Simple Approach for Selective Crystal Growth of Intermetallic Clathrates

We report on a new method for the synthesis of single-crystal inter-metallic clathrates. Alkali-metal is slowly removed from an alkali-silicide precursor by reaction of the vapor phase with spatially separated graphite, in a closed volume under uniaxial pressure, to form single-crystals of the binary intermetallic clathrates Na 8 Si 46 and Na 24 Si 136 . Single-crystal structure refinement for Na 8 Si 46 is reported for the first time. For both Na 8 Si 46 and Na 24 Si 136, full occupation of all Si framework sites as well as full Na occupancy in both polyhedra was observed. In addition to comprising a simple method for selective, phase-pure crystal growth of clathrates such as Na 8 Si 46 or Na 24 Si 136 which was previously challenging to achieve, this or similar approaches are applicable in the preparation of new compositions from different alkali-metal precursors.