α-Lead tellurite from single-crystal data

The crystal structure of the title compound, α-PbTeO₃ (PTO), has been reported previously by Mariolacos [Anz. Oesterr. Akad. Wiss. Math. Naturwiss. Kl. (1969), 106, 128–130], refined on powder data. The current determination at room temperature from data obtained from single crystals grown by the Czochralski method shows a significant improvement in the precision of the geometric parameters when all atoms have been refined anisotropically. The selection of a centrosymmetric (C2/c) structure model was confirmed by the second harmonic generation test. The asymmetric unit contains three formula units. The structure of PTO is built up of three types of distorted [PbO_x] polyhedra (x = 7 and 9) which share their O atoms with TeO₃ pyramidal units. These main anionic polyhedra are responsible for establishing the two types of tunnel required for the stereochemical activity of the lone pairs of the Pb²⁺ and Te⁴⁺ cations.

Related literature

Single crystals of PTO were grown by the Czochralski technique (Kosse, Politova, Bush et al., 1983). For the temperature dependence of the physical properties of PTO, see: Kosse, Politova, Astafiev et al. (1983). For the polymorphism of PTO, see: Tananaeva et al. (1977), Robertson et al. (1976), Young (1979). Several different polymorphs were previously described as monoclinic (Mariolacos, 1969), triclinic (Williams, 1979), ortho­rhom­bic (Spiridonov & Tananaeva, 1982), tetra­gonal (Sciau et al., 1986) and cubic (Gaitan et al., 1987). For related literature, see: Brown (1974); Galy et al. (1975); Gillespie (1972); Tananaeva & Novoselova (1977).

Experimental

Crystal data

• PbTeO₃

• M_r = 382.79

• Monoclinic, C 2/c

• a = 26.555 (5) Å

• b = 4.593 (1) Å

• c = 17.958 (4) Å

• β = 106.97 (3)°

• V = 2094.9 (7) ų

• Z = 24

• Mo Kα radiation

• μ = 56.32 mm⁻¹

• T = 295 (2) K

• 0.14 × 0.04 × 0.02 mm

Data collection

• Enraf–Nonius CAD-4 diffractometer

• Absorption correction: refined from ΔF (Walker & Stuart, 1983) T_min = 0.234, T_max = 0.695 (expected range = 0.109–0.324)

• 3717 measured reflections

• 3608 independent reflections

• 1676 reflections with I > 2σ(I)

• R_int = 0.054

• 3 standard reflections frequency: 60 min intensity decay: none

Refinement

• R[F² > 2σ(F²)] = 0.026

• wR(F²) = 0.063

• S = 0.77

• 3608 reflections

• 137 parameters

• Δρ_max = 2.31 e Å⁻³

• Δρ_min = −2.06 e Å⁻³

Data collection: CAD-4-PC (Enraf–Nonius, 1993); cell refinement: CAD-4-PC; data reduction: CAD-4-PC; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: CIFTAB97 (Sheldrick, 2008).

