Synthesis and crystal structures of new mixed-valence terbium (III/IV) fluorides with a random distribution between Tb3+ and Tb4+

The thermal decomposition of terbium tetrafluoride was already used for the synthesis of mixed-valence terbium fluorides such as KTb3F12 or K2Tb4F17. In this paper, new mixed-valence terbium (III/IV) fluorides with formula Rb2AlTb3F16 and RbAl2Tb4F22 with a statistical distribution between Tb(III) and Tb(IV) are presented. The Rb2AlTb3F16 compound which crystallizes in the hexagonal system, space group P63/mmc (n deg. 194) with unit cell parameters a=7.556(6) A and c=12.397(3) A exhibits an uncommon structural unit [Tb3F22]11- resulting from the association of three monocapped square antiprisms TbF9 by sharing edges about a common corner. The RbAl2Tb4F22 compound crystallizes in the tetragonal system, space group I4/mmm (n deg. 139) with unit cell parameters a=10.662(2) A and c=6.911(3) A. The structure is built of chains of edge-sharing TbF8 dodecahedra. These chains arranged in a tetragonal packing are further held together by sharing vertices to form the three-dimensional framework. This polyhedral linking delimits two kinds of channels parallel to the tetragonal c-axis. The first ones, star shaped, are occupied by isolated zigzag chains of trans corner-sharing [AlF6]3- octahedra whereas the second ones accommodate the Rb+ ions in 8-coordination