Pressure-induced evolution of structures and promising superconductivity of ScB₆

Unique multicenter bonding in boron-rich materials leads to the formation of complicated structures and intriguing properties. ScB₆, as a sister compound, possibly possesses high hardness and superconducting critical temperature in this family under ambient pressure. Here, phase transitions, chemical bonding states and electronic properties of ScB₆ at high pressure are uncovered using particle swarm optimization (PSO) combined with first-principles calculations. The phase sequence of P2₁/m → C2/m → Cmcm for ScB₆ has been identified under high pressure. Interestingly, the evolution of a boron framework is from a graphene-like layer to a planar B₄ ring, B₆ and B₇ cycle, and non-planar B₈ cycle, which interconnect a graphene-like network. These phases of ScB₆ are expected to be hard materials due to the excellent mechanical behaviors by the mechanical property calculations. Although the metallic features of the three phases reduce their hardness, the further electron-phonon coupling calculations indicate that the three phases of ScB₆ are superconducting phases under high pressures.