Supporting information

S1. Comment

Crystals with the Pb²⁺ and Te⁴⁺ cations having stereochemically active lone-pairs are very attractive materials for ferroelectric and non-linear optical applications. The knowledge of the crystal structures of these compounds should provide important information about the unusual mechanism of formation of their polar properties. The investigation of the PbO-TeO₂ system (Robertson et al., 1976; Young, 1979) has provided evidence of a large number of different phases. The polymorphism, crystal structure and thermodynamic status of PbTeO₃ (PTO) are not fully established and literature reports give conflicting statements (Tananaeva et al.,1977; Robertson et al.,1976; Young, 1979). Several different polymorphs have previously been described: monoclinic (Mariolacos, 1969), triclinic (Williams, 1979), tetragonal (Sciau et al., 1986) and cubic (Gaitan et al.,1987). It should be mentioned that Spiridonov & Tananaeva (1982) described α-PbTeO₃ as orthorhombic. The tetragonal phase was shown to be ferroelectric. The phase change from the tetragonal to the monoclinic form at 783 K has been shown to be irreversible (Young, 1979). The present paper deals with the crystal structure determination of α-PTO. This structure can be described in terms of complex irregular Pb²⁺ polyhedra with 7 and 9 apices and separate Te⁴⁺O₃ groups (Fig. 1,2). Three kinds of Pb—O distances can be distinguished: three short contacts (2.25–2.53 Å), three longer distances (2.63–2.96 Å) and three abnormally long distances (3.02–3.26 Å). The different Pb polyhedra are connected by face, edge and corner sharing through the Pb—O bonds forming the network with the honeycomb-like chains parallel to c axis. The Te⁴⁺ cations coordinate to three O atoms in a one-sided pyramidal coordination TeO₃E (E are lone-pair electrons). The Te—O distances are in the range 1.85–1.90 Å. The O—Te—O angles are close to 100°. The next-nearest anions are located at distances greater than 2.7 Å. In accordance with Brown (1974) these additional weak contacts are important for the determination of the correct coordination geometry of the Te cations. Depending on the type of Te⁴⁺O₃ E units, two types of tunnels are formed running along [010], which represent the required space for the electron lone pairs within the structure. According to Gillespie (1972), Galy et al. (1975) the electronic lone pair E is sitting inside these non-bonding regions.

Related literature

Single crystals of PTO were grown by the Czochralski technique (Kosse, Politova, Bush et al., 1983). For the temperature dependence of the physical properties of PTO, see: Kosse, Politova, Astafiev et al. (1983). For the polymorphism of PTO, see: Tananaeva et al. (1977), Robertson et al. (1976), Young (1979). Several different polymorphs were previously described as monoclinic (Mariolacos, 1969), triclinic (Williams, 1979), orthorhombic (Spiridonov & Tananaeva, 1982), tetragonal (Sciau et al., 1986) and cubic (Gaitan et al., 1987). For related literature, see: Brown (1974); Galy et al. (1975); Gillespie (1972); Tananaeva & Novoselova (1977).

S2. Experimental

Single crystals of PTO were grown by the Czochralski technique as described earlier (Kosse, Politova, Bush et al., 1983; Kosse, Politova, Astafiev et al., 1983). The chemical composition of tested crystals was confirmed with energy-dispersive spectrometry analysis (LINK AN10000). Second harmonic generation (SHG) measurements showed no positive signals at room temperature which is in accordance with the given space group.

S3. Refinement

The structure of PTO was solved by the direct method in space group C2/c where the atomic coordinates of all Pb and Te cations were found. The oxygen atoms were localized by difference Fourier maps.

The very high absorption coefficient (µ=56.32 mm^-1) and imperfect shape of crystal are the reason why the program DIFABS (Walker & Stuart, 1983) was used for absorption correction.

The highest residual electron density peak (2.31 e A^-3) is located 1.00Å from atom Pb1 and the deepest hole

(-2.06 e A^-3) is located 1.46Å from atom O4.

Computing details

Data collection: CAD-4-PC (Enraf–Nonius, 1993); cell refinement: CAD-4-PC (Enraf–Nonius, 1993); data reduction: CAD-4-PC (Enraf–Nonius, 1993); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: CIFTAB97 (Sheldrick, 2008).

Figures

	Fig. 1. Polyhedral representation of the structure of α-PbTeO3.
	Fig. 2. Coordination polyhedra of the different Pb cations. Displacement ellipsoids are drawn at the 50% probability level. The labeling scheme for symmetry-related atoms is the following: (i) 0.5 - x,1/2 + y,1.5 - z˛ (ii) x,1 + y,z˛ (iii) -x,-y,1 - z˛ (iv) 0.5 - x,-1/2 + y,1.5 - z˛ (v) x,-1 - y,1/2 + z˛ (vi) x,-y,1/2 + z.

