SYNTHESIS AND CHARACTERIZATION OF 2 NEW ORDERED CRYSTALLINE PHASES CLOSE TO THF4-ZRF4 FLUORIDE GLASSES .2. CRYSTAL-STRUCTURE OF THZR2F12

ThZr2F12 crystallizes in the non-conventional space group: I-2/m with the cell parameters a = 9.895(2) Angstrom, 10.488(2) Angstrom, c= 7.85 6(1) Angstrom, beta= 90.73(1)degrees, Z= 4. Its crystal structure is i nvestigated by powder X-ray diffraction from reflections recorded on a Siemens D5000 diffractometer with CuK alpha radiation [Rietveld refin ement: Fullprof program; structure solution by direct methods: Shelxs- 86 program; R(B) = 0.039, R(p) = 0.064, R(WP) = 0.090]. The crystal st ructure of ThZr2F12 results from the stacking of alternate sheets of o n one hand, ThF9 tricapped trigonal prisms, on the other hand, corner- and edge-shared ZrF7 and ZrF8 monocapped and bicapped trigonal prisms . It closely derives from beta-ZrF4 and PrZr2F11 structure-types. As t hese last phases and as ThZrF8, it can be used as a structural model f or the homologous fluoride glasses: the analogy between PrZr2F11 and T hZr2F12 structures strongly suggests the similarity of the structure o f the fluoride glasses isolated in the LnF(3)-ZrF4 and ThF4-ZrF4 syste ms. These glasses appear as structurally the closest to the ideal << Z rF4 >> one, with a moderate modifying role for the Th and Ln cations. As already proposed, the presence of similar cationic frameworks in ma ny glassforming fluorozirconates evidences the strong influence of the se frameworks an the existence and the stability of the homologous gla sses.