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 Na8Si46 and Na24Si136. Single-crystal structure refinement for Na8Si46 is reported for the first time. For both Na8Si46 and Na24Si136, 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 Na8Si46 or Na24Si136 which was previously challenging to achieve, this or similar approaches are applicable in the preparation of new compositions from different alkali-metal precursors.