The crystal structure of a new CdTe2O5 polymorph, isotypic with ∊-CaTe2O5

Eder, F.; Weil, M.

doi:10.1107/S2056989020006283

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Volume 76| Part 6| June 2020| Pages 831-834

https://doi.org/10.1107/S2056989020006283

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The crystal structure of a new CdTe₂O₅ polymorph, isotypic with ∊-CaTe₂O₅

Felix Eder ^a ^* and 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: felix.eder@tuwien.ac.at

Edited by W. T. A. Harrison, University of Aberdeen, Scotland (Received 7 May 2020; accepted 8 May 2020; online 15 May 2020)

Single crystals of cadmium pentaoxidoditellurate(IV), CdTe₂O₅, were obtained as by-products in a hydrothermal reaction of Cd(NO₃)₂·4H₂O, TeO₂, H₆TeO₆ and NH₃ (molar ratios 2:1:1:6) at 483 K for seven days. The crystals represent a different polymorph (henceforth referred to as the β-form) than the α-CdTe₂O₅ crystals grown from the melt, and are isotypic with hydrothermally grown ∊-CaTe₂O₅. The asymmetric unit of β-CdTe₂O₅ comprises one Cd, two Te and five O sites, all of which are located in general positions (Wyckoff position 4 e). The cadmium(II) atom is coordinated by seven oxygen atoms, forming ²_∞[CdO_6/2O_1/1] (100) layers. Both tellurium sites are surrounded by four oxygen atoms with one of them being at a significantly longer distance than the other three. The resulting bisphenoidal [TeO₄] units also form layers propagating parallel to (100) by sharing edges with each other. The stereochemically active 5s² lone pair of the Te^IV atoms leads to the formation of large channels extending along [011] and smaller ones along [010]. A quantitative comparison between the crystal structures of β-CdTe₂O₅ and ∊-CaTe₂O₅ is made.

Keywords: crystal structure; oxidotellurate(IV); cadmium tellurite; isotypism; structure comparison.

CCDC reference: 2002758

1. Chemical context

Cadmium pentaoxidoditellurate(IV), better known under its common name cadmium ditellurite, CdTe₂O₅, has been the subject of numerous investigations during the past decades with different emphases. In this regard, the CdO–TeO₂ phase diagram was elucidated by Robertson et al. (1978 ), or the formation of glasses in the Cd–Te–O system by Karaduman et al. (2012 ). Other studies focused on electric and ferroelastic properties of CdTe₂O₅ (Redman et al., 1970 ), with its ferroelasticity remaining up to the melting point (Sadovskaya et al., 1983 ; Gorbenko et al., 1990 ). Single crystals of CdTe₂O₅ are usually grown from the melt utilizing the Czochralski or Bridgeman techniques as crystal growth methods (Nawash, 2015 ). Even though single crystals of CdTe₂O₅ have been grown for decades this way, a satisfactory structure model for this phase had never been published so far, and only lattice parameters of a sub-cell were given (Redman et al., 1970). Other phases in the Cd–Te–O system that are compiled in the Inorganic Crystal Structure Database (ICSD; Zagorac et al., 2019 ) include two polymorphs of CdTe^IVO₃ (Krämer & Brandt, 1985 ; Poupon et al., 2017 ), two polymorphs of Cd₃Te^VIO₆ (Burckhardt et al., 1982 ; Weil & Veyer, 2018 ) and the mixed Te^IV/VI-compounds Cd₂Te₂O₇ and Cd₂Te₃O₉ (Weil, 2004 ).

The lack of a reasonable structure model for CdTe₂O₅ might be caused by the micaceous appearance of the grown crystals (Redman et al., 1970), as well as by its ferroelastic properties, which often are correlated with the formation of twins or multiple domains. The new CdTe₂O₅ phase discovered during the present study originally intended to synthesize new mixed-valent cadmium oxidotellurates(IV,VI), however, belongs to a different polymorph, hereafter referred to as the β-form of CdTe₂O₅.

In this communication we report on the synthesis and crystal structure analysis of β-CdTe₂O₅ and compare it quantitatively with the isotypic structure of ∊-CaTe₂O₅ (Weil & Stöger, 2008 ; Barrier et al., 2009 ).

