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 R3m, Pearson symbol hR30, a = 5.0735(1) & Aring; and c = 16.0004(7) & Aring;). The atomic arrangement consists of a unique framework formed by electron-precise octahedral closo (B6)(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-))(Si0)(2) reveals an electron precise Zintl-Wade phase, which is in line with electronic band structure calculations predicting semiconducting behavior.