This week Dias and Silvera have reported the discovery of metallic hydrogen in the journal Science and not without controversy (DOI). A diamond anvil cell was used to compress hydrogen gas at cryogenic temperatures and a record-high pressure. Problems to overcome were diamond tip failure and hydrogen diffusion into the diamond tip.Solutions: really low operating temperature, special treatment of the diamond tip and coating it with alumina. The laser used for illumination and for measuring the pressure inside the anvil was of low power in order to minimise damage to the sample. At 335 GPa the pressure was increased further by manually rotating the screw and at 495 GPa the hydrogen sample (8 by 10 by 1 micron in volume) as viewed through a microscope turned from black to reflecting, indicating metallic properties.
But there is controversy. Davide Castelvecchi in a Nature article sums up the criticism. Pressure miscalculated, the actual material observed is not hydrogen metal but the alumina. Most of all, the experiment is apparently only performed once, Dias and Silvera have been holding on to their precious sample in the anvil for several months until their publication was out. Well, as of this week they are back in business. The really interesting premise is that with heating up and de-pressurising the anvil, the hydrogen sample is predicted to be metastable and metallic properties should be retained. To be continued!
Update 23-02-2017: crap, the sample disappeared (The Independent article)