Ca2Te3O8, a new phase in the CaO–TeO2 system

Weil, M.

doi:10.1107/S2056989018017310

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Volume 75| Part 1| January 2019| Pages 26-29

https://doi.org/10.1107/S2056989018017310

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Ca₂Te₃O₈, a new phase in the CaO–TeO₂ system

Matthias Weil ^a ^*

^aInstitute for Chemical Technologies and Analytics, Division of Structural Chemistry, TU Wien, Getreidemarkt 9/164-SC, A-1060 Vienna, Austria
^*Correspondence e-mail: matthias.weil@tuwien.ac.at

Edited by S. Parkin, University of Kentucky, USA (Received 3 December 2018; accepted 5 December 2018; online 1 January 2019)

Single crystals of dicalcium octaoxidotritellurate(IV), Ca₂Te₃O₈, were obtained from a CsCl/NaCl melt with CaO and TeO₂ as educts in the molar ratio of 1:2. Ca₂Te₃O₈ crystallizes isotypically with Pb₃Te₂O₈ and is comprised of two unique Ca, four Te and eight O sites. One calcium cation has eight and the other nine coordination partners. Both coordination polyhedra are considerably distorted. Two kinds of oxotellurate(IV) anions with the same formula [Te₃O₈]⁴⁻ are present. One is an infinite zigzag chain anion consisting of pairs of [TeO₄] bisphenoids linked to a trigonal–pyramidal [TeO₃] group with a connectivity of [(TeO_1/1O_2/2)(TeO_2/1O_2/2)₂]_n, while the other is a finite anion made up of one central [TeO₄] bisphenoid linked to two [TeO₃] trigonal pyramids and has a connectivity of [(TeO_2/1O_1/2)₂(TeO_2/2O_2/1)]. In the crystal, the anions are organized in layers extending parallel to (100). Adjacent layers are held together by the calcium cations to define a three-dimensional framework structure.

Keywords: crystal structure; oxotellurate(IV); calcium tellurite; isotypism; compstru.

CCDC reference: 1883382

1. Chemical context

A partial phase diagram for the pseudo-binary system CaO–TeO₂ has been determined for the composition range 50–100 mol% TeO₂ to contain the 1:1 phase CaTeO₃ and the 1:2 phase CaTe₂O₅ (Mishra et al., 1998 ). Another phase not reported during the original study of Mishra et al. (1998) is the 4:5 phase Ca₄Te₅O₁₄, for which full structural details were determined for the normal-pressure and high-pressure forms (Weil, 2004 ; Weil et al., 2016 ). For compositions CaTeO₃ and CaTe₂O₅, polymorphism was reported on the basis of differential thermal analysis and temperature-dependent X-ray diffracion (Mishra et al., 1998; Tripathi et al., 2001 ), however, without structural details of the corresponding phases. Whereas crystal structure determinations were subsequently performed for four polymorphic forms of CaTeO₃ (Stöger et al., 2009 ; Poupon et al., 2015 ), our present knowledge of the CaTe₂O₅ structures is restricted to only one form (Weil & Stöger, 2008 ; Barrier et al., 2009 ) that is not related to the mica-like CaTe₂O₅ phase reported nearly 50 years ago (Redman et al., 1970 ). In an attempt to grow single crystals of the latter from a salt melt at comparatively low temperatures, a heretofore unknown phase in the CaO–TeO₂ system was obtained, viz. the 2:3 phase Ca₂Te₃O₈.

In this article, preparation conditions, crystal structure and the relation to the isotypic lead(II) analogue Pb₂Te₃O₈ (Champarnaud-Mesjard et al., 2001 ) are reported.

2. Structural commentary

The asymmetric unit of Ca₂Te₃O₈ comprises two Ca sites, four Te sites and eight O sites. One Ca site (Ca2) is located on Wyckoff position 8g (site symmetry ..m), sites Te1 on 4c (m2m), Te2 on 8f (m..), Te4 on 8g, O1 on 8f, O2 on 8e (2..) and O7 and O8 both on 8g; all other sites are on general positions 16h.

The two Ca²⁺ cations are surrounded by eight (Ca1) and nine (Ca2) O atoms, considering a cut-off value of 3.1 Å for relevant Ca—O distances (Table 1). The bond valence sums (Brown, 2002 ) computed with the parameters of Brown & Altermatt (1985 ) are 1.89 valence units (v.u.) for Ca1 and 2.02 v.u. for Ca2, in good agreement with the expected value of 2. Likewise, the mean Ca—O bond length of 2.55 Å for Ca1 and 2.57 Å for Ca2 are in accord with the values for eight- and nine-coordinate Ca of 2.50 (15) and 2.56 (20) Å, respectively (Gagné & Hawthorne, 2016 ). Whereas the [Ca1O₈] polyhedron is difficult to derive from a simple geometric figure, [Ca2O₉] can be best described as a monocapped square antiprism (Fig. 1).

