Fig. 1. Proposed (artistic) P–T phase diagram of H₂. Solid phase lines are a combination of static compression studies of solid hydrogen^9–13 and dynamic compression studies of fluid deuterium.^14,15 Dashed lines represent extrapolations of these combined results. The dark brown color of phases III and V at higher pressures suggests closing of the bandgap.

Fig. 2. Artistic representation of the gaseous and solid states of hydrogen under different pressures at room temperature (300 K): (a) gaseous molecular state; (b) phase I, with hcp structure; (c) phase IV, with mixed molecular and atomic state; (d) purely atomic and metallic state.