Hydrogen, one of the most basic and well-studied elements, still has surprises in store. At pressures exceeding several million atmospheres, hydrogen is predicted to become metallic, superconducting, and may even exhibit superfluidity.

In the November 13, 2011 online edition of Nature Materials (DOI: 10.1038/NMAT3175), M.I. Eremets and I.A. Troyan at the Max Planck Institute for Chemistry describe experiments in which molecular hydrogen undergoes transformation to dense hydrogen and then a conductive, metallic state under the megabar pressures exerted by a diamond anvil cell (DAC).

The hydrogen sample first becomes opaque at a pressure of about 220 GPa, and is a semiconductor, as shown by photoconductivity measurements where the samples conduct on illumination with a He-Ne laser (photon energy of 1.96 eV). As the pressure is increased, the width of the bandgap decreases, and the samples can conduct without illumination. Finally, the bandgap closes at an applied pressure of about 270 GPa. Eremets and Troyan propose that a first-order phase transformation to a metallic, monatomic liquid state occurs at that pressure, since the resistance drops precipitously and exhibits little pressure dependence at higher applied pressures. This metallic state was confirmed by cooling the sample down to about 30 K, and noting that the resistance remained low (in contrast with a nonmetal, which insulates at sufficiently low temperatures, because charge carriers cannot be thermally excited across the bandgap). Further proof was obtained from the presence of significant hysteresis in the structure/pressure relationship. On reducing the applied pressure below 270 GPa, a back transformation to molecular hydrogen takes place at about 200 GPa.

Previous DAC experiments have been carried out at low temperatures (~100 K) to mitigate the hydrogen diffusing into and ultimately destroying the diamonds. The current experiments were performed at room temperature (295 K) by covering the diamond culets with diffusion barriers consisting of ultrathin layers of Cu, Au, or alumina. These semitransparent layers also enabled structural changes in the hydrogen to be studied using optical techniques, and Raman spectroscopy confirmed that molecular hydrogen becomes opaque and conducting under pressure and that it ultimately transforms into a monatomic liquid. Troyan and Eremets also noted that new Raman bands appeared in the 220–270 GPa range, which suggests either that the hydrogen retains its Phase I structure until it transforms to a metallic monatomic liquid state, or that it transitions to another phase at 220 GPa.

“Our results on the transformation of normal molecular hydrogen at room temperature to a conductive and metallic state,” said the researchers, “open the door to more comprehensive studies of metallic hydrogen, and further understanding of hydrogen’s phase diagram.”