Synthesis of ThH10 in a diamond anvil cell: X-ray diffraction pattern, dependence of electrical resistance of the sample on temperature and external magnetic field.Room-temperature superconductivity has come a step closer with the discovery of new chemical compounds based on thorium and hydrogen that conduct electricity without resistance at relatively high temperatures [Semenok et al., Materials Today (2019), https://doi.org/10.1016/j.mattod.2019.10.005].
“This is well-known material science problem, also called the problem of ‘room-temperature’ superconductivity, to find materials for superconducting devices that do not require cooling,” explains first author of the study, Dmitry V. Semenok of Skolkovo Institute of Science and Technology.
Nearly 100 years ago it was predicted that hydrogen, the most abundant element in the universe, could form a metallic phase under very high pressures. More recently it has been suggested that similar properties could be found at lower pressures in hydrogen-rich compounds – metal polyhydrides. Now a team of Russian scientists led by Artem R. Oganov and Ivan A. Troyan of Shubnikov Institute of Crystallography, along with colleagues at the National Research Nuclear University, P. N. Lebedev Physical Institute, and the ID27 High Pressure Beamline in Grenoble, France, has succeeded in synthesizing two novel thorium hydride compounds with high superconducting transition (or ‘critical’) temperatures up to 161 K, the existence of which they had predicted using an evolutionary computer algorithm called USPEX.
“These materials are unique superconductors – only the third and fourth reported metal superhydrides to date that have such high critical parameters of superconductivity and hydrogen content (90 and 91 at.% hydrogen, respectively),” says Semenok.
The new compounds were synthesized under high pressure in a diamond anvil cell. First, a small amount of thorium is placed in the cell and compressed with ammonium borane (AB), which is used as the hydrogen source. Laser heating delivered in sequences of powerful pulses drives the formation of metal hydrides. In the two compounds, ThH9 and ThH10, heavy thorium atoms are encapsulated in a polyhedral cage of hydrogen atoms.
“This metallic hydrogen cage is responsible for the unique superconducting properties, although the nature of the metal atom is important too,” explains Semenok.
The critical temperature of 161 K shown by ThH10 is one of the highest achieved experimentally in any compound, point out the scientists. Thorium hydrides are also attractive because of their stability – ThH10 decomposes at pressures below 90 GPa, making it unique among high critical temperature metal hydrides.
“This work is a step in the development of a new simplified method for the synthesis of superhydrides under high pressure using AB as a hydrogen source,” says Semenok. “It will help us to find the best superconductors among all possible metal superhydrides.”
The researchers have also been exploring the current-voltage characteristics of the new compounds, which is an important stage towards the development of practical applications of these materials in superconducting electronic devices.