Fabrication of alkali metal boride. Honeycomb-like structured NaB${}_4$ with high hardness and excellent electrical conductivity

The design and fabrication of novel hard materials with excellent electrical conductivity is attractive in scientific and technological application under extreme conditions. Metal borides have brought substantial interest for decades in material science because of their strong covalent BB bonding network for intrinsic incompressibility and MM bonding for electron transportation. Here the successful synthesis of a novel hard alkali metal boride as NaB${}_4$ with high thermal stability (873 K) and metallic behaviors is reported. The relatively low synthetic P/T conditions (lowest at 1.5 GPa and 1000 K) enable the easy fabrication of large bulk materials (several centimeters in diameter). The studies reveal that the Vickers hardness value of NaB${}_4$ can reach up to 26 GPa, associated with superior incompressibility along (001) direction of honeycomb-like boron structure that exhibits the highest shear modulus up to 96 GPa for borides. The NaB${}_4$ structure undergoes an interesting metallic-semiconducting transition under compression up to 61 GPa. This new form of hard metal boride material with pressure-tunable electrical properties enables the development of industrial applications as future electrical devices. (© 2021 Wiley‐VCH GmbH)