Na 2 B 6 Si 2 : A Prototype Silico-boride with Closo (B 6 ) 2- Clusters.

The compound Na 2 B 6 Si 2 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 R 3̅ m , Pearson symbol hR 30, a = 5.0735(1) Å and c = 16.0004(7) Å). The atomic arrangement consists of a unique framework formed by electron-precise octahedral closo (B 6 ) 2- clusters connected via ethane-like (Si 2 ) 0 dumbbells. The Na + cations occupy cavities in the hierarchical variation of a Heusler-type framework. The balance (Na + ) 2 ([B 6 ] 2- )(Si 0 ) 2 reveals an electron precise Zintl-Wade phase, which is in line with electronic band structure calculations predicting semiconducting behavior.