Crystal structure of dilead(II) oxochromate(VI) oxotellurate(IV)

The crystal structure of Pb2(CrO4)(TeO3) is isotypic with its sulfate analogue Pb2(SO4)(TeO3). Comparison between the structures is made with the COMPSTRU program.


Chemical context
Pb 3 Fe 2 Te 2 O 12 is an oxotellurate(VI) with interesting structural features. It crystallizes in the non-centrosymmetric space group Cc and has Te VI and Fe III atoms occupationally disordered at the same sites (Mü ller-Buschbaum & Wedel, 1997). This compound has been prepared by solid-state reactions from a PbO, Fe 2 O 3 and TeO 2 mixture in air, which led to oxidation of Te IV to Te VI . During an attempt to replace iron(III) by chromium(III) to prepare a possible phase with composition 'Pb 3 Cr 2 Te 2 O 12 ', the title compound, Pb 2 (CrO 4 )(TeO 3 ), was obtained instead while working under similar conditions. Interestingly, chromium was then oxidized (Cr III ! Cr VI ) while tellurium remained its oxidation state of IV. Pb 2 (CrO 4 )(TeO 3 ) is isotypic with its sulfate analogue Pb 2 (SO 4 )(TeO 3 ) (Weil & Shirkhanlou, 2017).

Structural commentary
All atoms in the asymmetric unit, viz. two Pb, one Cr, one Te and seven O sites, are located on general positions.
The coordination environments of the two Pb 2+ cations are markedly different. If only Pb-O bond lengths < 2.8 Å are considered, atom Pb1 is surrounded by six O atoms in the range 2.4-2.8 Å whereas atom Pb2 has four oxygen atoms as coordination partners, three at $2.38 Å and one at 2.75 Å . Taking into account the more remote oxygen atoms as well, the coordination numbers are increased to nine for both Pb 2+ cations ( Fig. 1, Table 1).
The chromium atom shows a tetrahedral and the tellurium a trigonal-pyramidal coordination by oxygen atoms. These two coordination polyhedra and the corresponding bond lengths ISSN 2056-9890 ranges are typical for oxochromates(VI) (Pressprich et al., 1988) and oxotellurates(IV) (Christy et al., 2016), respectively.
In the crystal structure, the Pb 2+ cations are arranged in layers parallel to (001) at z $ 0, 1 2 and in turn are stacked into columns extending along [010]. The two types of anion polyhedra are isolated and are likewise arranged into columnar arrangements along [010], forming anion layers situated at z $ 1 4 and 3 4 . The metal cation and anion layers alternate along [001] and build up the three-dimensional framework of the crystal structure. The 5s 2 and 6s 2 electron lone pairs of the Te IV atoms of the oxotellurate anions and of the Pb 2+ cations, respectively, are stereochemically active and point into channels running parallel to the two types of columns along [010] (Fig. 2).

Synthesis and crystallization
Cr(NO 3 ) 3 Á9H 2 O, PbF 2 , PbO and TeO 2 were mixed thoroughly in a stoichiometric ratio of 1:1:2:1 and heated in an open alumina crucible to 1033 K within six h, held at this temperature for 30 h and cooled within eight h to room temperature. Most of the material had evaporated, and only a few orange plates of the title compound were left.
Alternatively, replacement of Cr(NO 3 ) 3 Á9H 2 O with Cr 2 O 3 under the same reaction conditions likewise led to the formation of Pb 2 (CrO 4 )(TeO 3 ).

Crystal data
Pb 2  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.