Synthesis and Structural Characterization of Single-Crystal K_7.5Si₄₆ and K_17.8Si₁₃₆ Clathrates

Selective single-crystal growth of clathrate-I K_7.5Si₄₆ and clathrate-II K_17.8Si₁₃₆ was achieved from the precursor K₄Si₄ by reaction of the vapor phase with spatially separated graphite in a closed volume under pressure. K_7.5Si₄₆ was prepared at a temperature of 550 °C, and K_17.8Si₁₃₆ was prepared at 625 °C. Structural properties of both compositions were investigated using single-crystal X-ray diffraction. We compare the structural features of K_7.5Si₄₆ and K_17.8Si₁₃₆ with that of Na₈Si₄₆ and Na₂₄Si₁₃₆. A Na_14.7Si₁₃₆ single-crystal was also synthesized and used for comparison with K_17.8Si₁₃₆. K@Si₂₈ was best refined in the center of the Si₂₈ polyhedron for K_17.8Si₁₃₆, in contrast to the off-center refinement of Na@Si₂₈ for Na_xSi₁₃₆.