Synthesis and crystal structure of the mixed-valent Te^IV/VI compound PbTe₃O₈

The crystal structure of the mixed-valent compound PbTe₃O₈ (= Pb^IITe^IV₂Te^VIO₈), lead(II) bis­[oxidotellurate(IV)] tellurate(VI), is isotypic with that of SrTe₃O₈. It comprises ¹_∞[Te₃O₈]^2– chains sharing O atoms to form a framework with two types of square channels extending parallel to [001]. The smaller channels (diameter ∼4 Å) are occupied by 12-coordinate Pb^II atoms in the centre of the channel, whereas the slightly larger channels (diameter ∼4.5 Å) accommodate the stereoactive 5s² electron lone pair of Te^IV.

1. Chemical context

In addition to the inter­est in the crystal-chemical peculiarities of compounds containing Pb^II and Te^IV atoms that are characterized by their lone pair electrons (Walsh et al., 2011), phases in the Pb/Te/O system are also relevant in terms of application-related problems. This applies, for example, to PbTe-based thermoelectrics (Kanatzidis, 2010) or to Pb/Te/O-based high-refractive index glasses (Alazoumi et al., 2018), where they can occur as unwanted surfacial oxidation products and as (re)crystallized phases, respectively. It is therefore important to have a thorough knowledge of the formation conditions and crystal structures of such phases.

Taking lead(II) oxidotellurate(VI), PbTeO₄, as an example, polycrystalline material from it was prepared at a temperature of 973 K and a pressure of 3000 atm (Sleight et al., 1972). In an attempt to grow PbTeO₄ crystals under more moderate hydro­thermal conditions following a modified procedure reported by Mathers & Graham (1929), crystals of the so far unknown compound PbTe₃O₈ were obtained serendipitously, and its crystal structure is reported in the present communication. As it turned out, PbTeO₄ can only be prepared at higher pressure and temperature conditions (Hladik et al., 2024).

2. Structural commentary

PbTe₃O₈ expands our knowledge of ternary phases in the Pb/Te/O system (all contain lead solely in oxidation state +II), of which the following are currently known. For oxidotellurates(IV): α-PbTeO₃ (Mariolacos, 1969; Zavodnik et al., 2008; Missen et al., 2019b), β-PbTeO₃ (Sciau et al., 1986; Ma et al., 2017; Weil et al., 2018), γ-PbTeO₃ (Weil et al., 2018; Kampf et al., 2022), Pb₂Te₃O₈ (Dewan et al., 1978; Champarnaud-Mesjard et al., 2001), PbTe₅O₁₁ (Oufkir et al., 2001); for oxidotellurates(VI): PbTeO₄ (Hladik et al., 2024), Pb₂TeO₅ (Kampf et al., 2010; Artner & Weil, 2013), Pb₅TeO₈ (Artner & Weil, 2013). This makes PbTe₃O₈ the first mixed-valent Te^IV/VI compound (= Pb^IITe^IV₂Te^VIO₈) in the ternary Pb/Te/O system. It belongs to class I of the Robin & Day (1967) classification, with the different valence states each clearly located on their own atomic sites.

The crystal structure of PbTe₃O₈ is isotypic with that of SrTe₃O₈ (Barrier et al., 2006; Weil & Stöger, 2007). It can be considered as being composed of _∞¹[Te₂^IVTe^VIO₈]^2– chains, with adjacent chains sharing O atoms to form a framework with two types of square channels extending parallel to [001] (Fig. 1).

Figure 1 View of the crystal structure of PbTe3O8 in a projection along [001]. Displacement ellipsoids are drawn at the 90% probability level. Colour code: Pb green, O colourless, {Te1O5} square pyramid orange, {Te2O6} octa­hedron red; the 5s^2 lone pair at the Te1 site is indicated as a blue sphere of arbitrary size.

