Molecular hydrogen is expected to exhibit metallic properties
under megabar pressures. This metal is predicted to be
superconducting with a very high critical temperature, Tc, of
200–400 K (ref. 1), and it may acquire a new quantum state as
a metallic superfluid and a superconducting superfluid2. It may
potentially be recovered metastably at ambient pressures3.
However, experiments carried out at low temperatures,
T < 100 K (refs 4,5), showed that at record pressures of
300 GPa, hydrogen remains in the molecular insulating state.
Here we report on the transformation of normal molecular
hydrogen at room temperature (295 K) to a conductive
and metallic state. At 200 GPa the Raman frequency of
the molecular vibron strongly decreased and the spectral
width increased, evidencing a strong interaction between
molecules. Deuterium behaved similarly. Above 220 GPa,
hydrogen became opaque and electrically conductive. At
260–270 GPa, hydrogen transformed into a metal as the
conductance of hydrogen sharply increased and changed
little on further pressurizing up to 300 GPa or cooling to
at least 30 K; and the sample reflected light well. The
metallic phase transformed back at 295 K into molecular
hydrogen at 200 GPa. This significant hysteresis indicates
that the transformation of molecular hydrogen into a metal is
accompanied by a first-order structural transition presumably
into a monatomic liquid state. Our findings open an avenue for
detailed and comprehensive studies of metallic hydrogen.
