It might be possible to find truly high-temperature superconductors in the future thanks to the discovery of a "law within a law" by researchers at the Skolkovo and Moscow Institutes of Physics and Technology. They have demonstrated that there is a link between the position of an element in the Periodic Table and its likelihood of forming a high-temperature superconducting hydride. [Semenok, D. et al. Curr Opinion Solid State Mater Sci (2020); DOI: 10.1016/j.cossms.2020.100808]
Materials with zero electrical resistance dissipate no energy as heat and are therefore an important target for research in materials science and have been for many years. They could revolutions power transmission, and of course, applications such as magnetic resonance imaging (MRI) where they are essential for the powerful magnetic fields required for this medical technology. Currently, there are two ways to observe superconductivity. The first requires chilling a material to close to absolute zero or applying extremely high pressures. The warmest of the superconductors still require cryogenic temperatures. Theory predicts that metallic hydrogen might be superconducting at close to room temperature but under 4 million atmospheres, which lies at the technical limit.
Might hydrides be the new hope for superconductors at less extreme temperatures and pressures? The current record is -23 degrees Celsius for lanthanum decahydride at a less pressing 1.7 million atmospheres. Now, Skoltech's Dmitrii Semenok and MIPT's Artem Oganov and their colleagues have hit on a new rule that could map the way to novel metal hydrides based only on the electronic structure of metal atoms.
"The connection between superconductivity and the Periodic Table was puzzling at first," explains Oganov. "We are still not fully certain about its origin, but we think that it is because elements at the boundary between s- and p- or s- and d-elements (roughly between 2nd and 3rd groups of the Table) have electronic structure unusually sensitive to the crystal field, and this is perfect for the electron-phonon coupling, which is the cause of superconductivity in hydrides."
The team has now trained a neural network to help them search the chemical space for new superconducting compounds. For some elements, previously published critical temperatures for hydrides seemed to deviate from regular behavior. For elements where the published values of maxTc were (based on the discovered rule) too low or too high, the group performed systematic searches for stable hydrides. The new data confirmed the discovered rule and yielded new hydrides for magnesium, strontium, barium, cesium, and rubidium.
Among the new leads was a novel strontium hexahydride, with a max Tc of 189 K at about 1 million atmospheres. Also found was a barium superhydride, with a putative critical temperature of 214 K.