Crystalline diborane at high pressures

High-pressure ground-state phases of crystalline diborane (B ₂H₆) and their stability against decomposition into B and H are investigated by calculations within density-functional theory. Although B₂H₆ is thermodynamically unstable to phase separation into B and H in the intermediate pressure range, it is restabilized beyond 350 GPa. The candidate structures are then Pbcn and Cmcm, though at the level of the harmonic approximation the latter has some imaginary-frequency phonons. Both structures are metallic with quite high density of states at the Fermi energy. An estimate of superconducting transition temperature T_c is carried out for the Pbcn structure by using the extended McMillan formula, and the resulting T_c reaches around 100 K.