The _∞¹[Te₂^IVTe^VIO₈]^2– chains consist of {Te1₂O₈} double polyhedra, which are formed by two symmetry-related {Te1^IVO₅} pyramids (Te1, site symmetry m..), and of {Te2^VIO₆} octa­hedra (Te2, 2/m..) by sharing corners and edges (Fig. 2). In the framework, each {Te2O₆} octa­hedron is connected to four {Te1₂O₈} double polyhedra, two via shared edges and two via shared corners, and each {Te1₂O₈} double polyhedron is connected in the same way to four {Te2O₆} octa­hedra. The {Te1O₅} pyramid deviates somewhat from an ideal square-pyramidal coordination geometry (the τ₅ index is 0.23, with extreme values of 0 for ideal square-pyramidal and 1 for ideal trigonal-bipyramidal geometry; Addison et al., 1984). The Te—O bond lengths span a range from 1.948 (10) to 2.170 (9) Å (Table 1). The {Te2O₆} octa­hedron shows no variance in bond lengths (Table 1) and only minor angular distortions for the equatorial O atoms [range 85.2 (4)–94.8 (4)°]. All Te—O bond lengths are in characteristic ranges for tetra- and hexa­valent tellurium atoms (Christy et al., 2016). The LPLoc software (Hamani et al., 2020) revealed the position of the presumed 5s² electron lone pair at the Te1^IV site with resulting fractional coordinates of x = 0.49158, y = 0.35089, z = 0. The radius of the electron lone pair was calculated to be 1.03 Å with a distance of 1.027 Å from the Te1 position. The electron lone pair protrudes into the larger of the two channels (diameter ∼4.5 Å), and thus is stereochemically active, as shown in Fig. 1.

Table 1
Selected bond lengths (Å)

Pb1—O2	2.570 (7)	Te1—O1	2.080 (7)
Pb1—O3^i	2.842 (8)	Te1—O1^iii	2.080 (7)
Pb1—O1	3.147 (8)	Te1—O2	2.170 (9)
Te1—O3^ii	1.948 (10)	Te2—O1^iv	1.927 (7)
Te1—O3	2.027 (9)	Te2—O2	1.927 (9)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Figure 2 Part of a ∞^1[Te2^IVTe^VIO8]^2– chain. Displacement ellipsoids are drawn at the 90% probability level. Symmetry codes refer to Table 1.

The Pb^II cation is located in the smaller channel (diameter ∼4 Å, Fig. 1). It exhibits site symmetry .. and has a coordination number of 12, with three groups of short, medium and long Pb—O bond lengths (Table 1). The corresponding [PbO₁₂] coordination polyhedron was analysed using the Polynator program (Link & Niewa, 2023). It has a volume of 52.82 ų and can be described as a distorted cubocta­hedron that differs from the ideal polyhedron (point group 2m) by a δ value (Link & Niewa, 2023) of 5.736. The average Pb^[12]—O distance (2.853 Å) is in good agreement with the value of 2.827 (113) Å provided by Gagné & Hawthorne (2018). Localization of the electron lone pair at the Pb^II site using LPLoc shows that it is situated directly at the metal site (calculated lone pair radius 1.16 Å) and apparently exhibits no stereochemical activity.

A qu­anti­tative comparison between the isotypic MTe₃O₈ (M = Pb, Sr) structures 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 Å for Pb1/Sr1, 0 Å for Te2, 0.0235 Å for Te1, 0.0247 Å for O3, 0.0252 Å for O1, and 0.0600 Å for O2; the arithmetic mean of the distance between paired atoms is 0.0264 Å. The degree of lattice distortion (0.0070), and the low value for the measure of similarity (0.024) indicate a high degree of simiularity for the two crystal structures.

In addition to the two isotypic MTe₃O₈ (M = Pb, Sr) structures, there is another compound with the same formula type (M = Ca) that occurs in nature as the rare mineral carlfriesite. Its monoclinic crystal structure (Effenberger et al., 1978; Missen et al., 2019a) can also be described as a nanoporous framework, although it exhibits a different topology for the [Te₂^IVTe^VIO₈]^2– anion. Here, the metal cation is situated in slightly larger channels with a diameter of approximately 6 Å and exhibits a coordination number of eight with a distorted tetra­gonal anti­prismatic oxygen coordination.