Table 1
Comparison of bond lengths (Å) in Ca₂Te₃O₈ and isotypic Pb₂Te₃O₈

	Ca₂Te₃O₈^a	Pb₂Te₃O₈^b
M1—O3ⁱ	2.3198 (11)	2.372 (8)
M1—O6ⁱⁱ	2.3903 (13)	2.440 (8)
M1—O5	2.3924 (11)	2.470 (6)
M1—O3	2.4691 (12)	2.636 (8)
M1—O5ⁱⁱⁱ	2.4712 (11)	2.934 (8)
M1—O3^iv	2.6212 (12)	3.032 (8)
M1—O4	2.7186 (13)
M1—O8	3.0360 (4)	3.069 (2)
M2—O8^v	2.3089 (18)	2.439 (9)
M2—O7^vi	2.3660 (17)	2.374 (10)
M2—O5^v	2.4497 (11)	2.556 (6)
M2—O5ⁱⁱⁱ	2.4497 (11)	2.556 (6)
M2—O8	2.4749 (19)	3.080 (11)
M2—O4^vi	2.6335 (13)	2.732 (6)
M2—O4^iv	2.6335 (13)	2.732 (6)
M2—O4	2.9154 (13)	3.342 (7)
M2—O4ⁱⁱ	2.9154 (13)	3.342 (7)
Te1—O7^vii	1.8369 (17)	1.852 (10)
Te1—O7^viii	1.8369 (17)	1.852 (10)
Te1—O1	2.1608 (17)	2.160 (9)
Te1—O1ⁱⁱ	2.1608 (17)	2.160 (9)
Te2—O6^ix	1.8522 (12)	1.859 (8)
Te2—O6	1.8522 (12)	1.859 (8)
Te2—O1	1.8902 (17)	1.883 (10)
Te3—O3	1.8602 (11)	1.868 (8)
Te3—O5^x	1.8743 (10)	1.856 (7)
Te3—O2^x	2.0123 (5)	2.008 (3)
Te3—O4	2.3222 (12)	2.338 (6)
Te4—O8	1.8694 (17)	1.857 (10)
Te4—O4	1.8994 (11)	1.900 (7)
Te4—O4ⁱⁱ	1.8994 (11)	1.900 (7)
Notes: (a) this study; (b) Champarnaud-Mesjard et al. (2001); single-crystal data with a = 19.522 (4), b = 7.121 (1) and c = 18.813 (4) Å. Symmetry codes: (i) x, −y + 1, −z + 1; (ii) x, y, −z + $[{1\over 2}]$ ; (iii) −x + $[{1\over 2}]$ , y + $[{1\over 2}]$ , z; (iv) −x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , z; (v) −x + $[{1\over 2}]$ , y + $[{1\over 2}]$ , −z + $[{1\over 2}]$ ; (vi) −x + $[{1\over 2}]$ , y − $[{1\over 2}]$ , −z + $[{1\over 2}]$ ; (vii) x, y − 1, z; (viii) −x + 1, y − 1, −z + $[{1\over 2}]$ ; (ix) −x + 1, y, z; (x) x, y + 1, z.

Figure 1
(a) The [Ca1O₈] and (b) the [Ca1O₉] polyhedra in the crystal structure of Ca₂Te₃O₈. Displacement ellipsoids are drawn at the 90% probability level. Symmetry codes refer to Table 1

All four Te atoms have an oxidation state of +IV and can be divided into two pairs with the most commonly observed three-coordination in the form of a trigonal pyramid (Te2 and Te4) and four-coordination in the form of a bisphenoid (Te1 and Te3). The Te—O bond lengths within the [TeO₃] trigonal pyramids are only slightly spread, ranging from 1.8522 (12) to 1.8994 (11) Å. The two [TeO₄] bisphenoids are characterised by two short bonds of < 2 Å and two longer bonds of > 2 Å, with the maximum at 2.3222 (12) Å for Te3. All Te—O bond lengths (Table 1) are in characteristic ranges for oxotellurates(IV) with three- and four-coordinate tellurium, as reviewed recently by Christy et al. (2016 ).

Bond valence sums for the four Te atoms computed with the parameters of Brese and O'Keeffe (1991 ) are 4.14, 4.07, 3.99 and 3.81 v.u., but are considerably lower when the revised parameters of Mills & Christy (2013 ) are used, i.e. 3.93, 3.80, 3.81 and 3.57 v.u.

The oxotellurium(IV) network is built up from two different anions, both with composition [Te₃O₈]⁴⁻. One anion is made up from an infinite zigzag chain that extends parallel to [001] and consists of a pair of corner-sharing [Te3O₄] bisphenoids linked alternately to a [Te4O₃] trigonal pyramid {= [(Te4O_1/1O_2/2)(Te3O_2/1O_2/2)₂]_n} (Fig. 2a). The second oxotellurate(IV) anion is finite and is situated between neighbouring chain anions. It is comprised of a curved [Te₃O₈]⁴⁻ unit with a central Te1O₄ bisphenoid linked to two [Te2O₃] trigonal pyramids {= [(Te1O_2/1O_1/2)₂(Te2O_2/2O_2/1)]} (Fig. 2b).

