Synthesis and crystal structure of sodium borosilicide, Na ₈B_74.5Si_17.5

Ternary sodium borosilicide, Na₈B_74.5Si _17.5, was newly synthesized by heating a mixture of sodium, silicon and amorphous or crystalline boron at 1073-1273 K. The crystal structure of the black hexagonal prismatic single crystal obtained at 1273 K was analyzed. The X-ray diffraction reflections of the crystal were indexed with hexagonal cell parameters a = 10.2392(3) Å and c = 10.9215(4) Å (space group P6₃/mmc, No. 194). The structural formula could be represented as Na₈(B₁₂)₆Si₁₆[BSi] _1.5[B₂]_0.5. B₁₂ icosahedra form a three-dimensional framework having small triangular and large hexagonal channels along the c axis. Chains of [-Si-(Si-Si)₃-Si-] surrounded by Na atoms are located in the large channels. Trigonal prism cages for Na atoms and for atom pairs of Si-B or B-B are alternately arrayed in the small channels. The ratio of Si-B and B-B pairs in the cage is around 3:1.