3. Synthesis and crystallization

The chemicals were supplied by Merck and were used without further purification. Equimolar amounts of PbO₂ and TeO₂ (total load 0.3 g) were thoroughly mixed and placed in a Teflon container. The container was filled to about two-thirds of its inner volume (5 ml) with water, closed with a Teflon lid and placed in a steel autoclave, which was heated at 483 K for two days. The greyish solid product was filtered off and washed with water and ethanol. A few colourless crystals with a rod-like form of the title compound were isolated under a polarising microscope. Qualitative phase analysis of the bulk product from laboratory X-ray powder diffraction data revealed the starting materials TeO₂ and PbO₂ as well as PbTe₃O₈ as products, together with unassigned reflections. This shows that, in a redox reaction, parts of Pb^IV and Te^IV have reacted to form Pb^II and Te^VI.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. For better comparison with isotypic SrTe₃O₈ (Weil & Stöger, 2007), the same atom numbering scheme and starting coordinates for refinement were used. The residual maximum and minimum difference-electron densities are located 0.61 Å from Te2 and 0.49 Å from Pb1, respectively.

Table 2
Experimental details

Crystal data
Chemical formula	PbTe3O8
Mr	717.99
Crystal system, space group	Tetragonal, P42/m
Temperature (K)	296
a, c (Å)	6.9345 (3), 6.7369 (4)
V (Å^3)	323.96 (3)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	39.29
Crystal size (mm)	0.10 × 0.02 × 0.01
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.155, 0.349
No. of measured, independent and observed [I > 2σ(I)] reflections	7237, 514, 499
Rint	0.031
(sin θ/λ)max (Å^−1)	0.702
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.037, 0.112, 1.12
No. of reflections	514
No. of parameters	34
Δρmax, Δρmin (e Å^−3)	3.82, −4.85
Computer programs: APEX3 and SAINT (Bruker, 2018), SHELXL (Sheldrick, 2015), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010).

Compared with the other atoms, the displacement parameters of the Pb site are large. The same phenomenon is observed in the isotypic Sr structure (Weil & Stöger, 2007). We assume that this property is characteristic of this type of structure and is primarily caused by the position of the metal site within the large pores. It is also possible that the Pb site shows positional order around the 4 axis or is under-occupied with a potential Pb^II/IV co-occupation, but this cannot be conclusively demonstrated by the available X-ray data. To obtain a more detailed picture, supplementary analytical data are required, for example through wet chemical methods or microprobe analysis.

Supporting information

Computing details

Lead(II) bis[oxidotellurate(IV)] tellurate(VI)

Crystal data

PbTe3O8	Dx = 7.360 Mg m^−3
Mr = 717.99	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P42/m	Cell parameters from 4370 reflections
a = 6.9345 (3) Å	θ = 3.0–29.4°
c = 6.7369 (4) Å	µ = 39.29 mm^−1
V = 323.96 (3) Å^3	T = 296 K
Z = 2	Fragment, colourless
F(000) = 604	0.10 × 0.02 × 0.01 mm

Data collection

Bruker APEXII CCD diffractometer	499 reflections with I > 2σ(I)
ω– and φ–scans	Rint = 0.031
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θmax = 29.9°, θmin = 2.9°
Tmin = 0.155, Tmax = 0.349	h = −9→9
7237 measured reflections	k = −9→9
514 independent reflections	l = −9→9

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: isomorphous structure methods
R[F^2 > 2σ(F^2)] = 0.037	w = 1/[σ^2(Fo^2) + (0.0619P)^2 + 13.4507P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.112	(Δ/σ)max < 0.001
S = 1.12	Δρmax = 3.82 e Å^−3
514 reflections	Δρmin = −4.85 e Å^−3
34 parameters

