Crystal structure of RbAl 2Tb 4F 22: a second example of mixed-valence fluoroterbate with a random distribution of Tb 3+ and Tb 4+ ions

Abstract

A new mixed-valence (III/IV) terbium fluoride RbAl 2Tb 4F 22 was obtained by direct fluorination of the stoichiometric mixture 1 RbCl + 2 AlF 3 + 4 TbF 4 at 650 °C under pure fluorine gas flow. The structure of RbAl 2Tb 4F 22 has been determined from single-crystal X-ray diffraction data and refined to a conventional R = 0.020 ( R ω = 0.023 ) from 676 independent reflections (29 parameters). This compound crystallizes in the tetragonal system, space group I 4 / m m m (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 TbF 8 dodecahedra surrounding the Tb 3+ and Tb 4+ 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 [AlF 6] 3− octahedra linked to four Tb chains by sharing equatorial fluorines. A structural relationship between RbAl 2Tb 4F 22 and BaY 2F 8 has been established.