Synthesis and crystal structure of sodium borosilicide, Na8B74.5Si17.5

Abstract

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.