Hard Copper Boride with Exceptional Conductivity

Copper and boron seldom engage in reaction at ambient pressure. The few reports on copper-doped boron compounds that exist in the literature often lack definitive stoichiometry. Here, we report successful synthesis of Cu₂-⏓B₂₅ single crystals (δ∼0. 03, indicating Cu understoichiometry) via a high-pressure melting method using copper and β-rhombohedral boron as precursors. Crystals thus synthesized are characterized by a tetragonal boron sublattice, within which Cu atoms are either partially or fully situated at different interstices between B₁₂ icosahedra. The crystals possess a high Vickers hardness of 26. 5 GPa and an unusually high electrical conductivity of 1. 19×10^5 S/m-the highest conductivity among the icosahedron-based borides. Hall measurements reveal a notable p-n conduction type transition around 30 GPa. This transition, alongside the remarkable conductivity, is potentially modulated by the copper content and its valence states within the structure. The synthesis of Cu₂-⏓B₂₅ not only broadens the spectrum of hard materials but also opens new avenues for innovative modulation of electronic properties in boron-rich compounds, with promising technological implications.