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 [PbOx] 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.


Data collection
Enraf-Nonius CAD-4 diffractometer Absorption correction: refined from ÁF (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 > 2(I) R int = 0.054 3 standard reflections frequency: 60 min intensity decay: none Refinement R[F 2 > 2(F 2 )] = 0.026 wR(F 2 ) = 0.063 S = 0.77 3608 reflections 137 parameters Á max = 2.31 e Å À3 Á min = À2.06 e Å À3 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 Comment Crystals with the Pb 2+ and Te 4+ 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-TeO 2 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 PbTeO 3 (PTO) are not fully established and literature reports give conflicting statements 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-PbTeO 3 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 Pb 2+ polyhedra with 7 and 9 apices and separate Te 4+ O 3 groups ( Fig. 1,2). , 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.

Experimental
Single crystals of PTO were grown by the Czochralski technique as described earlier (Kosse, Politova, Bush 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.

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.
supplementary materials sup-2 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.

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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Geometric parameters (Å, °)