Crystal structure of RbAl2Tb4F22: a second example of mixed-valence fluoroterbate with a random distribution of Tb3+ and Tb4+ ions

A new mixed-valence (III/IV) terbium fluoride RbAl2Tb4F22 was obtained by direct fluorination of the stoichiometric mixture 1 RbCl + 2 AlF3 + 4 TbF4 at 650 °C under pure fluorine gas flow. The structure of RbAl2Tb4F22 has been determined from single-crystal X-ray diffraction data and refined to a conventional R=0.020(Rω=0.023) from 676 independent reflections (29 parameters). This compound crystallizes in the tetragonal system, space group I4/mmm (No. 139) with a=10.662(2)Å and c=6.911(3)Å, Z=2. The structure is built of chains of edge-sharing TbF8 dodecahedra surrounding the Tb3+ and Tb4+ ions randomly distributed over a unique crystallographic site. These chains arranged in a tetragonal packing are further held together by sharing vertices to form the three-dimensional framework which encompasses two kinds of channels parallel to the c tetragonal axis. The first ones accommodate the Rb+ ions in 8-coordination. The second ones are occupied by isolated zigzag chains of trans-corner-sharing [AlF6]3− octahedra linked to four Tb chains by sharing equatorial fluorines. A structural relationship between RbAl2Tb4F22 and BaY2F8 has been established.