2. Structural commentary

All atoms in the asymmetric unit, viz. one Cd site, two Te sites and five O sites, are located on general Wyckoff positions 4 e (site symmetry 1). The cadmium atom is coordinated by seven oxygen atoms with distances in a range of 2.235 (3)–2.688 (3) Å. The average Cd—O bond length is 2.389 Å, which is in accordance with the sum of ionic radii for Cd^II (CN 7; 1.17 Å) and O (CN 3; 1.22 Å) compiled by Shannon (1976 ). The bond-valence sum (BVS; Brown, 2002 ) of Cd is 2.07 valence units (v.u.) using the values of Brese & O'Keeffe (1991 ) for calculation. The [CdO₇] polyhedron is best described as a distorted pentagonal bipyramid (Fig. 1). The [CdO₇] polyhedra are connected to each other by sharing three edges with other [CdO₇] units, thereby forming ²_∞[CdO_6/2O_1/1] layers oriented parallel to (100) with the Cd^II atoms located at x ≃ 0;1.

Figure 1
The [CdO₇] polyhedron in the crystal structure of β-CdTe₂O₅. Displacement ellipsoids are drawn at the 90% probability level. Symmetry codes refer to Table 1

The two tellurium(IV) atoms are both coordinated by four oxygen atoms with three of them being closer than 2 Å (Table 1) and the fourth one at a distance of 2.285 (3) Å (Te1) and 2.204 (3) Å (Te2), respectively. The oxygen atoms are located to one side of the Te^IV atoms due to the large amount of space the 5s² electron lone pair requires. The corresponding coordination polyhedra can be derived from a trigonal bipyramid, [ΨTeO₄], with the lone pair occupying one of the equatorial positions in each case. The shapes of the polyhedra without the contribution of the lone pair correspond to [TeO₄] bisphenoids (Fig. 2). The bond-valence sums for the tellurium(IV) atoms were calculated to be 4.15 and 4.10 v.u. for Te1 and Te2, respectively, using the values of Brese & O'Keeffe (1991). When applying the revised parameters of Mills & Christy (2013 ), BVS of 3.94 and 3.92 v.u. were obtained.

Table 1
Comparison of Te—O and M—O (M = Cd, Ca) bond lengths (Å) in the isotypic β-CdTe₂O₅ and ∊-CaTe₂O₅ structures

	β-CdTe₂O₅	∊-CaTe₂O₅^a
Te1—O2	1.843 (3)	1.832 (4)
Te1—O1	1.875 (3)	1.852 (4)
Te1—O3	1.991 (3)	1.980 (5)
Te1—O4	2.285 (3)	2.450 (5)
Te2—O4ⁱ	1.864 (3)	1.854 (4)
Te2—O5	1.897 (3)	1.898 (4)
Te2—O3	1.990 (3)	2.009 (5)
Te2—O5ⁱⁱ	2.204 (5)	2.178 (5)
M1—O1ⁱⁱⁱ	2.235 (3)	2.305 (4)
M1—O2^iv	2.238 (3)	2.326 (4)
M1—O1	2.256 (3)	2.360 (5)
M1—O2^v	2.296 (3)	2.358 (5)
M1—O3	2.424 (3)	2.476 (5)
M1—O4^v	2.589 (3)	2.554 (5)
M1—O4ⁱⁱⁱ	2.688 (3)	2.682 (5)
(a) Lattice parameters: a = 9.382 (2), b = 5.7095 (14), c = 11.132 (3) Å, β = 115.109 (4)°, V = 539.95 Å³ (Weil & Stöger, 2008). [Symmetry codes: (i) x, −y + $[{1\over 2}]$ , z − $[{1\over 2}]$ ; (ii) −x + 1, −y + 1, −z + 1; (iii) −x, y + $[{1\over 2}]$ , −z + 1; (iv) −x, −y + 1, −z + 1; (v) x, y + 1, z.]

