RbLu[SiO]F, a tubular chain silicate

Single crystals of RbLu[SiO]F (dirubidium lutetium tetrasilicate fluoride) were obtained in flux-synthesis experiments in the system SiO-LuO-RbF. The compound belongs to the group of tubular chain silicates, i.e. it is based on multiple chains of condensed [SiO4] 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 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.