Figure 2
(a) The chain [Te₃O₈]⁴⁻ anion and (b) the finite [Te₃O₈]⁴⁻ anion in the crystal structure of Ca₂Te₃O₈. Displacement ellipsoids are drawn at the 90% probability level. Symmetry codes refer to Table 1

In the crystal, the two types of [Te₃O₈]⁴⁻ anions are arranged in layers parallel to (100). Approximately at x ≃ 1/4 and 3/4, the calcium cations link adjacent layers into the three-dimensional framework (Fig. 3).

Figure 3
The crystal structure of Ca₂Te₃O₈ in a projection along [0 $[\overline{1}]$ 0]. Displacement ellipsoids and colour code as in Figs. 1

and 2

Ca₂Te₃O₈ is isotypic with Pb₂Te₃O₈ (Champarnaud-Mesjard et al., 2001), but not with its higher alkaline earth homologue Sr₂Te₃O₈, which is reported to have a different orthorhombic cell, with details of the structure not known (Elerman & Koçak, 1986 ). Comparison of the bond lengths of the [MO_x] (M = Ca, Pb) polyhedra and the [TeO₃] and [TeO₄] units in the isotypic structures of Ca₂Te₃O₈ and Pb₂Te₃O₈ (Table 1) reveals nearly identical values for the individual oxotellurate(IV) units, but differences up to 0.6 Å for the metal–oxygen polyhedra. On one hand, this behaviour is ascribed to the different ionic radii for eight-coordinate Ca^II and Pb^II of 1.12 and 1.29 Å, respectively (Shannon, 1976 ), and, on the other hand, to the stereochemical activity (Galy et al., 1975 ) of the 6s² free-electron lone pair located at Pb^II that is responsible for the formation of off-centred lead–oxygen polyhedra with either holo- or hemidirected oxygen ligands (Shimoni-Livny et al., 1998 ).

For a quantitative structural comparison of the isotypic M₂Te₃O₈ (M = Ca, Pb) structures, the program compstru (de la Flor et al., 2016 ), available at the Bilbao Crystallographic Server (Aroyo et al., 2006 ), was used. The degree of lattice distortion is 0.0205, the maximum distance between the atomic positions of paired atoms is 0.403 Å for pair O8, the arithmetic mean of all distances is 0.195 Å and the measure of similarity is 0.05.

3. Synthesis and crystallization

Crystals of Ca₂Te₃O₈ were obtained as one of the products from a flux synthesis using a CsCl/NaCl salt mixture (molar ratio 0.65/0.35). To 1.5 g of the salt mixture were added CaO (0.075 g; freshly prepared by heating CaCO₃ at 1473 K for 1 d) and TeO₂ (0.425 g) according to a molar ratio of 1:2. The reaction mixture was placed in a silica ampoule that was subsequently evacuated and sealed. The ampoule was placed vertically in a furnace and heated from room temperature within 3 h to 793 K, kept at that temperature for 90 h and cooled within 10 h to room temperature. The silica ampoule was broken and the solidified melt leached out with water for two h. The colourless product was filtered off, washed with water and was dried in a stream of air. The title compound was present in the form of a few crystals that were distinguishable from the other crystals due to their characteristic square form (maximum edge length 1.5 mm). Other phases identified by single-crystal X-ray diffraction measurements of selected crystals and by powder X-ray diffraction measurements of the bulk were CaTe₂O₅ in the mica-like modification reported by Redman et al. (1970) as the main phase (tiny colourless plates) and Ca₄Te₅O₁₄ (small colourless pinacoids; Weil, 2004).

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Starting coordinates for the refinement were taken from isotypic Pb₂Te₃O₈ (Champarnaud-Mesjard et al., 2001). Both remaining maximum and minimum electron-density peaks are located 0.64 and 0.30 Å from the Te2 site.

Table 2
Experimental details

Crystal data
Chemical formula	Ca₂Te₃O₈
M_r	590.95
Crystal system, space group	Orthorhombic, Cmcm
Temperature (K)	297
a, b, c (Å)	18.7368 (15), 6.8399 (6), 18.5652 (15)
V (Å³)	2379.3 (3)
Z	12
Radiation type	Mo Kα
μ (mm⁻¹)	12.27
Crystal size (mm)	0.25 × 0.15 × 0.10

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.472, 0.750
No. of measured, independent and observed [I > 2σ(I)] reflections	47523, 6097, 5325
R_int	0.043
(sin θ/λ)_max (Å⁻¹)	1.057

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.023, 0.043, 1.12
No. of reflections	6097
No. of parameters	101
Δρ_max, Δρ_min (e Å⁻³)	4.13, −2.08
Computer programs: APEX3 and SAINT (Bruker, 2016 ), SHELXL2017 (Sheldrick, 2015 ), ATOMS (Dowty, 2006 ) and publCIF (Westrip, 2010 ). Starting coordinates taken from an isotypic compound.

