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Computational Chemistry

Metallic hydrogen simulations reconcile experimental results

Calculations suggest that recent studies found a semimetallic state. The models could predict how soon scientists will make the real thing.

by Sam Lemonick
September 10, 2020 | APPEARED IN VOLUME 98, ISSUE 35

 

09835-scicon1-metal.jpg
Credit: Paul Loubeyre
Calculations of hydrogen's behavior at high pressures track with experimental results, like the change in infrared light absorption observed above 400 GPa, and suggest the material enters a semimetallic state.

Several groups have claimed experimental evidence of metallic hydrogen at very high pressures over the past few years. The material is theorized to be a near-room temperature superconductor, but the difficulty of making and analyzing it means that those experiments have not produced definitive proof of its existence or nature. A new computational model that incorporates quantum mechanical effects appears to paint a more cohesive picture of hydrogen’s behavior (Nat. Phys. 2020, DOI: 10.1038/s41567-020-1009-3).

Scientists predicted hydrogen’s metallic phase in the 1930s and its high-temperature superconductivity—which could mean more-efficient electronics, among other things—in the 1960s. Only in the past 5 years have researchers been able to squeeze hydrogen to the very high pressures at which it might become metallic, about 4 million times ambient air pressure, roughly 400 GPa. Two accepted signatures of a metal are electrical conductivity and optical reflectivity (put plainly: shininess). Researchers reported optical evidence of metallic hydrogen at 495 GPa in 2017 (Science 2017, DOI: 10.1126/science.aal1579), although those results were widely met with skepticism. More recently, separate groups described conductivity and optical evidence of a metallic phase at 360 GPa and 420 GPa, respectively (Nat. Phys. 2019, DOI: 10.1038/s41567-019-0646-x and Nature 2020, DOI: 10.1038/s41586-019-1927-3). These reports bolstered the idea that metallic hydrogen was within reach but didn’t present a comprehensive model of the element at high pressure.

A new computational model of molecular hydrogen under pressure fits the experimental evidence and seems to fill in the gaps. Francesco Mauri of Sapienza University of Rome and colleagues calculated structural properties and Raman, infrared, and visible spectra of hydrogen compressed to 155, 260, 355, and 460 GPa. Importantly, the group was able to account for quantum wiggling of hydrogen nuclei in their calculations, something previous models have omitted.

The model suggests that molecular hydrogen enters a semimetallic state at around 380 GPa. That explains the conductivity and black color observed in the 2019 and 2020 studies and would make this state analogous to graphite, Mauri says. Unlike graphite, however, hydrogen seems to be transparent in the IR up to about 420 GPa. Though IR opacity was taken as a sign of a transition to a metal phase in the 2020 Nature paper, this new model confirms conductivity arises at lower pressure. Mauri says the group’s model reconciles the “apparently contradicting properties” seen in experiments.

The researchers did not test their model at the pressures used in the 2017 experiments. But considering the low reflectivity the calculations predict at 460 GPa, the group posits that those scientists may have observed atomic metallic hydrogen, which may be a room temperature superconductor.

The group also predicts deuterium has a semi-metallic phase transition just 70 GPa higher than hydrogen’s. Laboratory confirmation of that would boost confidence in this model. “This would be an extremely exciting experiment to carry out,” University at Buffalo computational chemist Eva Zurek says.

Physicist Mikhail Eremets of the Max Planck Institute for Chemistry, who led the 2019 study, says the calculations’ agreement with experimental results implies the model is accurate. That will be important as Mauri’s group attempts to predict hydrogen’s behavior above 460 GPa. Mauri says their simulations could confirm that the 2017 experiments produced atomic metallic hydrogen, or reveal if that material is still months or years away.

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