Crystalline diborane at high pressures

High-pressure ground-state phases of crystalline diborane (B₂H₆) 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 Tc is carried out for the $Pbcn$ structure by using the extended McMillan formula, and the resulting Tc reaches around 100 K.