Exploration of stable compounds, crystal structures, and superconductivity in the Be-H system

Using first-principles variable-composition evolutionary methodology, we explored the high-pressure structures of beryllium hydrides between 0 and 400 GPa. We found that BeH₂ remains the only stable compound in this pressure range. The pressure-induced transformations are predicted as I b a m → P 3 ¯ m 1 → R 3 ¯ m → C m c m → P 4 / n m m, which occur at 24, 139, 204 and 349 GPa, respectively. P 3 ¯ m 1 and R 3 ¯ m structures are layered polytypes based on close packings of H atoms with Be atoms filling all octahedral voids in alternating layers. Cmcm and P4/nmm contain two-dimensional triangular networks with each layer forming a kinked slab in the ab-plane. P 3 ¯ m 1 and R 3 ¯ m are semiconductors while Cmcm and P4/nmm are metallic. We have explored superconductivity of both metal phases, and found large electron-phonon coupling parameters of λ = 0.63 for Cmcm with a T_c of 32.1-44.1 K at 250 GPa and λ = 0.65 for P4/nmm with a T_c of 46.1-62.4 K at 400 GPa. The dependence of T_c on pressure indicates that T_c initially increases to a maximum of 45.1 K for Cmcm at 275 GPa and 97.0 K for P4/nmm at 365 GPa, and then decreases with increasing pressure for both phases.