Supporting information

CCDC reference: 1883382

Crystal structure: contains datablocks I, general. DOI: https://doi.org/10.1107/S2056989018017310/pk2611sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989018017310/pk2611Isup2.hkl

checkCIF report

Computing details top

Data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: coordinates from isotypic compound; program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Dicalcium octaoxidotritellurate(IV) top

Crystal data top

Ca₂Te₃O₈	D_x = 4.949 Mg m⁻³
M_r = 590.95	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Cmcm	Cell parameters from 9248 reflections
a = 18.7368 (15) Å	θ = 3.2–48.6°
b = 6.8399 (6) Å	µ = 12.27 mm⁻¹
c = 18.5652 (15) Å	T = 297 K
V = 2379.3 (3) Å³	Plate, colourless
Z = 12	0.25 × 0.15 × 0.10 mm
F(000) = 3120

Data collection top

Bruker APEXII CCD diffractometer	5325 reflections with I > 2σ(I)
ω scans	R_int = 0.043
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θ_max = 48.7°, θ_min = 2.2°
T_min = 0.472, T_max = 0.750	h = −37→39
47523 measured reflections	k = −14→14
6097 independent reflections	l = −36→39

Refinement top

Refinement on F²	Primary atom site location: isomorphous structure methods
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.0102P)² + 6.3377P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.023	(Δ/σ)_max = 0.002
wR(F²) = 0.043	Δρ_max = 4.13 e Å⁻³
S = 1.12	Δρ_min = −2.08 e Å⁻³
6097 reflections	Extinction correction: SHELXL2017 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
101 parameters	Extinction coefficient: 0.00143 (3)
0 restraints

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq
Ca1	0.29994 (2)	0.36584 (4)	0.41210 (2)	0.00884 (4)
Ca2	0.19996 (2)	0.49754 (6)	0.250000	0.00819 (5)
Te1	0.500000	0.08952 (3)	0.250000	0.00866 (3)
Te2	0.500000	0.29743 (2)	0.07448 (2)	0.00865 (2)
Te3	0.37898 (2)	0.84140 (2)	0.41362 (2)	0.00685 (2)
Te4	0.37986 (2)	0.54478 (2)	0.250000	0.00616 (2)
O1	0.500000	0.0728 (2)	0.13377 (9)	0.0150 (3)
O2	0.40826 (9)	0.000000	0.500000	0.0146 (3)
O3	0.31332 (6)	0.69421 (16)	0.46567 (6)	0.01057 (16)
O4	0.32340 (7)	0.67071 (18)	0.32118 (6)	0.01226 (17)
O5	0.31362 (6)	0.02866 (15)	0.38074 (6)	0.00918 (15)
O6	0.42263 (7)	0.42092 (19)	0.11674 (8)	0.0165 (2)
O7	0.42329 (9)	0.9223 (3)	0.250000	0.0135 (3)
O8	0.31736 (10)	0.3318 (3)	0.250000	0.0209 (4)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ca1	0.00841 (9)	0.00807 (9)	0.01005 (9)	0.00102 (7)	−0.00031 (7)	0.00024 (7)
Ca2	0.00857 (13)	0.00778 (12)	0.00821 (12)	0.00113 (10)	0.000	0.000
Te1	0.00648 (6)	0.00643 (6)	0.01305 (7)	0.000	0.000	0.000
Te2	0.00773 (4)	0.00848 (4)	0.00974 (4)	0.000	0.000	−0.00001 (3)
Te3	0.00550 (3)	0.00710 (3)	0.00794 (3)	0.00067 (2)	0.00072 (2)	0.00095 (2)
Te4	0.00569 (4)	0.00572 (4)	0.00706 (4)	0.00012 (3)	0.000	0.000
O1	0.0227 (8)	0.0108 (6)	0.0116 (6)	0.000	0.000	0.0018 (5)
O2	0.0110 (6)	0.0209 (7)	0.0119 (6)	0.000	0.000	−0.0089 (5)
O3	0.0123 (4)	0.0103 (4)	0.0091 (4)	−0.0023 (3)	0.0028 (3)	0.0010 (3)
O4	0.0122 (4)	0.0157 (4)	0.0088 (4)	0.0031 (3)	0.0022 (3)	−0.0022 (3)
O5	0.0096 (4)	0.0072 (3)	0.0107 (4)	0.0021 (3)	−0.0006 (3)	0.0000 (3)
O6	0.0091 (4)	0.0175 (5)	0.0228 (6)	0.0037 (4)	0.0000 (4)	−0.0049 (4)
O7	0.0072 (5)	0.0142 (6)	0.0193 (7)	−0.0022 (5)	0.000	0.000
O8	0.0099 (7)	0.0077 (6)	0.0451 (12)	−0.0030 (5)	0.000	0.000

