Pressure-induced evolution of structures and promising superconductivity of ScB6.

Abstract

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.