Progress towards hydrogen-based solid superconductivity
Scientists uncover the surprising hydrogen content variability in novel lanthanum hydrides.
It has been a long-standing goal of physicists, chemists and material scientists to find a superconductor which works at room temperature. Such a discovery would transform our societies by allowing us to achieve more work while using less natural resources and creating reduced waste.
There are many applications which currently use materials with superconductors, such as MRI machines, radio and television broadcasting and wireless communications, electric motors and generators. At present, the highest temperature superconductors can only work under extremely high pressures, such as those near the centre of the Earth.
Recent research suggests that hydrogen-based solids are among the top contenders for superconductors. Since 2015, high-pressure sciences have been at the leading edge of the synthesis of materials with high superconducting critical temperature (Tc), namely with a breakthrough Tc of 260 K reported in the lanthanum hydride (La-H) system at 1,600,000 times atmospheric pressure. Yet, incongruities in experimental measurements suggested other phases to be formed along with LaH10+δ. These undiscovered lanthanum hydrides, which also anticipated to be very high temperature superconductors, were expected to play an active role in the interpretation of the data to assess superconductivity.
Hydrogen content variability
Dr Dominique Laniel led a team of international collaborators from the University of Bayreuth and Linköping University in employing extreme pressure and temperature conditions to synthesize seven lanthanum hydrides, including five of which were previously unobserved.
A detailed investigation of these hydrides determined that their hydrogen content for a given lanthanum atoms’ arrangement varies depending on pressure and their synthesis conditions—an observation key to understanding pressure-formed hydrogen-based superconductors.
The team synthesized five novel lanthanum hydrides: LaH~4, LaH4+δ, La4H23, LaH6+δ and LaH9+δ, along with the previously known LaH3 and LaH10+δ. To accomplish this, lanthanum and paraffin—i.e. a hydrogen reservoir—were first squeezed to enormous pressures of 0.96 to 1.76 million times the atmospheric pressure in between two tiny diamond anvils and heated to temperatures of more than 2200°C using high power lasers. Then, to characterise the atomic arrangement adopted by the compounds under these conditions, the samples were illuminated by an intense X-ray beam at two particle accelerators, the German PETRA III and the United States APS synchrotrons.
Dr Laniel commented:
Given the extensive investigations that the La-H system had already underwent, we were surprised to see the formation of this many previously unknown lanthanum hydrides. But the most unexpected result was that their hydrogen-content seemed to greatly vary based on their synthesis condition and pressure—without any noticeable changes in the lanthanum atoms’ arrangement.
This discovery, which is published in Nature Communications, has profound ramifications. Largely due to hydrogen having a single electron, this element cannot directly be detected by X-ray diffraction: a method commonly used to study these hydrides. As such, scientists instead rely on theoretical calculations to determine the position and content of hydrogen atoms in these solids. However, such calculations often assume a single possible configuration of hydrogen atoms for a given arrangement of lanthanum atoms—a hypothesis now demonstrated to be incorrect. This realisation is especially crucial given the very strong dependence of a solid’s superconducting temperature on the hydrogen atoms, i.e. having the wrong model for the hydrogen atoms’ is likely to completely change the calculated superconducting temperature, in turn greatly undermining the reliability of theoretical calculations.
Upon re-analysing published data from other groups, Dr Laniel and collaborators found this hydrogen content variability to be a recurrent phenomenon, thereby generalizing their observations to other systems.
This established hydrogen content variability will undoubtedly provide the impetus for revising theoretical models of high pressure hydrides, ultimately leading to an intimate understanding of their superconductivity and bringing us one step closer to a room temperature superconductor.