General principles found to speed up the hunt for room temperature superconductors

Submitted by Freedomman on Wed, 04/25/2018 - 23:27

MOSCOW, Russia (PNN) - April 17, 2018 - Scientists from Moscow Institute of Physics and Technology and Skoltech have demonstrated the high-temperature superconductivity of actinium hydrides and discovered a general principle for calculating the superconductivity of hydrides based only on the periodic table.

They have used a new algorithm to discover a material that could become a superconductor at close to room temperature. It will superconduct at minus 20°C (minus 4°F), although it still needs to be squeezed under high pressure.

The actinides are a series of 15 metals with large atomic numbers 89 to 103 (actinium to lawrencium) sitting alongside that other weird “outside” block of elements, the lanthanides.

The stability of numerous unexpected actinium hydrides was predicted via the evolutionary algorithm USPEX. The electron–phonon interaction was investigated for the hydrogen-richest and most symmetric phases: R3̅m-AcH10, I4/mmm-AcH12, and P6̅m2-AcH16. Predicted structures of actinium hydrides are consistent with all previously studied Ac–H phases and demonstrate phonon-mediated high-temperature superconductivity with TC in the range of 204–251 K for R3̅m-AcH10 at 200 GPa and 199–241 K for P6̅m2-AcH16 at 150 GPa, which was estimated by directly solving the Eliashberg equation. Actinium belongs to the series of d1 elements (Sc–Y–La–Ac) that form high-TC superconducting (HTSC) hydrides. Combining this observation with previous predictions of p0-HTSC hydrides (MgH6 and CaH6), we propose that p0 and d1 metals with low-lying empty orbitals tend to form phonon-mediated HTSC metal polyhydrides.

High-temperature superconductivity is a phenomenon of zero electrical resistance in certain materials at temperatures above -196°C (the temperature of liquid nitrogen) that physicists, chemists and materials scientists worldwide have been intensely researching for decades, as room-temperature superconductors open up vast prospects for the power industry, transport, and other technology-driven sectors. Currently, the record holder in high-temperature superconductivity is hydrogen sulfide (H3S), which functions as a superconductor at 1.5 million atmospheres and temperatures of down to -70 oC. Such pressure levels can only be attained in a lab environment, not in real life, and the temperature is way below room temperature, so the search continues for new superconductors. Perhaps an even higher-temperature superconductivity can be attained in metal-hydrogen compounds. Yet the link between chemical composition and superconductivity was unclear, leaving scientists to puzzle out by trial and error.

A group of chemists led by Artem R. Oganov, Professor at Moscow Institute of Physics and Technology and Skoltech, discovered that certain elements capable of forming superconducting compounds are arranged in a specific pattern in the periodic table. It was established that high-temperature superconductivity develops in substances containing metal atoms that come close to populating a new electronic subshell. Metal atoms inside the crystal are assumed to become highly sensitive to the positions of the neighboring atoms, which would result in strong electron-phonon interaction ‒ the underlying effect of conventional superconductivity. Based on this assumption, the scientists supposed that high-temperature superconductivity could occur in actinium hydrides. Their supposition was verified and confirmed: superconductivity was predicted for AcH16 at temperatures of -69-22 oC at 1.5 million atmospheres.

“The very idea of a connection between superconductivity and the periodic table was first put forward by Dmitry Semenok, a student at my lab. The principle he discovered is very simple and it is really amazing that no one had hit upon it before,” says Oganov.