SYNTHESIS AND CHARACTERIZATION OF 2 NEW ORDERED CRYSTALLINE PHASES CLOSE TO THF4-ZRF4 FLUORIDE GLASSES .1. CRYSTAL-STRUCTURE OF THZRF8

A study of the ThF4-ZrF4 system reveals the presence, besides an alrea dy known vitreous domain, of two stoichiometric crystalline phases. Bo th are monoclinic, ThZrF8 [space group: P2(1)/c, a= 8.232(1) Angstrom, b= 8.510(1) Angstrom, c= 16.355(2) Angstrom, beta= 100.79(1)degrees; Z= 8], and ThZr2F12 [(non-conventional) space group: I2/m, Im or I2; a = 9.895(2) Angstrom, 10.488(2) Angstrom, c= 7.856(1) Angstrom, beta= 9 0.73(1)degrees, Z= 4]. Their composition closely corresponds to the li mits of the vitreous domain. The unit cell of ThZr2F12 presents close analogies with PrZr2F11 and beta-ZrF4 ones. The crystal structure of T hZrF8 is investigated by X-ray diffraction on a single crystal measure d on a Siemens P4 diffractometer with MoK alpha radiation [refinement: Shelxl-93 program, structure solution by direct methods: Shelxs-86 pr ogram, R(1) = 0.040, wR(2) = 0.087 for all reflections]. It correspond s to a 3d-framework of corner-shared ThF9 and ZrF7 polyhedra. The stru cture of ThZrF8 derives from alpha-ZrF4 structure-type by a long-range ordering of the ThF9 and ZrF7 polyhedra replacing ZrF8 dodecahedra. T he topological transformations between the cationic plane nets of alph a-ZrF4 and ThZrF8 occur through structural mechanisms (e.g.: anionic b ridging) similar to those previously described in the so-called << ani on-excess ReO3-related phases >>.