Rb(2)Lu[Si(4)O(10)]F, a tubular chain silicate

Single crystals of Rb(2)Lu[Si(4)O(10)]F (dirubidium lutetium tetra­silicate fluoride) were obtained in flux-synthesis experiments in the system SiO(2)–Lu(2)O(3)–RbF. The compound belongs to the group of tubular chain silicates, i.e. it is based on multiple chains of condensed [SiO(4)] tetra­hedra forming closed columns. The periodicity of the unbranched multiple chains is four and corresponds to the length of the b axis. Adjacent columns are connected by Lu(3+) cations, which are coordinated by four oxide and two fluoride anions in the form of slightly distorted octa­hedra. By sharing common fluoride corners, the single octa­hedra are linked into chains running parallel to the silicate tubes. Electroneutrality is achieved by the incorporation of additional Rb(+) cations. All four symmetrically independent rubidium ions, four out of twelve oxide as well as the two fluoride anions are located on mirror planes. The remaining atoms reside on general positions.