High-Pressure Evolution of Unexpected Chemical Bonding and Promising Superconducting Properties of YB6

As the prototype of MB6 (M = alkaline-earth and rare-earth metals) compounds, YB6 possesses the highest superconducting critical temperature in this family at the ambient pressure. Here, we performed a first principle exploration on the chemical bonding states and the corresponding electronic properties of YB6 at high pressure. Two phases with Cmcm and I4/mmm space groups are predicted using CALYPSO method, energetically more stable than the previously proposed structures. The B covalent network is eventually evolved from B6 octahedron in the cubic Pm-3m phase to B-24 unit in I4/mmm phase. The calculated electron-phonon coupling parameters show that the contribution of B is significantly increased by the high-pressure effect in Cmcm and I4/mmm structure, contrary to that in Pm-3m structure. Further calculations of electron-phonon coupling indicate that the high-pressure phases are likely superconducting with the major contribution by boron phonon vibration.