supplementary materials

Continuing investigations of crystalline complex compounds of lead and 3d metal cation fluorides having chemical formulations similar to some fluoride glasses for structure modelling purposes (Le Bail, 2000), such as Pb₈MnFe₂F₂₄ (Le Bail & Mercier, 1992) or NaPbFe₂F₉ (Le Bail, 1989), the title compound, Pb₂CrF₇, was synthesized and characterized from single crystal X-ray data. The results confirm that it is isostructural with Pb₂RhF₇ (Domesle & Hoppe, 1983), as mentioned previously by de Kozak et al. (1999). The monoclinic structure is built up of CrF₆^3- octahedra (Fig. 1) isolated from each other, inserted in a fluorite-related matrix of PbF₆ distorted octahedra and PbF₈ square antiprisms sharing edges and corners. The seventh fluorine atom is "independent", connected only to 3 Pb atoms in FPb₃ triangles, sharing an edge and building an almost planar Pb₄F₂ unit (Fig. 2), so that the formula can be alternatively written as Pb₂F(CrF₆). There are some differences observed with the Pb₂RhF₇ structure-type. The Pb1 atom appears to be six-coordinated in Pb₂CrF₇ with Pb—F distances ranging from 2.324 (5) to 2.899 (6) Å, there are then 5 next F atoms being between 3.067 (7) and 3.364 (6) Å whereas in Pb₂RhF₇, Pb1 looks eightfold coordinated (Pb—F distances between 2.375 and 2.744 Å), with two next F atoms at 3.105 and 3.219 Å. The bond valence calculations show that the first 6 F atoms around Pb1 cannot really satisfy the Pb²⁺ charge and that the 5 next F atoms contribute to the overall bond valence (Table 2). The behaviour of the cell parameters is surprising in the series of the three isostructural compounds. If all cell parameters are logically smaller in Sr₂RhF₇ (Grosse & Hoppe, 1987) than in Pb₂RhF₇, due to the smaller Sr²⁺ size, in Pb₂CrF₇ one observes that only a and b are smaller but that c is much larger, in spite of the Cr³⁺ cation being smaller than Rh³⁺ (Table 3). Finally, the cell volume of Pb₂CrF₇ is even slightly larger than that of Pb₂RhF₇.

Usually, such fluorinated phases with high A^II/M ratio (A^II = Ca, Pb, Sr ; M = 3d element or In, Nb, Zr, etc), are found to be related to the fluorite structure adopted by PbF₂ or SrF₂. The relation is in general easy to establish due to the presence of "independent" F atoms (not bonded to M) which are found to form characteristic FPb₄ tetrahedra. But this is not obvious here (Fig. 3) since no such FPb₄ tetrahedron is observed. Other compounds containing "independent" fluorine atoms coordinated to three cations (Ca or Sr) in a plane are Ca₂AlF₇ (Domesle & Hoppe, 1980), Sr₅Zr₃F₂₂ (Le Bail, 1996) and Sr₅(VOF₅)₃F(H₂O)₃ (Le Bail et al., 2009), the last two being strongly related to the fluorite structure in which the FSr₃ triangles were found to interconnect the fluorite-related blocks. An examination of the Pb coordinates in Pb₂CrF₇ along the a axis (which is close to the PbF₂ fluorite cell parameter) shows that they alternate at values x~1/4 and 3/4, forming fluorite-related strips corrugating in the ac plane where highly distorted PbF₈ cubes as expected in the fluorite structure were obtained by small displacements of some of the F atoms as evidenced in Fig. 4 . Similar corrugated strips were observed for the Pb₂ZrF₈ structure (Le Bail & Laval, 1998), where the Zr⁴⁺ cations occupy bicapped trigonal prisms at positions similar to those of Cr³⁺ in the title compound. In Pb₂ZrF₈, the Pb²⁺ lone pair position was suggested by comparison of the Pb coordination with that of Ba in the isostructural α-Ba₂ZrF₈ structure, observing some strong distorsions, the stereochemichally active lone pair repelling clearly some F atoms. The same exercice is not so obvious here, even by comparing Sr₂RhF₇ with Pb₂RhF₇. However, there is a specific pattern of differences in the bond lengths (3 adjacent short Pb—F bonds and 3 adjacent long ones for Pb1; 4 short and 4 long for Pb2) which could be attributed to repulsion effects involving a somehow weak stereochemically active lone pair producing the longer Pb—F distances. The Pb1 lone pair is thus probably oriented towards the barycenter of the (F5—F6—F2) face of the Pb1F₆ octahedron, a similar reasoning applying to the Pb2 lone pair.