Geometric parameters (Å, º) top

Ca1—O3ⁱ	2.3198 (11)	Ca2—O4	2.9154 (13)
Ca1—O6ⁱⁱ	2.3903 (13)	Ca2—O4ⁱⁱ	2.9154 (13)
Ca1—O5	2.3924 (11)	Ca2—Te4	3.3863 (5)
Ca1—O3	2.4691 (12)	Ca2—Te4^vi	3.4390 (5)
Ca1—O5ⁱⁱⁱ	2.4712 (11)	Ca2—Te3^vi	3.5434 (3)
Ca1—O3^iv	2.6212 (12)	Te1—O7^vii	1.8369 (17)
Ca1—O4	2.7186 (13)	Te1—O7^viii	1.8369 (17)
Ca1—O8	3.0360 (4)	Te1—O1	2.1608 (17)
Ca1—Te3^iv	3.3567 (4)	Te1—O1ⁱⁱ	2.1609 (17)
Ca1—Te3	3.5742 (4)	Te2—O6^ix	1.8522 (12)
Ca1—Te4	3.5773 (4)	Te2—O6	1.8522 (12)
Ca1—Ca2	3.6575 (4)	Te2—O1	1.8902 (17)
Ca2—O8^v	2.3089 (18)	Te3—O3	1.8602 (11)
Ca2—O7^vi	2.3660 (17)	Te3—O5^x	1.8743 (10)
Ca2—O5^v	2.4497 (11)	Te3—O2^x	2.0123 (5)
Ca2—O5ⁱⁱⁱ	2.4497 (11)	Te3—O4	2.3222 (12)
Ca2—O8	2.4749 (19)	Te4—O8	1.8694 (17)
Ca2—O4^vi	2.6335 (13)	Te4—O4	1.8994 (11)
Ca2—O4^iv	2.6335 (13)	Te4—O4ⁱⁱ	1.8994 (11)

