A Perspective on Hydrogen Near the Liquid-Liquid Phase Transition and Metallization of Fluid H

Metallic hydrogen has been a major issue in physical chemistry since its prediction in 1935. Its predicted density implies 100 GPa (10(6) bar = Mbar) pressures P are needed to make metallic H with the Fermi temperature T-F = 220 000 K. Temperatures T can be several 1000 K and still be "very low" with T/T-F << 1. In 1996, metallic fluid H was made under dynamic compression at P = 140 GPa and calculated T approximate to 3000 K generated with a two-stage light-gas gun. Those T's place metallic H in the liquid-liquid phase transition region. The purpose of this Perspective is to place the phase curve measured in laser-heated diamond anvil cells in context with those measured electrical conductivities. That phase curve is probably caused by dissociation of H-2 to H starting near 90 GPa/1600 K. Metallic H then forms in a crossover as a semiconductor up to 140 GPa/3000 K. Dynamic quasi-isentropic pressure was tuned to make metallic H by design in those conductivity experiments.