Crystal structure and superconductivity in atomic hydrogen: Deformation between I4₁/amd and Fddd

We investigated crystal structures of solid metallic hydrogen using the potential energy surface trekking for structure search. We applied this technique to a tetragonal I4₁/amd structure at pressures of 500 and 600 GPa and obtained the transformation into multiple orthorhombic Fddd structures, which are formed by distortions in the ab plane of I4₁/amd. The potential barriers are easily surmounted by few trekking steps, which indicates that in solid metallic hydrogen crystal structure is softened with respect to the distortion and is easily fluctuated among the I4₁/amd and Fddd structures. Calculated superconducting critical temperatures show 269 K for I4₁/amd and 263 K for Fddd at 500 GPa. The structures are softened and the electron-phonon coupling are enhanced with pressurization to 600 GPa. As the results, the superconducting critical temperature is increased to 281 K for I4₁/amd, whereas it is decreased to 252 K for Fddd owing to its larger phonon softening than that of I4₁/amd.