STRUCTURE REFINEMENT OF TRIVALENT THALLIUM FLUOROZIRCONATES

In the TlF3-ZrF4 SYSteM, two phases have been detected : a cubic one formulated Tl0.67Zr0.33F3.33 and crystallizing with the Pm3mBAR space group (a = 4.061(1) angstrom) and a rhombohedral one, TlZr3F15 (R3cBAR space group with a = 12.196(1) angstrom, c = 11.161(1) angstrom). The structures have been refined using the Rietveld method. The refinement converges to R(l) = 0.038 and 0.042 for Tl0.67Zr0.33F3.33 and TlZr3F15 respectively. The structures derive from the ReO3-type by anionic insertion. In Tl0.67Zr0.33F3.33 this mechanism involves two anion positions: one, F1, is slightly shifted from the ideal ReO3 anionic site and the other, F2, is displaced further away. The anion-excess may be explained by a substitution of some F1 atoms by F2-F2 pairs. The anionic polyhedra are constituted of pentagonal bipyramids and distorted octahedra. The anion-excess in TlZr3F15 results from the insertion of fluorine atoms on the two opposite faces of the unit cell of the ReO3 prototype. This structure can be described as a three-dimensional framework of corner-sharing dodecahedra and distorted octahedra.