin, Tmax0.495, 0.800
No. of measured, independent and
observed [I > 2σ(I)] reflections
5694, 1569, 1352
(sin θ/λ)max1)0.703
R[F2 > 2σ(F2)], wR(F2), S 0.036, 0.079, 1.07
No. of reflections1569
No. of parameters73
Δρmax, Δρmin (e Å3)1.96, 1.15

Computer programs: SMART (Bruker, 2002), SAINT (Bruker, 2002), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ATOMS (Dowty, 2006).

Selected geometric parameters (Å, º) top
Ca—O1i2.305 (4)Te1—O11.852 (4)
Ca—O2ii2.326 (5)Te1—O31.980 (5)
Ca—O2iii2.358 (5)Te1—O42.450 (5)
Ca—O12.360 (5)Te2—O4iv1.854 (4)
Ca—O32.476 (5)Te2—O51.898 (4)
Ca—O4iii2.554 (5)Te2—O32.009 (5)
Ca—O4i2.682 (5)Te2—O5v2.178 (5)
Te1—O21.832 (4)
Te1—O3—Te2123.4 (2)Te2—O5—Te2v103.5 (2)
Te2vi—O4—Te1125.6 (2)
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, z1/2; (v) x+1, y+1, z+1; (vi) x, y+1/2, z+1/2.

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