Sr3ZrF10, a new type of anion-excess fluorite superstructure. Comparison with Pb3ZrF10 and Pb3HfF10. Influence on defect structure models of M1-xZrxF2+2x fluorite solid solutions

The triclinic P 1 ¯ structure of Sr 3 ZrF 10 derives from the fluorite MF 2 type by accommodation of the anionic excess (MX 2.50 ) in ZrF 8 polyhedra, independent one from another but organized in double columns aligned along [01-1] axis. Contrary to the orthorhombic Cmcm Pb 3 ZrF 10 which also comprises double columns of square antiprisms which are aligned along [001] axis, the successive ZrF 8 polyhedra in a column of Sr 3 ZrF 10 take two different orientations in order to decrease the steric constraints resulting from the lower size of Sr 2+ compared to Pb 2+ . In complement, the structure of Pb 3 ZrF 10 is redetermined and the structure of isostructural Pb 3 HfF 10 is refined. A small defect, corresponding to the statistical reorientation of a small proportion of Zr(Hf)F 8 polyhedra, in a way similar to that described in Sr 3 ZrF 10 , is observed for the first time in both Pb phases but is absent in the isotypic structure of Ba 3 HfF 10 with a higher size Ba 2+ cation. The possibility of similar reorientations of the ZrF 8 polyhedra in the homologous Pb 1-x Zr x F 2+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 19 F NMR measurements of the Pb 1-x Zr x F 2+2x solid solution and of the Pb 3 ZrF 10 ordered superstructure. Double atomic fluorite layer in the crystal structure of Sr 3 ZrF 10 , showing the various orientations of columns of ZrF 8 bicapped trigonal prisms. • Sr 3 Zr 3 F 10 structure is a new triclinic type of anion-excess fluorite superstructures. • It derives from orthorhombic Pb 3 ZrF 10 by an ordered reorientation of some ZrF 8 polyhedra. • Pb 3 ZrF 10 structure is re-refined and isotypic Pb 3 HfF 10 structure is determined. They contain a small defect corresponding to a reorientation of some MF 8 polyhedra as in Sr 3 Zr 3 F 10 . • orthorhombic Ba 3 HfF 10 and likely Ba 3 ZrF 10 do not present this small defect. • The reorientations of the MF 8 polyhedra allow to better understand the defect structure and the properties of the disordered cubic fluorite M 1-x Zr(Hf) x F 2+2x . solid solutions.