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
Pb1	0.000000	0.000000	0.250000	0.0409 (4)
Te1	0.43446 (12)	0.21418 (12)	0.000000	0.0105 (3)
Te2	0.000000	0.500000	0.000000	0.0066 (3)
O1	0.4112 (10)	0.1842 (10)	0.3063 (10)	0.0125 (12)
O2	0.1237 (14)	0.2511 (13)	0.000000	0.0111 (17)
O3	0.6731 (15)	0.0454 (14)	0.000000	0.0129 (18)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.0373 (4)	0.0373 (4)	0.0480 (7)	0.000	0.000	0.000
Te1	0.0105 (4)	0.0101 (4)	0.0109 (4)	0.0020 (3)	0.000	0.000
Te2	0.0086 (5)	0.0052 (5)	0.0061 (5)	0.0025 (3)	0.000	0.000
O1	0.019 (3)	0.012 (3)	0.007 (3)	0.007 (2)	0.001 (2)	0.001 (2)
O2	0.009 (4)	0.007 (4)	0.017 (4)	0.003 (3)	0.000	0.000
O3	0.012 (4)	0.013 (4)	0.014 (4)	0.007 (3)	0.000	0.000

Geometric parameters (Å, º)

Pb1—O2^i	2.570 (7)	Te1—O3^vii	1.948 (10)
Pb1—O2^ii	2.570 (7)	Te1—O3	2.027 (9)
Pb1—O2^iii	2.570 (7)	Te1—O1	2.080 (7)
Pb1—O2	2.570 (7)	Te1—O1^x	2.080 (7)
Pb1—O3^iv	2.842 (8)	Te1—O2	2.170 (9)
Pb1—O3^v	2.842 (8)	Te2—O1^iv	1.927 (7)
Pb1—O3^vi	2.842 (8)	Te2—O1^xi	1.927 (7)
Pb1—O3^vii	2.842 (8)	Te2—O1^ix	1.927 (7)
Pb1—O1	3.147 (8)	Te2—O1^xii	1.927 (7)
Pb1—O1^viii	3.147 (8)	Te2—O2	1.927 (9)
Pb1—O1^ii	3.147 (7)	Te2—O2^xiii	1.927 (9)
Pb1—O1^ix	3.147 (7)
O2^i—Pb1—O2^ii	115.43 (15)	O1—Pb1—O1^ii	90.83 (3)
O2^i—Pb1—O2^iii	115.43 (15)	O1^viii—Pb1—O1^ii	90.83 (3)
O2^ii—Pb1—O2^iii	98.1 (3)	O2^i—Pb1—O1^ix	113.5 (2)
O2^i—Pb1—O2	98.1 (3)	O2^ii—Pb1—O1^ix	131.0 (2)
O2^ii—Pb1—O2	115.43 (15)	O2^iii—Pb1—O1^ix	60.1 (2)
O2^iii—Pb1—O2	115.43 (15)	O2—Pb1—O1^ix	56.1 (2)
O2^i—Pb1—O3^iv	165.0 (3)	O3^iv—Pb1—O1^ix	52.7 (2)
O2^ii—Pb1—O3^iv	78.6 (3)	O3^v—Pb1—O1^ix	60.3 (2)
O2^iii—Pb1—O3^iv	54.6 (3)	O3^vi—Pb1—O1^ix	138.5 (2)
O2—Pb1—O3^iv	79.10 (19)	O3^vii—Pb1—O1^ix	110.6 (2)
O2^i—Pb1—O3^v	54.6 (3)	O1—Pb1—O1^ix	90.83 (3)
O2^ii—Pb1—O3^v	165.0 (3)	O1^viii—Pb1—O1^ix	90.83 (3)
O2^iii—Pb1—O3^v	79.10 (19)	O1^ii—Pb1—O1^ix	166.2 (3)
O2—Pb1—O3^v	78.6 (3)	O3^vii—Te1—O3	77.2 (5)
O3^iv—Pb1—O3^v	110.57 (12)	O3^vii—Te1—O1	83.0 (2)
