Theoretical considerations of superconducting HfBH2 and HfB2H under high pressure

AlB2-type metal diborides represented by MgB2 become an important type of conventional superconductors. However, AlB2-type HfB2 is not superconductive. Pressure-induced HfH10 are predicted to be high-temperature superconductors. Here, first-principles swarm-intelligence structural search calculations identified two ternary compounds, HfBH2 and HfB2H, showing superconductivity with the predicted superconducting transition temperatures of 2.1–14.9 and 2.2–15.0 K for a wide range of Coulomb pseudopotential μ* = 0.15–0.05 at 300 GPa, respectively. The two compounds exhibit interesting structural features, such as an Hf–H layer, covalent B–H framework, and graphene-type boron layer, as observed in AlB2-type HfB2. Besides atomic hydrogen and covalent skeleton contributing to superconducting transition, H-activated hafnium also plays a certain role.