RESEARCHERS from the United States, Russia, and China have bent the rules of classical chemistry and synthesised a “forbidden” compound of cerium and hydrogen—CeH9—which exhibits superconductivity at a relatively low pressure of 1 million atmospheres. The work has been published in “Nature Communications”.
Superconductors known today can unfortunately only work at very low temperatures (below –138oC), and the latest record (–13oC) requires extremely high pressures of nearly 2 million atmospheres. This, therefore, limits the scope of their possible applications and makes the available superconducting technologies expensive as maintaining such fairly extreme operating conditions is challenging.
Theoretical predictions suggest hydrogen as a potential candidate for room-temperature superconductivity. However, coaxing hydrogen into a superconductive state would take a tremendous pressure of some 5 million atmospheres—compare it with 3.6 million atmospheres at the centre of the earth. Compressed so hard, it would turn into a metal, but that would defeat the purpose of operating at room temperature conditions.
“The alternative to metallising hydrogen is the synthesis of so-called ‘forbidden’ compounds of some element—lanthanum, sulphur, uranium, cerium, and so on—and hydrogen, with more atoms of the latter than classical chemistry allows for. Thus normally, we might talk about a substance with a formula like CeH2 or CeH3. But our cerium superhydride—CeH9—packs considerably more hydrogen, endowing it with exciting properties,” said Artem R. Oganov of Skoltech and the Moscow Institute of Physics and Technology (MIPT), one of the authors of the study.
As materials scientists pursue superconductivity at higher temperatures and lower pressures, one may come at the cost of the other. “While cerium superhydride only becomes superconductive once cooled to –200oC, this material is remarkable in that it is stable at a pressure of 1 million atmospheres—less than what the previously synthesised sulphur and lanthanum superhydrides require. On the other hand, uranium superhydride is stable at an even lower pressure, but needs considerably more cooling,” explained co-author Ivan Kruglov of MIPT and Dukhov Research Institute of Automatics.
To synthesise their “impossible” superconductor, the scientists placed a microscopic sample of the metal cerium into a diamond anvil cell, along with a chemical that releases hydrogen when heated, which they did using a laser. The cerium sample was squeezed between two flat diamonds to enable the pressure needed for the reaction. As the pressure grew, cerium hydrides with a progressively larger proportion of hydrogen formed in the reactor: CeH2, CeH3, and so on.
The CeH9 crystal lattice is composed of cages of 29 hydrogen atoms in a near-spherical formation. The atoms in each cage are held together by covalent bonds, not unlike those in the familiar H2 molecule of hydrogen gas, but somewhat weaker. Each cage provides a cavity that houses one cerium atom.
Using USPEX, a crystal structure predicting code developed by Oganov’s laboratory in 2004, and other computer algorithms predicting the crystal structure of previously unheard of “forbidden” compounds, researchers have been able to study the single-metal hydrides in minute detail. The next step is adding a third element into the mix: The triple compounds of hydrogen and two different metals are an uncharted territory. Since the number of possible combinations is great, researchers are considering using AI algorithms to select the most promising candidates.