Figure 2
The [TeO₄] polyhedra in the crystal structure of β-CdTe₂O₅. Displacement ellipsoids are drawn at the 90% probability level. Symmetry codes refer to Table 1

The [TeO₄] polyhedra are connected to each other to form layers oriented parallel to (100). These layers have a distinct undulating shape (Fig. 3) and are built up by chains of [Te1O₄] and [Te2O₄] units arranged alternately by sharing corners with two neighbours. These chains are cross-linked by two [Te2O₄]-polyhedra by sharing an edge consisting of two O5 atoms. The [Te1O₄] units are located very close to the Cd–O layer and share edges with three [CdO₇] polyhedra and one corner with a fourth one. The [Te2O₄] units are positioned in the centre of the layer and only share two corners with three [CdO₇] polyhedra. The rather loose arrangement of [TeO₄] units in the layer can be explained by the stereochemically active 5s² electron lone pair situated at each of the two Te^IV atoms. The space requirements of the non-bonding electron pairs lead to the undulating shape of the layer, which results in the presence of large channels in the structure, which are oriented parallel to [011] (Fig. 4). Smaller channels are also realized and propagate parallel to [010] (Fig. 5).

Figure 3
The crystal structure of β-CdTe₂O₅ in a projection along [001]. Displacement ellipsoids are drawn at the 90% probability level.

Figure 4
Large channels in the β-CdTe₂O₅ structure running parallel to [011]. Displacement ellipsoids are drawn at the 90% probability level.

Figure 5
Smaller channels in the β-CdTe₂O₅ structure running parallel to [010]. Displacement ellipsoids are drawn at the 90% probability level.

The arrangement of such an undulating ²_∞[Te₂O₅]^2– layer was reported for the first time for the ∊-polymorph of CaTe₂O₅ (Weil & Stöger, 2008), which is isotypic with β-CdTe₂O₅. CaTe₂O₅ is likewise reported to crystallize in a mica-like form from the melt (Redman et al., 1970). Although several high-temperature polymorphs have also been reported for this phase (Tripathi et al., 2001 ), ∊-CaTe₂O₅ is the only polymorph for which a crystal-structure determination has been performed (Weil & Stöger, 2008; Barrier et al., 2009). The close similarity between the two structures can be explained by the very similar ionic radii (Shannon, 1976) of Ca (CN 7: 1.20 Å) and Cd (CN 7: 1.17 Å). The corresponding bond lengths in the isotypic structures (Table 1) differ only slightly with one exception: the Te1—O4 bond, which is 0.165 Å longer in the Ca structure, shows by far the biggest difference. A quantitative comparison between β-CdTe₂O₅ and ∊-CaTe₂O₅ was carried out using the compstru software (de la Flor et al., 2016 ), available at the Bilbao Crystallographic Server (Aroyo et al., 2006 ). The absolute distances between paired atoms are 0.0303 Å for Cd/Ca1, 0.0628 Å for Te1, 0.0178 Å for Te2, 0.1426 Å for O1, 0.0791 Å for O2, 0.0384 Å for O3, 0.0788 Å for O4 and 0.0635 Å for O5. The degree of lattice distortion is 0.0118, the arithmetic mean of the distance between paired atoms is 0.0642 Å, and the measure of similarity is 0.077.

3. Synthesis and crystallization

Crystals of CdTe₂O₅ were obtained under hydrothermal conditions. The reactants, 0.1890 g (0.613 mmol) Cd(NO₃)₃·2H₂O, 0.0484 g (0.303 mmol) TeO₂, 0.0710 g (0.309 mmol) H₆TeO₆ and 0.12 g (1.8 mmol) 25%_wt NH_3(aq) were weighed into a small teflon vessel with a volume of ca 3 ml. Deionized water was added until the vessel was filled to about two thirds of its volume. Then the vessel was heated to 483 K in a steel autoclave for 7 d under autogenous pressure. Afterwards, the autoclave was cooled to room temperature within about 4 h. The reaction product was a light-yellow, almost white solid. In the X-ray powder pattern of the bulk, α-Cd₃TeO₆ (Burckhardt et al., 1982) and β-CdTe₂O₅ were found. Under a polarising microscope a few small shiny colourless blocks of β-CdTe₂O₅ were isolated for single-crystal measurements.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Atom labels and starting coordinates for refinement were adopted from the isotypic ∊-CaTe₂O₅ structure (Weil & Stöger, 2008).

