The Metallic Hydrogen Wigner–Huntington transition is only a piece of the story, – the problem of high pressure hydrogen metallization is much more interesting than we thought just a few years ago, with intriguing intermediate phases with unexpected physical properties that continue to be discovered along the way.
In February 2017, it was reported that the sample of claimed metallic hydrogen (at Harvard) was lost, after the diamond anvils it was contained between broke.
High pressure researcher Paul Loubeyre of the Atomic Energy Research Centre for Military Applications in France, calls the new results ‘much ado about nothing’. In particular, he sees no reason to believe that Dias and Silvera really achieved such high pressures. Earlier work, he says, has shown that diamond anvil cells can’t attain pressures higher than 350GPa. ‘It has been demonstrated that to explore the pressure domain above 400GPa will require new types of anvils,’ he says. ‘If you’re going to claim that using a standard high pressure cell configuration you can obtain 50% higher pressure than previously, you should at least do a benchmark experiment [to prove it].’
Loubeyre also believes that ‘the analysis of reflectivity is wrong – the correction for diamond absorption was incorrectly done’. He and his co-workers have explained their reservations in detail in a preprint.
Eremets and his Mainz colleague Alexander Drozdov also say the pressure was overestimated. They say that the Harvard duo stopped raising the pressure as soon as the sample became reflective – which can happen, according to earlier measurements by the Mainz group, above just 350GPa. In a third preprint criticising the Harvard work4, Alexander Goncharov and Viktor Struzhkin of the Carnegie Institution of Washington reiterate several of these criticisms and say that the observations ‘have nothing to do with the properties of metallic hydrogen’. They also charge that ‘the proposed implications [for example, for “energy and rocketry”] are highly speculative, making this paper very confusing for a broad audience’.
Silvera and Dias have rebutted these criticisms. They say that their methods of estimating the pressure by looking at the shift in the vibrational spectra of the diamond and of a small added ruby grain are widely used and well established, although they had stated that they ‘can have large systematic uncertainties’. They defend their calculations of reflectance and argue that the possibility of the alumina coating becoming metallic doesn’t fit with what is currently known about this material. ‘There is no doubt that metallic hydrogen was produced at the highest pressures,’ the pair insist.