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.

Pressure experiment offers tantalising glimpse of metallic hydrogen

For some fuel-cell manufacturers, compressing hydrogen to a liquid has become routine. But for decades, materials scientists have dreamed of going further and forcing hydrogen into a solid metal phase. Metal hydrogen is believed to exist at the heart of gas giant planets like Saturn and Jupiter, and if replicated on Earth could form the basis for high-temperature superconductors.

Superconducting materials have traditionally had to be kept extremely cold, but since the 1970s scientists have theorised that, under extreme pressure conditions, hydrogen coupled with alkali metals could form a stable superconductor. Now a team of researchers under Viktor Struzhkin at the Carnegie Institution of Washington claim to have confirmed those predictions.

The research is laid out in Nature Communications. The team mixed hydrogen and sodium samples in a diamond anvil cell, then subjected them to extreme pressures of up to 70 GPa. On top of that, the team used lasers to superheat the samples. At 2100 degrees Kelvin, instruments indicated a series of chemical reactions that further heated the area to 4000-6000 degrees Kelvin.

In the aftermath of this reaction, the team were able to observe polyhydride structures of NaH3 and NaH7. In the latter, three-atom hydrogen chains formed, which would be key to future use of hydrogen as a superconducting material. The NaH7 became thermodynamically stable at a balmy 300 degrees Kelvin.

Of course, in terms of superconducting, this research merely replaces a temperature with a pressure problem. Nonetheless, as well as firming up theories of planetary formation, this research could have practical repercussions on the use of hydrogen as a material and energy carrier in future.

You can read the full paper, ‘Synthesis of sodium polyhydrides at high pressures’ at Nature Communications.