Table 2
Experimental details

Crystal data
Chemical formula	CdTe₂O₅
M_r	447.60
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	9.4535 (5), 5.5806 (3), 10.8607 (5)
β (°)	114.430 (1)
V (Å³)	521.67 (5)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	15.08
Crystal size (mm)	0.10 × 0.06 × 0.05

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.600, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	9236, 1827, 1462
R_int	0.045
(sin θ/λ)_max (Å⁻¹)	0.747

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.024, 0.045, 1.00
No. of reflections	1827
No. of parameters	73
Δρ_max, Δρ_min (e Å⁻³)	1.20, −1.04
Computer programs: APEX3 (Bruker, 2016 ), SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a ), SHELXL (Sheldrick, 2015b ), ATOMS (Dowty, 2006 ), publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 2002758

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989020006283/hb7913sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989020006283/hb7913Isup2.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: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Cadmium pentaoxidoditellurate(IV) top

Crystal data top

CdTe₂O₅	F(000) = 768
M_r = 447.60	D_x = 5.699 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
a = 9.4535 (5) Å	Cell parameters from 2645 reflections
b = 5.5806 (3) Å	θ = 2.4–32.1°
c = 10.8607 (5) Å	µ = 15.08 mm⁻¹
β = 114.430 (1)°	T = 100 K
V = 521.67 (5) Å³	Block, colourless
Z = 4	0.10 × 0.06 × 0.05 mm

Data collection top

Bruker APEXII CCD diffractometer	1462 reflections with I > 2σ(I)
ω– and φ–scan	R_int = 0.045
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θ_max = 32.1°, θ_min = 3.8°
T_min = 0.600, T_max = 0.746	h = −14→14
9236 measured reflections	k = −8→8
1827 independent reflections	l = −15→16

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	Primary atom site location: isomorphous structure methods
R[F² > 2σ(F²)] = 0.024	w = 1/[σ²(F_o²) + (0.0171P)²] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.045	(Δ/σ)_max = 0.001
S = 1.00	Δρ_max = 1.20 e Å⁻³
1827 reflections	Δρ_min = −1.03 e Å⁻³
73 parameters

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
Te1	0.25911 (3)	0.32034 (5)	0.72297 (3)	0.01195 (7)
Te2	0.34935 (3)	0.64780 (5)	0.49078 (3)	0.01181 (7)
Cd1	0.01900 (4)	0.79762 (6)	0.63620 (3)	0.01416 (8)
O1	0.1293 (4)	0.4948 (5)	0.7821 (3)	0.0141 (6)
O2	0.1216 (4)	0.1359 (5)	0.5843 (3)	0.0188 (7)
O3	0.2239 (4)	0.5960 (5)	0.5973 (3)	0.0163 (7)
O4	0.2081 (4)	0.0287 (6)	0.8466 (3)	0.0162 (7)
O5	0.4791 (4)	0.3979 (6)	0.5962 (3)	0.0176 (7)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Te1	0.00997 (13)	0.01304 (14)	0.01191 (13)	0.00123 (10)	0.00360 (10)	−0.00069 (10)
Te2	0.01214 (13)	0.01187 (13)	0.01070 (13)	−0.00178 (10)	0.00399 (10)	−0.00077 (10)
Cd1	0.01824 (16)	0.01311 (16)	0.01254 (15)	0.00370 (12)	0.00778 (13)	0.00255 (12)
O1	0.0171 (16)	0.0124 (15)	0.0175 (15)	0.0035 (12)	0.0116 (13)	0.0054 (12)
O2	0.0256 (18)	0.0146 (16)	0.0109 (15)	−0.0055 (14)	0.0023 (13)	−0.0008 (13)
O3	0.0199 (17)	0.0142 (15)	0.0209 (16)	0.0035 (13)	0.0144 (14)	0.0032 (13)
O4	0.0162 (16)	0.0142 (15)	0.0159 (15)	0.0016 (13)	0.0044 (13)	0.0068 (13)
O5	0.0162 (16)	0.0198 (16)	0.0188 (16)	0.0060 (13)	0.0094 (13)	0.0072 (13)

Geometric parameters (Å, º) top

Te1—O2	1.843 (3)	Te2—Te2ⁱⁱ	3.2253 (6)
Te1—O1	1.875 (3)	Cd1—O1ⁱⁱⁱ	2.235 (3)
Te1—O3	1.991 (3)	Cd1—O2^iv	2.238 (3)
Te1—O4	2.285 (3)	Cd1—O2^v	2.296 (3)
Te2—O4ⁱ	1.864 (3)	Cd1—O1	2.255 (3)
Te2—O5	1.897 (3)	Cd1—O3	2.424 (3)
Te2—O3	1.990 (3)	Cd1—O4^v	2.589 (3)
Te2—O5ⁱⁱ	2.204 (3)	Cd1—O4ⁱⁱⁱ	2.688 (3)

