Na₂B₆Si₂: A Prototype Silico-boride with Closo (B₆)^2- Clusters

The compound Na₂B₆Si₂ 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 R3̅m, Pearson symbol hR30, 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 (B₆)^2- clusters connected via ethane-like (Si₂)⁰ dumbbells. The Na⁺ cations occupy cavities in the hierarchical variation of a Heusler-type framework. The balance (Na⁺)₂([B₆]^2-)(Si⁰)₂ reveals an electron precise Zintl-Wade phase, which is in line with electronic band structure calculations predicting semiconducting behavior.