Under pressure

I vaguely recall some of my undergraduate chemistry lectures with more than Spartan detail. I remember the one about relativity and how it makes mercury liquid and gold gold in color, I mentioned this in a previous comment on the site. I also remember the scent (odor?) worn by our resident sulfur specialist, he was an incredibly enthusiastic and inspirational prof, definitely a good egg, but aromatically speaking not so much. I also remember an inorganic lecture in which the position of hydrogen in the periodic table of the chemical elements was discussed. The lecturer pointed out that in so many ways, despite it being a diatomic gas at ambient temperatures and pressures, hydrogen is actually a metallic element and so should sit atop the alkali metals, lithium, potassium, sodium etc.

So, it was with great interest that I read of recent work that corroborated this notion. More specifically, it supported the 1935 theoretical work by Eugene Wigner and Hillard Bell Huntington who predicted a degenerate electrically conducting phase of hydrogen that should exist under immense pressures. In January 2017, a team from Harvard University led by Isaac Silvera published details in the journal Science of how they had employed the crushing power of a diamond anvil cell to expose hydrogen to almost 500 gigapascals, almost 5 million times atmospheric pressure. At this immense pressure, the diatomic gas, solidified, breaks apart into atomic hydrogen...in real terms forming a metal.

The Harvard Crimson reports Silver's discovery: "As we were pressing [the diamonds], we found that at lower pressures the sample is transparent," Silver said. "When you get to about 4 million atmospheres it turns black and we've seen [black hydrogen] before. We continued to turn the pressure up and suddenly it became lustrous, reflecting like metal."

However, almost as soon as the paper was published and the popular science news media had grabbed the press release and turned out their metallic hydrogen headlines, the scientific skeptics were out in force, suggesting that the finding was flawed. They picked holes in the approach and the details. Not least they pointed to the fact that this isn't a real metallic element, it is simply a meta-stable phase. Mikhail Eremets, who also studies the concept of metallic hydrogen at Max Planck Institute for Chemistry in Germany was more than a little worried that there was insufficient data in the paper to warrant the claims pointing out that the original team had not replicated their own experiment.

Meanwhile, another amazing piece of science was bubbling up to the surface. Artem Oganov and Xiao Dong of State University of New York and their colleagues had also used a diamond anvil to press helium, that most inert of chemical elements, to react with sodium. I reported on this discovery elsewhere. Oganov had plenty to tell me about the importance of making the first compound containing helium, Na₂He and the possibility of Na₂HeO. There are implications for helium storage, given how this gas is critical to medicine (cooling the magnets in MRI machines, for instance). There are also implications for understanding of the evolution of stars and gas giant planets. There might also be insights into how bubbles form in alloys used in the nuclear power industry.

With that item written, I was also intrigued to hear Oganov's thoughts on the metallic hydrogen "discovery", whether it were valid or not. "I tend to believe that report on metallic hydrogen," he told me. "They presented only minimal data, but intuitively I tend to accept their claims." He adds that, "Actually, there is a general conclusion that at sufficient pressure all matter should transform to nearly free-electron like metals. Hydrogen is no exception - so it's no wonder that it will become metallic at some pressure."

As ever in science, it's a slow iterative process with occasional flurries of activity. The so-called "breakthroughs" come and go, of course. Sometimes they are replicated by independent laboratories and persist, sometimes they are not. The nature of the popular science headline is that the amazing discovery will get all the exposure, any subsequent work whether corroborating or retracting the original claim will likely not. It will be interesting to see whether another team confirms my memory of undergraduate chemistry back in the day. I have my fingers crossed; it is a fascinating notion...metallic hydrogen. No pressure, folks!

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".