O2—Te1—O1	103.25 (15)	O1ⁱⁱⁱ—Cd1—O3	167.39 (11)
O2—Te1—O3	90.59 (13)	O2^iv—Cd1—O3	93.09 (12)
O1—Te1—O3	83.46 (12)	O2^v—Cd1—O3	83.66 (11)
O2—Te1—O4	80.40 (12)	O1—Cd1—O3	66.67 (10)
O1—Te1—O4	80.89 (12)	O1ⁱⁱⁱ—Cd1—O4^v	73.85 (11)
O3—Te1—O4	159.60 (12)	O2^iv—Cd1—O4^v	138.57 (10)
O4ⁱ—Te2—O5	100.25 (14)	O2^v—Cd1—O4^v	66.40 (10)
O4ⁱ—Te2—O3	91.11 (13)	O1—Cd1—O4^v	78.65 (10)
O5—Te2—O3	86.24 (13)	O3—Cd1—O4^v	94.31 (10)
O4ⁱ—Te2—O5ⁱⁱ	88.71 (13)	Te1—O1—Cd1^vi	119.15 (14)
O5—Te2—O5ⁱⁱ	76.52 (13)	Te1—O1—Cd1	109.06 (13)
O3—Te2—O5ⁱⁱ	162.43 (12)	Cd1^vi—O1—Cd1	117.64 (13)
O4ⁱ—Te2—Te2ⁱⁱ	95.12 (10)	Te1—O2—Cd1^iv	133.44 (16)
O5—Te2—Te2ⁱⁱ	41.63 (9)	Te1—O2—Cd1^vii	119.05 (14)
O3—Te2—Te2ⁱⁱ	127.79 (9)	Cd1^iv—O2—Cd1^vii	105.95 (12)
O5ⁱⁱ—Te2—Te2ⁱⁱ	34.89 (8)	Te1—O3—Te2	122.67 (15)
O1ⁱⁱⁱ—Cd1—O2^iv	98.69 (12)	Te1—O3—Cd1	99.09 (12)
O1ⁱⁱⁱ—Cd1—O2^v	95.20 (12)	Te2—O3—Cd1	138.18 (14)
O2^iv—Cd1—O2^v	74.05 (13)	Te2^viii—O4—Te1	128.18 (16)
O1ⁱⁱⁱ—Cd1—O1	105.89 (7)	Te2^viii—O4—Cd1^vii	118.13 (14)
O2^iv—Cd1—O1	140.67 (11)	Te1—O4—Cd1^vii	94.15 (10)
O2^v—Cd1—O1	131.97 (12)	Te2—O5—Te2ⁱⁱ	103.48 (13)

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

Comparison of Te—O and M—O (M = Cd, Ca) bond lengths (Å) in the isotypic β-CdTe₂O₅ and ε-CaTe₂O₅ structures top

	β-CdTe₂O₅	ε-CaTe₂O₅^a
Te1—O2	1.843 (3)	1.832 (4)
Te1—O1	1.875 (3)	1.852 (4)
Te1—O3	1.991 (3)	1.980 (5)
Te1—O4	2.285 (3)	2.450 (5)
Te2—O4ⁱ	1.864 (3)	1.854 (4)
Te2—O5	1.897 (3)	1.898 (4)
Te2—O3	1.990 (3)	2.009 (5)
Te2—O5ⁱⁱ	2.204 (5)	2.178 (5)
M1—O1ⁱⁱⁱ	2.235 (3)	2.305 (4)
M1—O2^iv	2.238 (3)	2.326 (4)
M1—O1	2.256 (3)	2.360 (5)
M1—O2^v	2.296 (3)	2.358 (5)
M1—O3	2.424 (3)	2.476 (5)
M1—O4^v	2.589 (3)	2.554 (5)
M1—O4ⁱⁱⁱ	2.688 (3)	2.682 (5)

(a) Lattice parameters: a = 9.382 (2), b = 5.7095 (14), c = 11.132 (3) Å, β = 115.109 (4)°, V = 539.95 Å³ (Weil & Stöger, 2008). [Symmetry codes: (i) x, -y + 1/2, z - 1/2; (ii) -x + 1, -y + 1, -z + 1; (iii) -x, y + 1/2, -z + 1; (iv) -x, -y + 1, -z + 1; (v) 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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