The quest to make metallic hydrogen has been long and arduous. Victory has been declared several times, and sometimes retracted. Scientists predicted hydrogen’s metallic state—which could be useful as a superconductor, rocket fuel, and more—almost 85 years ago, but it wasn’t until the 2010s that research groups announced results that looked to put the field on the long-sought material’s doorstep. In the opening weeks of a new decade, another team has reported they have made a metallic form of hydrogen using a new experimental design (Nature 2020, DOI: 10.1038/s41586-019-1927-3).
Like other elements, hydrogen should become a metal under high pressures, when its electrons delocalize and move freely through a matrix of protons. But getting to almost 5 million times ambient pressure has made experimental attempts difficult . Researchers have had success with diamond anvils, made from carefully-produced synthetic diamonds with flattened tips between which small amounts of hydrogen get squeezed.
In the new study, Paul Loubeyre of the French Alternative Energies and Atomic Energy Commission and colleagues used an anvil with a new type of squeezing surface—a ring-shaped depression carved with an ion beam—and measured changes in their hydrogen sample’s infrared absorption. Above 400 GPa, Loubeyre’s group measured IR spectra indicating that the hydrogen sample had become metallic, which aligns with theoretical predictions.
The research team and others agree that the results are most consistent with a metallic state of diatomic hydrogen molecules, rather than a metal of hydrogen atoms, which is the state with the most-sought-after properties. To show that the measurements weren’t the result of the diamond anvil’s deformation—an issue in some previous studies—the researchers reported data suggesting that the changes also reverse as the pressure decreases.
The research has received measured praise from experts in the field, many of whom could be described as the Loubeyre group’s competitors or collaborators—or both—in the race to make metallic hydrogen. Several point out that while these optical measurements are consistent with a transition to a metallic state, there still could be other explanations, including another state of hydrogen, some change to the diamond’s structure, or an interaction between the hydrogen and the anvil. Similar optical results have also been reported by other groups. Loubeyre’s group appears to have reached a higher pressure than in those studies, although differences in how pressure is measured in these experiments make a clear determination difficult.
Mikhail Eremets of the Max Planck Institute for Chemistry says measuring the sample’s conductivity would be a more definitive way to determine if a group finally made metallic hydrogen. Measuring conductivity would determine whether the sample remained conductive to absolute zero—one definition of a metal—or was superconducting, which is the ultimate promise of metallic hydrogen. Eremets has performed conductivity experiments on pressurized hydrogen, but it is difficult to incorporate electrical leads into diamond anvil experiments, and conductivity measurements are invasive because the electrical current may affect the sample’s properties.
More attempts to make metallic hydrogen will come. Loubeyre says his group will attempt higher pressures with their set up. But Russell Hemley of the University of Illinois at Chicago points out that while metallic hydrogen remains an interesting research target, chemists have already delivered materials like lanthanum hydride that exhibit some of the properties—like near room-temperature superconductivity—that metallic hydrogen promises (Phys. Rev. Lett. 2019, DOI: 10.1103/PhysRevLett.122.027001).