Abstract
The triclinic P1¯ structure of Sr3ZrF10 derives from the fluorite MF2 type by accommodation of the anionic excess (MX2.50) in ZrF8 polyhedra, independent one from another but organized in double columns aligned along [01-1] axis. Contrary to the orthorhombic Cmcm Pb3ZrF10 which also comprises double columns of square antiprisms which are aligned along [001] axis, the successive ZrF8 polyhedra in a column of Sr3ZrF10 take two different orientations in order to decrease the steric constraints resulting from the lower size of Sr2+ compared to Pb2+.In complement, the structure of Pb3ZrF10 is redetermined and the structure of isostructural Pb3HfF10 is refined. A small defect, corresponding to the statistical reorientation of a small proportion of Zr(Hf)F8 polyhedra, in a way similar to that described in Sr3ZrF10, is observed for the first time in both Pb phases but is absent in the isotypic structure of Ba3HfF10 with a higher size Ba2+ cation.The possibility of similar reorientations of the ZrF8 polyhedra in the homologous Pb1-xZrxF2+2x anion excess fluorite solid solution is discussed in relation with the previous models of columnar clusters proposed for this solid solution from neutron diffraction experiments supported by ionic conductivity and 19F NMR measurements of the Pb1-xZrxF2+2x solid solution and of the Pb3ZrF10 ordered superstructure.
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