Metallic superhard CaB12 with novel waffle-like boron backbone

Combining features of complex bonding scheme and multifarious structures, metal borides have received extensive attention as appealing contenders for novel superhard materials with superior electrical conductivity. Based on first-principles calculations, we predicted a metallic superhard boride CaB12, consisted by a high-density B−B covalent sublattice arranged in waffle-like B32 clusters. This special B−B arrangement results in a hitherto maximum hardness value (Hvmax) of 50.1 GPa in binary metal borides. Since the Ca atoms provide electrons to the B32 cluster, it is beneficial to generating conductive channel in 3D boron network, playing a dominant role in conductivity for CaB12. Through the electronic regulation by substituting the alkaline atom at the Ca positions, a superconducting superhard compound Ca2KB36 (Tc = 16.9 K; Hv = 44.1 GPa) can be achieved. We then propose three strategies to design materials with superhard and excellent conductivity/superconductivity properties: (1) the strong average covalent bond strength; (2) the high density of covalent bonds; (3) with appropriate metal atoms to produce carriers. Our work provides innovative way to search for novel ultrahard compounds with favorable electrical conductivity/superconductivity.