Abstract
Metallic hydrogen has been a major issue in physical chemistry since its prediction in 1935. Its predicted density implies 100 GPa (106 bar = Mbar) pressures P are needed to make metallic H with the Fermi temperature TF = 220 000 K. Temperatures T can be several 1000 K and still be "very low" with T/TF ≪ 1. In 1996, metallic fluid H was made under dynamic compression at P = 140 GPa and calculated T ≈ 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 H2 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.
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