Hard Copper Boride with Exceptional Conductivity
Ming-Xing Huang, Changzeng Fan, Bo Xu, Jingyu Hou, Xi Shao, Xiao‐Ji Weng, Xiang Zhang, Wentao Hu, Yufei Gao, Lin Wang, Zhisheng Zhao, Guochun Yang, Dongzhou Zhang, Yanbin Wang, Zhongyuan Liu, Xiang‐Feng Zhou, Yongjun Tian
Abstract
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 ${\mathrm{Cu}}_{2\ensuremath{-}\ensuremath{\delta}}{\mathrm{B}}_{25}$ single crystals ($\ensuremath{\delta}\ensuremath{\sim}0.03$, indicating Cu understoichiometry) via a high-pressure melting method using copper and $\ensuremath{\beta}$-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 ${\mathrm{B}}_{12}$ icosahedra. The crystals possess a high Vickers hardness of 26.5 GPa and an unusually high electrical conductivity of $1.19\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{ }\mathrm{S}/\mathrm{m}$---the highest conductivity among the icosahedron-based borides. Hall measurements reveal a notable $p\text{\ensuremath{-}}n$ 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 ${\mathrm{Cu}}_{2\ensuremath{-}\ensuremath{\delta}}{\mathrm{B}}_{25}$ 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.