O3ⁱ—Ca1—O6ⁱⁱ	98.22 (5)	O8^v—Ca2—Te3^vi	103.87 (2)
O3ⁱ—Ca1—O5	93.20 (4)	O7^vi—Ca2—Te3^vi	61.771 (12)
O6ⁱⁱ—Ca1—O5	89.68 (4)	O5^v—Ca2—Te3^vi	29.94 (2)
O3ⁱ—Ca1—O3	75.88 (4)	O5ⁱⁱⁱ—Ca2—Te3^vi	146.33 (3)
O6ⁱⁱ—Ca1—O3	81.32 (4)	O8—Ca2—Te3^vi	103.47 (2)
O5—Ca1—O3	164.59 (4)	O4^vi—Ca2—Te3^vi	40.94 (3)
O3ⁱ—Ca1—O5ⁱⁱⁱ	113.79 (4)	O4^iv—Ca2—Te3^vi	96.03 (3)
O6ⁱⁱ—Ca1—O5ⁱⁱⁱ	134.77 (4)	O4—Ca2—Te3^vi	147.38 (2)
O5—Ca1—O5ⁱⁱⁱ	117.99 (4)	O4ⁱⁱ—Ca2—Te3^vi	93.73 (2)
O3—Ca1—O5ⁱⁱⁱ	76.84 (4)	Te4—Ca2—Te3^vi	116.377 (7)
O3ⁱ—Ca1—O3^iv	68.72 (4)	Te4^vi—Ca2—Te3^vi	63.068 (6)
O6ⁱⁱ—Ca1—O3^iv	159.11 (4)	O7^vii—Te1—O7^viii	102.97 (11)
O5—Ca1—O3^iv	75.37 (4)	O7^vii—Te1—O1	88.11 (3)
O3—Ca1—O3^iv	109.69 (4)	O7^viii—Te1—O1	88.11 (3)
O5ⁱⁱⁱ—Ca1—O3^iv	66.05 (4)	O7^vii—Te1—O1ⁱⁱ	88.11 (3)
O3ⁱ—Ca1—O4	136.52 (4)	O7^viii—Te1—O1ⁱⁱ	88.11 (3)
O6ⁱⁱ—Ca1—O4	65.45 (4)	O1—Te1—O1ⁱⁱ	173.92 (9)
O5—Ca1—O4	124.82 (4)	O7^vii—Te1—Ca2^vi	28.98 (5)
O3—Ca1—O4	62.35 (4)	O7^viii—Te1—Ca2^vi	131.95 (6)
O5ⁱⁱⁱ—Ca1—O4	69.34 (4)	O1—Te1—Ca2^vi	89.497 (8)
O3^iv—Ca1—O4	135.24 (4)	O1ⁱⁱ—Te1—Ca2^vi	89.497 (8)
O3ⁱ—Ca1—O8	160.81 (5)	O7^vii—Te1—Ca2^xi	131.95 (6)
O6ⁱⁱ—Ca1—O8	71.74 (5)	O7^viii—Te1—Ca2^xi	28.98 (5)
O5—Ca1—O8	70.93 (4)	O1—Te1—Ca2^xi	89.497 (8)
O3—Ca1—O8	117.27 (4)	O1ⁱⁱ—Te1—Ca2^xi	89.497 (8)
O5ⁱⁱⁱ—Ca1—O8	83.89 (4)	Ca2^vi—Te1—Ca2^xi	160.934 (13)
O3^iv—Ca1—O8	115.39 (4)	O6^ix—Te2—O6	103.01 (8)
O4—Ca1—O8	54.98 (4)	O6^ix—Te2—O1	97.12 (6)
O3ⁱ—Ca1—Te3^iv	95.22 (3)	O6—Te2—O1	97.12 (6)
O6ⁱⁱ—Ca1—Te3^iv	165.99 (3)	O6^ix—Te2—Ca1^xii	30.77 (4)
O5—Ca1—Te3^iv	93.50 (3)	O6—Te2—Ca1^xii	133.69 (4)
O3—Ca1—Te3^iv	98.24 (3)	O1—Te2—Ca1^xii	93.563 (8)
O5ⁱⁱⁱ—Ca1—Te3^iv	33.33 (2)	O6^ix—Te2—Ca1ⁱⁱ	133.69 (4)
O3^iv—Ca1—Te3^iv	33.48 (2)	O6—Te2—Ca1ⁱⁱ	30.77 (4)
O4—Ca1—Te3^iv	101.83 (3)	O1—Te2—Ca1ⁱⁱ	93.563 (8)
O8—Ca1—Te3^iv	96.42 (4)	Ca1^xii—Te2—Ca1ⁱⁱ	163.902 (10)
O3ⁱ—Ca1—Te3	96.23 (3)	O3—Te3—O5^x	96.14 (5)
O6ⁱⁱ—Ca1—Te3	57.30 (3)	O3—Te3—O2^x	93.33 (5)
O5—Ca1—Te3	146.60 (3)	O5^x—Te3—O2^x	93.97 (5)
O3—Ca1—Te3	29.18 (3)	O3—Te3—O4	79.35 (5)
O5ⁱⁱⁱ—Ca1—Te3	87.05 (3)	O5^x—Te3—O4	79.04 (5)
O3^iv—Ca1—Te3	137.74 (3)	O2^x—Te3—O4	169.18 (6)
O4—Ca1—Te3	40.52 (3)	O3—Te3—Ca1ⁱⁱⁱ	51.01 (4)
O8—Ca1—Te3	91.90 (3)	O5^x—Te3—Ca1ⁱⁱⁱ	46.42 (3)
Te3^iv—Ca1—Te3	117.328 (9)	O2^x—Te3—Ca1ⁱⁱⁱ	104.60 (5)
O3ⁱ—Ca1—Te4	146.96 (3)	O4—Te3—Ca1ⁱⁱⁱ	64.59 (3)
O6ⁱⁱ—Ca1—Te4	49.84 (3)	O3—Te3—Ca2^v	109.41 (4)
O5—Ca1—Te4	94.60 (3)	O5^x—Te3—Ca2^v	40.72 (3)
O3—Ca1—Te4	89.14 (3)	O2^x—Te3—Ca2^v	129.382 (18)
O5ⁱⁱⁱ—Ca1—Te4	90.46 (3)	O4—Te3—Ca2^v	47.99 (3)
O3^iv—Ca1—Te4	144.22 (3)	Ca1ⁱⁱⁱ—Te3—Ca2^v	63.955 (8)
O4—Ca1—Te4	31.52 (2)	O3—Te3—Ca1	40.32 (4)
O8—Ca1—Te4	31.50 (3)	O5^x—Te3—Ca1	110.40 (3)
Te3^iv—Ca1—Te4	116.244 (9)	O2^x—Te3—Ca1	127.580 (17)
Te3—Ca1—Te4	61.433 (6)	O4—Te3—Ca1	49.52 (3)
O3ⁱ—Ca1—Ca2	154.68 (3)	Ca1ⁱⁱⁱ—Te3—Ca1	68.372 (6)
O6ⁱⁱ—Ca1—Ca2	105.63 (3)	Ca2^v—Te3—Ca1	95.427 (9)
O5—Ca1—Ca2	95.28 (3)	O3—Te3—Ca1ⁱ	26.53 (4)
O3—Ca1—Ca2	99.17 (3)	O5^x—Te3—Ca1ⁱ	105.99 (3)
O5ⁱⁱⁱ—Ca1—Ca2	41.77 (3)	O2^x—Te3—Ca1ⁱ	68.39 (3)
