Solid metal hydrogen announcement makes global headlines

Have a pair of Harvard scientists created the most powerful form of energy storage known to humanity?

Hydrogen hit the headlines this weekend, as Harvard researchers Ranga Dias and Isaac Silvera published a paper in Science claiming to have transmuted hydrogen into a solid metal (Observation of the Wigner-Huntington transition to metallic hydrogen). The pair first announced their discovery in October last year, but the full details have not been made available until now and the breakthrough is causing quite a stir.

Researchers have been attempting to produce solid metal hydrogen since it was first theorised in 1935 (E. Wigner, H. B. Huntington, On the possibility of a metallic modification of hydrogen. J. Chem. Phys. 3, 764–770 (1935)). Silva and Dias claim to have at last achieved success by slowly ratcheting up the pressure in a diamond vice to 495 GPa, 50% higher than the pressure in the centre of the Earth. Under these conditions their team observed the material changing from transparent to black to a shiny red; evidence enough for a metallic solid, according to their paper.

There is nothing new in submitting hydrogen to extreme pressure, but Silva and Dias believe they succeeded where others failed by cutting back on high-intensity laser spectroscopy, which can destroy the diamond or the hydrogen it is trained on. Instead they initially used a low intensity laser to avoid damaging the sample:

For fear of diamond failure due to laser illumination and possible heating of the black sample, we only measured the Raman active phonon at the very highest pressure of the experiment (495 GPa) after the sample transformed to metallic hydrogen and reflectance measurements had been made.

The potential for metallic hydrogen could be huge, as it is predicted to be a room-temperature superconductor which could revolutionise materials science. Its potential for storing energy could also be phenomenal. In a previous paper, Silvera suggested that hydrogen compressed to a metal could pack so much energy that it could be ‘The Most Powerful Rocket Fuel Yet to Exist’.

Much of this potential depends on whether or not metallic hydrogen is metastable and would retain its solid form once extreme pressure was released. As it stands, the paper offers no answer to this question. Having reached the critical pressure required to create their sample, the team have not yet modified their set-up for fear of destroying the sample. This has left a lot of questions unanswered – is it really a solid? Is it stable?

Big claims require big evidence, and the team has come in for criticism from several quarters for a lack of follow-through on their experiment. Science’s online announcement of the news gave rise to the kind of heated comments threads usually found on political news reports. Nonetheless, Silver and Dias stand by their results, saying that they wanted to announce the news before a second-round of tests potentially destroy their sample. ‘If people disagree, they should go to measure it and try to show that it’s different than what was claimed’, Silvera suggested.

Teams across the world will undoubtedly be throwing themselves into that very task, so we can expect more news on this subject as the year unfolds. If nothing else, the Harvard group have our attention.