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.