O3^iv—Ca1—Ca2	90.45 (3)	O4—Te3—Ca1ⁱ	105.36 (3)
O4—Ca1—Ca2	51.91 (3)	Ca1ⁱⁱⁱ—Te3—Ca1ⁱ	68.861 (8)
O8—Ca1—Ca2	42.13 (4)	Ca2^v—Te3—Ca1ⁱ	132.402 (9)
Te3^iv—Ca1—Ca2	60.505 (8)	Ca1—Te3—Ca1ⁱ	60.638 (8)
Te3—Ca1—Ca2	89.690 (9)	O8—Te4—O4	90.26 (6)
Te4—Ca1—Ca2	55.803 (9)	O8—Te4—O4ⁱⁱ	90.26 (6)
O8^v—Ca2—O7^vi	94.49 (6)	O4—Te4—O4ⁱⁱ	88.18 (7)
O8^v—Ca2—O5^v	84.22 (3)	O8—Te4—Ca2	45.74 (6)
O7^vi—Ca2—O5^v	85.27 (3)	O4—Te4—Ca2	59.26 (4)
O8^v—Ca2—O5ⁱⁱⁱ	84.22 (3)	O4ⁱⁱ—Te4—Ca2	59.26 (4)
O7^vi—Ca2—O5ⁱⁱⁱ	85.27 (3)	O8—Te4—Ca2^v	115.43 (6)
O5^v—Ca2—O5ⁱⁱⁱ	164.44 (5)	O4—Te4—Ca2^v	49.41 (4)
O8^v—Ca2—O8	125.35 (5)	O4ⁱⁱ—Te4—Ca2^v	49.42 (4)
O7^vi—Ca2—O8	140.16 (6)	Ca2—Te4—Ca2^v	69.699 (8)
O5^v—Ca2—O8	97.59 (3)	O8—Te4—Ca1	58.067 (9)
O5ⁱⁱⁱ—Ca2—O8	97.59 (3)	O4—Te4—Ca1	48.45 (4)
O8^v—Ca2—O4^vi	144.79 (4)	O4ⁱⁱ—Te4—Ca1	120.49 (4)
O7^vi—Ca2—O4^vi	69.70 (5)	Ca2—Te4—Ca1	63.297 (6)
O5^v—Ca2—O4^vi	63.85 (3)	Ca2^v—Te4—Ca1	97.242 (7)
O5ⁱⁱⁱ—Ca2—O4^vi	123.62 (4)	O8—Te4—Ca1ⁱⁱ	58.067 (9)
O8—Ca2—O4^vi	76.04 (5)	O4—Te4—Ca1ⁱⁱ	120.49 (4)
O8^v—Ca2—O4^iv	144.79 (4)	O4ⁱⁱ—Te4—Ca1ⁱⁱ	48.45 (4)
O7^vi—Ca2—O4^iv	69.70 (5)	Ca2—Te4—Ca1ⁱⁱ	63.299 (6)
O5^v—Ca2—O4^iv	123.62 (4)	Ca2^v—Te4—Ca1ⁱⁱ	97.242 (7)
O5ⁱⁱⁱ—Ca2—O4^iv	63.85 (3)	Ca1—Te4—Ca1ⁱⁱ	114.545 (11)
O8—Ca2—O4^iv	76.04 (5)	Te2—O1—Te1	122.58 (9)
O4^vi—Ca2—O4^iv	60.24 (5)	Te3^vii—O2—Te3ⁱ	148.36 (9)
O8^v—Ca2—O4	73.10 (5)	Te3—O3—Ca1ⁱ	132.49 (6)
O7^vi—Ca2—O4	149.63 (3)	Te3—O3—Ca1	110.51 (5)
O5^v—Ca2—O4	119.75 (4)	Ca1ⁱ—O3—Ca1	102.82 (4)
O5ⁱⁱⁱ—Ca2—O4	66.29 (3)	Te3—O3—Ca1ⁱⁱⁱ	95.51 (5)
O8—Ca2—O4	58.73 (4)	Ca1ⁱ—O3—Ca1ⁱⁱⁱ	111.28 (4)
O4^vi—Ca2—O4	134.77 (3)	Ca1—O3—Ca1ⁱⁱⁱ	99.92 (4)
O4^iv—Ca2—O4	104.43 (4)	Te4—O4—Te3	119.50 (6)
O8^v—Ca2—O4ⁱⁱ	73.10 (5)	Te4—O4—Ca2^v	97.37 (5)
O7^vi—Ca2—O4ⁱⁱ	149.63 (3)	Te3—O4—Ca2^v	91.07 (4)
O5^v—Ca2—O4ⁱⁱ	66.29 (3)	Te4—O4—Ca1	100.03 (5)
O5ⁱⁱⁱ—Ca2—O4ⁱⁱ	119.75 (4)	Te3—O4—Ca1	89.96 (4)
O8—Ca2—O4ⁱⁱ	58.73 (4)	Ca2^v—O4—Ca1	159.36 (5)
O4^vi—Ca2—O4ⁱⁱ	104.43 (4)	Te4—O4—Ca2	86.69 (4)
O4^iv—Ca2—O4ⁱⁱ	134.77 (3)	Te3—O4—Ca2	153.51 (5)
O4—Ca2—O4ⁱⁱ	53.91 (5)	Ca2^v—O4—Ca2	89.17 (4)
O8^v—Ca2—Te4	92.60 (5)	Ca1—O4—Ca2	80.88 (3)
O7^vi—Ca2—Te4	172.91 (4)	Te3^vii—O5—Ca1	130.51 (5)
O5^v—Ca2—Te4	95.46 (3)	Te3^vii—O5—Ca2^vi	109.34 (5)
O5ⁱⁱⁱ—Ca2—Te4	95.46 (3)	Ca1—O5—Ca2^vi	108.29 (4)
O8—Ca2—Te4	32.75 (4)	Te3^vii—O5—Ca1^iv	100.25 (5)
O4^vi—Ca2—Te4	104.27 (3)	Ca1—O5—Ca1^iv	106.55 (4)
O4^iv—Ca2—Te4	104.27 (3)	Ca2^vi—O5—Ca1^iv	96.02 (4)
O4—Ca2—Te4	34.05 (2)	Te2—O6—Ca1ⁱⁱ	125.87 (7)
O4ⁱⁱ—Ca2—Te4	34.05 (2)	Te1^x—O7—Ca2^v	128.92 (9)
O8^v—Ca2—Te4^vi	146.15 (5)	Te4—O8—Ca2^vi	149.29 (10)
O7^vi—Ca2—Te4^vi	51.66 (4)	Te4—O8—Ca2	101.52 (8)
O5^v—Ca2—Te4^vi	91.90 (3)	Ca2^vi—O8—Ca2	109.19 (7)
O5ⁱⁱⁱ—Ca2—Te4^vi	91.90 (3)	Te4—O8—Ca1	90.43 (3)
O8—Ca2—Te4^vi	88.51 (4)	Ca2^vi—O8—Ca1	93.49 (3)
O4^vi—Ca2—Te4^vi	33.21 (2)	Ca2—O8—Ca1	82.49 (4)
O4^iv—Ca2—Te4^vi	33.21 (2)	Te4—O8—Ca1ⁱⁱ	90.43 (3)
O4—Ca2—Te4^vi	135.31 (3)	Ca2^vi—O8—Ca1ⁱⁱ	93.49 (3)
O4ⁱⁱ—Ca2—Te4^vi	135.31 (3)	Ca2—O8—Ca1ⁱⁱ	82.49 (4)
Te4—Ca2—Te4^vi	121.252 (12)	Ca1—O8—Ca1ⁱⁱ	164.82 (7)