α-lead tellurite

Crystal data

PbTeO3	F(000) = 3792
Mr = 382.79	Dx = 7.282 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 24 reflections
a = 26.555 (5) Å	θ = 12.1–14.5°
b = 4.593 (1) Å	µ = 56.32 mm^−1
c = 17.958 (4) Å	T = 295 K
β = 106.97 (3)°	Needle, colourless
V = 2094.9 (7) Å^3	0.14 × 0.04 × 0.02 mm
Z = 24

Data collection

Enraf–Nonius CAD-4 diffractometer	1676 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.054
β-filter monochromator	θmax = 32.0°, θmin = 1.6°
ω/2θ scans	h = −39→37
Absorption correction: part of the refinement model (ΔF) (Walker & Stuart, 1983)	k = −6→0
Tmin = 0.234, Tmax = 0.695	l = 0→26
3717 measured reflections	3 standard reflections every 60 min
3608 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.026	w = 1/[σ^2(Fo^2) + (0.0301P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.063	(Δ/σ)max = 0.001
S = 0.77	Δρmax = 2.31 e Å^−3
3608 reflections	Δρmin = −2.06 e Å^−3
137 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.000052 (5)

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
Pb1	0.183652 (15)	0.21366 (10)	0.73592 (2)	0.01749 (9)
Pb2	0.066493 (14)	0.22395 (12)	0.54345 (2)	0.01847 (9)
Pb3	0.162024 (14)	−0.24695 (10)	0.913902 (19)	0.01916 (9)
Te1	0.04734 (2)	−0.23056 (15)	0.36782 (3)	0.01331 (11)
Te2	0.06941 (2)	−0.25178 (16)	0.70793 (3)	0.01367 (11)
Te3	0.20758 (3)	−0.30201 (15)	0.59262 (4)	0.01439 (13)
O1	0.0204 (3)	−0.179 (2)	0.4532 (4)	0.0240 (17)
O2	0.1172 (3)	−0.156 (2)	0.4259 (5)	0.0285 (19)
O3	0.0559 (4)	−0.6348 (18)	0.3703 (6)	0.034 (2)
O4	0.1359 (3)	−0.221 (2)	0.7823 (4)	0.0201 (15)
O5	0.0952 (4)	−0.1694 (19)	0.6233 (4)	0.028 (2)
O6	0.0659 (4)	−0.6528 (17)	0.6935 (6)	0.035 (2)
O7	0.2214 (3)	−0.2120 (18)	0.6999 (4)	0.0180 (14)
O8	0.1858 (4)	−0.683 (2)	0.6000 (5)	0.033 (2)
O9	0.2781 (3)	−0.355 (2)	0.6016 (6)	0.034 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.01503 (16)	0.0168 (2)	0.02066 (16)	0.00061 (16)	0.00526 (13)	−0.00209 (16)
Pb2	0.01480 (15)	0.0215 (2)	0.01935 (15)	0.00049 (18)	0.00528 (12)	0.00186 (17)
Pb3	0.02372 (17)	0.02011 (19)	0.01346 (14)	0.00414 (19)	0.00511 (12)	0.00129 (16)
Te1	0.0161 (2)	0.0109 (3)	0.0145 (2)	0.0019 (3)	0.00698 (19)	−0.0003 (2)
Te2	0.0136 (2)	0.0106 (3)	0.0157 (2)	−0.0002 (3)	0.00254 (19)	−0.0021 (3)
Te3	0.0163 (3)	0.0122 (3)	0.0137 (2)	0.0009 (2)	0.0029 (2)	0.0001 (2)
O1	0.022 (4)	0.035 (5)	0.016 (3)	−0.007 (4)	0.008 (3)	−0.003 (3)
O2	0.013 (3)	0.032 (5)	0.037 (4)	0.005 (3)	0.002 (3)	0.007 (4)
O3	0.056 (7)	0.007 (4)	0.040 (5)	0.010 (4)	0.014 (5)	−0.002 (3)
O4	0.017 (3)	0.030 (4)	0.014 (3)	−0.013 (4)	0.006 (2)	−0.004 (3)
O5	0.050 (6)	0.022 (4)	0.014 (3)	0.015 (4)	0.012 (4)	0.010 (3)
O6	0.044 (6)	0.005 (3)	0.057 (6)	−0.004 (4)	0.019 (5)	−0.012 (4)
O7	0.020 (3)	0.014 (3)	0.020 (3)	0.006 (3)	0.007 (3)	0.006 (3)
O8	0.053 (6)	0.021 (4)	0.024 (4)	−0.016 (4)	0.013 (4)	0.000 (3)
O9	0.013 (3)	0.052 (6)	0.039 (5)	0.014 (4)	0.010 (3)	0.000 (4)

