NaBSi: A Prototype Silico-boride with Closo (B) Clusters.

The compound NaBSi was synthesized under high-pressure, high-temperature conditions at pressures ranging from 6 to 9.5 GPa and temperatures from 1070 to 1270 K before quenching to room temperature followed by slow decompression. The crystal structure was determined from microcrystals using precession-assisted electron diffraction tomography, validated by dynamical refinement and full-profile refinements using optimized coordinates from quantum chemical calculations (space group 3̅, Pearson symbol 30, = 5.0735(1) Å and = 16.0004(7) Å). The atomic arrangement consists of a unique framework formed by electron-precise octahedral closo (B) clusters connected via ethane-like (Si) dumbbells. The Na cations occupy cavities in the hierarchical variation of a Heusler-type framework. The balance (Na)([B])(Si) reveals an electron precise Zintl-Wade phase, which is in line with electronic band structure calculations predicting semiconducting behavior.