Ultrastrong Boron Frameworks in ZrB₁₂: A Highway for Electron Conducting

The elastic/plastic and electronic properties of single crystal and polycrystal ZrB₁₂ have been studied by experiments and first principle simulations. Under the load of 0.25 and 0.49 N, the Vickers hardness of polycrystalline ZrB₁₂ exceeds 40 GPa, suggesting the very hard property at small load. Under the load of 4.9 N, the measured Vickers hardness decreases and saturates at =27.0 GPa, which is slightly higher than the well-known ReB₂, WB₃. Revealed by first principle simulations, the ideal shear strength of ZrB₁₂ can be as high as 34.5 GP because the frameworks are constructed with very high symmetrical B-B clusters. In addition to the high strength, ZrB₁₂ also exhibits superior metallic behavior with ultralow electrical resistivity =18 μω.cm, and Seebeck coefficient =2.0 μV.K^-1 (RT) both of which are comparable to that of Pt. First principle simulations show that the extensive B-B covalent network in ZrB₁₂ can form delocalized π-bonds, which generate extensive conducting channels for valence electrons by contacting Zr 4d orbitals.