Tantalum polyhydride joins emerging class of high

Jul 17, 2023

Tantalum polyhydride becomes a superconductor at a temperature of 30 K and pressures of around 200 gigapascals (GPa), say researchers at the Chinese Academy of Science in Beijing. This marks the first time that superconductivity has been observed in a hydride made from a Group 5 transition metal, and members of the research team say the discovery could pave the way towards synthesizing metallic hydrogen.

Superconductors are materials that carry electrical current with no electrical resistance when cooled to below their superconducting transition temperature (Tc). They are used in applications ranging from the high-field magnets in MRI scanners and particle accelerators to quantum bits in quantum computers. However, Tc for most superconductors is just a few degrees above absolute zero, and even so-called high-temperature superconductors must be cooled to below 150 K before they can conduct electricity without resistance. Researchers are therefore seeking to develop materials that remain superconducting at higher temperatures, and ideally at room temperature.

In theory, the metallic state of hydrogen, which is expected to occur at extremely high pressures, should be a superconductor at room temperature. Unfortunately, it is very difficult to make pure hydrogen metallic. As an alternative, scientists have begun investigating hydrides, which are compounds consisting of hydrogen and a metal.

Within the past few years, sulphur hydride and polyhydrides have both been found to superconduct at temperatures above 200 K, albeit only at pressures more than a million times higher than atmospheric pressure at sea level. Other superconducting materials in this class include rare earth hydrides like LaH10 and YH9 and alkaline earth hydrides like CaH6. Hydrides containing zirconium, lutetium and tin have similarly been found to have a moderately high Tc.

Most 3d transition metals have local electron spins that tend to exhibit magnetic fluctuations that oppose superconductivity. For this reason, researchers have turned their attention to 5d transition metals such as Hf and Ta. Indeed, hafnium polyhydride becomes superconducting at around 83 K.

A team of researchers led by Changqing Jin have now synthesized an additional superconducting hydride, tantalum polyhydride (TaH3). They performed their experiment by placing the material in a diamond anvil cell, and at a pressure of 200 GPa, they observed that it superconducts at around 30 K. This makes it the first superconducting hydride to be made from the Group 5 metals, say Jin and colleagues, who investigated the superconducting phase using in situ electrical conductance as well X-ray diffraction measurements at high pressures.

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Jin explains that tantalum has a high tolerance for interstitial elements and can accommodate more than three hydrogen atoms in its lattice. However, these atoms are spaced too far from each other for electrons to hop directly between different sites in the material and produce a dissipationless electric current. “We suggest therefore that the superconductivity we have observed is related to the hybridization between the orbitals of Ta and H,” he tells Physics World. “This is totally different from what is expected for calcium hydride or rare earth polyhydride superconductors, in which electron can directly hop between adjacent H atoms that form a very dense cage.”

The observation of superconductivity in TaH3 implies that metallic hydrogen might be realized though metallizing a covalent bond between H and other elements. The researchers, who report their work in Chinese Physics Letters, say they now plan to explore other polyhydride superconductors containing these covalent bonds.