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Hydrogen has been the essential element in the development of atomic andmolecular physics1). Moving to the properties of dense hydrogen has appeared agood deal more complex than originally thought by Wigner and Hungtinton intheir seminal paper predicting metal hydrogen2): the electrons and the protonsare strongly coupled to each other and ultimately must be treated equally3)4).The determination of how and when molecular solid hydrogen will transform intoa metal is the stepping stone towards a full understanding of the quantum-manybody properties of dense hydrogen. The quest for metal hydrogen has pushedmajor developments of modern experimental high pressure physics, yet thevarious claims of its observation over the past 30 years have remainedcontroversial5)6)7). Here we show a first order phase transition near 425 GPafrom insulator molecular solid hydrogen to metal hydrogen. Pressure in excessof 400 GPa could be achieved by using the recently developed Toroidal DiamondAnvil Cell (T-DAC)8). The structural and electronic properties of dense solidhydrogen at 80 K have been characterized by synchrotron infrared spectroscopy.The continuous vibron frequency shift and the electronic band gap closure downto 0.5 eV, both linearly evolving with pressure, point to the stability of theinsulator C2/c-24 phase up to the metallic transition. Upon pressure release,the metallic state transforms back to the C2/c-24 phase with almost nohysteresis, hence suggesting that the metallization proceeds through astructural transformation within the molecular solid, presumably to the Cmca-12structure. Our results are in good agreement with the scenario recentlydisclosed by an advanced calculation able to capture many-body electroniccorrelations9).

Observation of a first order phase transition to metal hydrogen near 425 GPa