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

Comparison of bond lengths (Å) in Ca₂Te₃O₈ and isotypic Pb₂Te₃O₈ top

	Ca₂Te₃O₈^(a)	Pb₂Te₃O₈^(b)
M1—O3ⁱ	2.3198 (11)	2.372 (8)
M1—O6ⁱⁱ	2.3903 (13)	2.440 (8)
M1—O5	2.3924 (11)	2.470 (6)
M1—O3	2.4691 (12)	2.636 (8)
M1—O5ⁱⁱⁱ	2.4712 (11)	2.934 (8)
M1—O3^iv	2.6212 (12)	3.032 (8)
M1—O4	2.7186 (13)
M1—O8	3.0360 (4)	3.069 (2)
M2—O8^v	2.3089 (18)	2.439 (9)
M2—O7^vi	2.3660 (17)	2.374 (10)
M2—O5^v	2.4497 (11)	2.556 (6)
M2—O5ⁱⁱⁱ	2.4497 (11)	2.556 (6)
M2—O8	2.4749 (19)	3.080 (11)
M2—O4^vi	2.6335 (13)	2.732 (6)
M2—O4^iv	2.6335 (13)	2.732 (6)
M2—O4	2.9154 (13)	3.342 (7)
M2—O4ⁱⁱ	2.9154 (13)	3.342 (7)
Te1—O7^vii	1.8369 (17)	1.852 (10)
Te1—O7^viii	1.8369 (17)	1.852 (10)
Te1—O1	2.1608 (17)	2.160 (9)
Te1—O1ⁱⁱ	2.1608 (17)	2.160 (9)
Te2—O6^ix	1.8522 (12)	1.859 (8)
Te2—O6	1.8522 (12)	1.859 (8)
Te2—O1	1.8902 (17)	1.883 (10)
Te3—O3	1.8602 (11)	1.868 (8)
Te3—O5^x	1.8743 (10)	1.856 (7)
Te3—O2^x	2.0123 (5)	2.008 (3)
Te3—O4	2.3222 (12)	2.338 (6)
Te4—O8	1.8694 (17)	1.857 (10)
Te4—O4	1.8994 (11)	1.900 (7)
Te4—O4ⁱⁱ	1.8994 (11)	1.900 (7)

Notes: (a) this study; (b) Champarnaud-Mesjard et al. (2001); single-crystal data with a = 19.522 (4), b = 7.121 (1) and c = 18.813 (4) Å.

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

Acknowledgements

The X-ray centre of the TU Wien is acknowledged for financial support and for providing access to the single-crystal and powder X-ray diffractometers.

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