Geometric parameters (Å, º)

Pb1—O7	2.371 (8)	Pb3—O3^v	2.750 (11)
Pb1—O7^i	2.471 (7)	Te1—O3	1.870 (8)
Pb1—O8^ii	2.504 (8)	Te1—O2	1.876 (9)
Pb1—O4	2.628 (8)	Te1—O1	1.888 (7)
Pb2—O5	2.294 (8)	Te2—O6	1.859 (8)
Pb2—O1^iii	2.334 (8)	Te2—O5	1.878 (8)
Pb2—O1	2.528 (9)	Te2—O4	1.883 (7)
Pb2—O6^ii	2.758 (10)	Te3—O9	1.848 (8)
Pb3—O2^iv	2.246 (9)	Te3—O8	1.858 (9)
Pb3—O4	2.263 (6)	Te3—O7	1.899 (7)
Pb3—O9^i	2.471 (9)
O7—Pb1—O7^i	77.64 (19)	O6—Te2—O5	95.9 (4)
O7—Pb1—O8^ii	76.1 (3)	O6—Te2—O4	99.8 (5)
O7^i—Pb1—O8^ii	96.7 (3)	O5—Te2—O4	94.1 (4)
O7—Pb1—O4	75.0 (2)	O9—Te3—O8	101.4 (5)
O7^i—Pb1—O4	118.6 (2)	O9—Te3—O7	92.8 (4)
O8^ii—Pb1—O4	127.2 (3)	O8—Te3—O7	96.0 (4)
O5—Pb2—O1^iii	93.7 (3)	Te1—O1—Pb2^iii	128.4 (4)
O5—Pb2—O1	80.5 (3)	Te1—O1—Pb2	112.6 (4)
O1^iii—Pb2—O1	69.8 (3)	Pb2^iii—O1—Pb2	110.2 (3)
O5—Pb2—O6^ii	69.3 (3)	Te1—O2—Pb3^vi	124.2 (4)
O1^iii—Pb2—O6^ii	72.9 (3)	Te1—O3—Pb3^vii	108.1 (5)
O1—Pb2—O6^ii	129.5 (3)	Te2—O4—Pb3	132.5 (3)
O2^iv—Pb3—O4	92.7 (3)	Te2—O4—Pb1	105.7 (3)
O2^iv—Pb3—O9^i	77.7 (3)	Pb3—O4—Pb1	110.0 (3)
O4—Pb3—O9^i	81.7 (3)	Te2—O5—Pb2	122.1 (4)
O2^iv—Pb3—O3^v	70.3 (3)	Te2—O6—Pb2^viii	109.0 (4)
O4—Pb3—O3^v	74.8 (3)	Te3—O7—Pb1	119.0 (3)
O9^i—Pb3—O3^v	138.7 (3)	Te3—O7—Pb1^ix	108.2 (3)
O3—Te1—O2	94.2 (4)	Pb1—O7—Pb1^ix	116.2 (3)
O3—Te1—O1	100.2 (4)	Te3—O8—Pb1^viii	110.2 (4)
O2—Te1—O1	94.2 (4)	Te3—O9—Pb3^ix	139.0 (6)

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