O2^i—Pb1—O3^vi	79.10 (19)	O3—Te1—O1	90.32 (18)
O2^ii—Pb1—O3^vi	54.6 (3)	O3^vii—Te1—O1^x	83.0 (2)
O2^iii—Pb1—O3^vi	78.6 (3)	O3—Te1—O1^x	90.32 (18)
O2—Pb1—O3^vi	165.0 (3)	O1—Te1—O1^x	165.5 (4)
O3^iv—Pb1—O3^vi	107.3 (2)	O3^vii—Te1—O2	74.3 (4)
O3^v—Pb1—O3^vi	110.57 (12)	O3—Te1—O2	151.5 (4)
O2^i—Pb1—O3^vii	78.6 (3)	O1—Te1—O2	86.3 (2)
O2^ii—Pb1—O3^vii	79.10 (19)	O1^x—Te1—O2	86.3 (2)
O2^iii—Pb1—O3^vii	165.0 (3)	O1^iv—Te2—O1^xi	180.0
O2—Pb1—O3^vii	54.6 (3)	O1^iv—Te2—O1^ix	94.8 (4)
O3^iv—Pb1—O3^vii	110.57 (12)	O1^xi—Te2—O1^ix	85.2 (4)
O3^v—Pb1—O3^vii	107.3 (2)	O1^iv—Te2—O1^xii	85.2 (4)
O3^vi—Pb1—O3^vii	110.57 (12)	O1^xi—Te2—O1^xii	94.8 (4)
O2^i—Pb1—O1	131.0 (2)	O1^ix—Te2—O1^xii	180.0 (4)
O2^ii—Pb1—O1	56.1 (2)	O1^iv—Te2—O2	89.5 (3)
O2^iii—Pb1—O1	113.5 (2)	O1^xi—Te2—O2	90.5 (3)
O2—Pb1—O1	60.1 (2)	O1^ix—Te2—O2	90.5 (3)
O3^iv—Pb1—O1	60.3 (2)	O1^xii—Te2—O2	89.5 (3)
O3^v—Pb1—O1	138.5 (2)	O1^iv—Te2—O2^xiii	90.5 (3)
O3^vi—Pb1—O1	110.6 (2)	O1^xi—Te2—O2^xiii	89.5 (3)
O3^vii—Pb1—O1	52.7 (2)	O1^ix—Te2—O2^xiii	89.5 (3)
O2^i—Pb1—O1^viii	60.1 (2)	O1^xii—Te2—O2^xiii	90.5 (3)
O2^ii—Pb1—O1^viii	113.5 (2)	O2—Te2—O2^xiii	180.0
O2^iii—Pb1—O1^viii	56.1 (2)	Te2^xiv—O1—Te1	135.5 (3)
O2—Pb1—O1^viii	131.0 (2)	Te2^xiv—O1—Pb1	95.9 (3)
O3^iv—Pb1—O1^viii	110.6 (2)	Te1—O1—Pb1	89.5 (2)
O3^v—Pb1—O1^viii	52.7 (2)	Te2—O2—Te1	123.2 (5)
O3^vi—Pb1—O1^viii	60.3 (2)	Te2—O2—Pb1^i	117.3 (3)
O3^vii—Pb1—O1^viii	138.5 (2)	Te1—O2—Pb1^i	104.6 (3)
O1—Pb1—O1^viii	166.2 (3)	Te2—O2—Pb1	117.3 (3)
O2^i—Pb1—O1^ii	56.1 (2)	Te1—O2—Pb1	104.6 (3)
O2^ii—Pb1—O1^ii	60.1 (2)	Pb1^i—O2—Pb1	81.9 (3)
O2^iii—Pb1—O1^ii	131.0 (2)	Te1^vii—O3—Te1	102.8 (5)
O2—Pb1—O1^ii	113.5 (2)	Te1^vii—O3—Pb1^xv	101.7 (3)
O3^iv—Pb1—O1^ii	138.5 (2)	Te1—O3—Pb1^xv	135.7 (3)
O3^v—Pb1—O1^ii	110.6 (2)	Te1^vii—O3—Pb1^vii	101.7 (3)
O3^vi—Pb1—O1^ii	52.7 (2)	Te1—O3—Pb1^vii	135.7 (3)
O3^vii—Pb1—O1^ii	60.3 (2)	Pb1^xv—O3—Pb1^vii	72.7 (2)

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