A group of scientists from Jilin University, the Center for High Pressure Science and Technology Advanced Research, and Skoltech have successfully synthesized lanthanum-cerium polyhydride, a material that shows promise in advancing the study of superconductivity at near-room temperatures. This breakthrough offers a compromise between lanthanum and cerium polyhydrides, reducing the cooling and pressure requirements and making experiments more accessible. This development brings scientists closer to achieving the long-awaited goal of discovering materials that exhibit zero electrical resistance under ambient conditions, which would revolutionize power grids, microchip technology, and electromagnets. The research, published in Nature Communications, addresses one of the most intriguing questions in modern physics.
Scientists have been exploring various classes of materials in their quest for high-temperature superconductors, gradually raising the temperature at which superconductivity occurs while reducing the required pressure for stability. Polyhydrides, which are compounds with high hydrogen content, have emerged as one such group of materials. Currently, the leading contender for high-temperature superconductivity is lanthanum polyhydride (LaH10), which operates at -23°C but necessitates a pressure of 1.5 million atmospheres. On the other end of the spectrum, cuprates are materials that superconduct at normal atmospheric pressure but at much colder temperatures, typically no higher than -140°C.
To alleviate the pressure requirements of polyhydride superconductors, the team of Skoltech researchers collaborated with their Chinese counterparts and modified the lanthanum-hydrogen system by introducing cerium. This involved creating a lanthanum-cerium alloy and subjecting it to high pressure while reacting it with ammonia borane, a substance that releases hydrogen as it decomposes.
Lanthanum and cerium are chemically similar atoms that form analogous compounds and can often replace one another. While superconductivity has been observed in lanthanum polyhydrides (LaH10) and cerium polyhydrides (CeH10 and CeH9), researchers have rarely observed the corresponding LaH9 phase. The scientists hypothesized that they could stabilize LaH9 by incorporating an appropriate additive, such as cerium, which would modify the structure of the original material. Their hypothesis proved successful.
When pure lanthanum and hydrogen are subjected to very high pressure, they form the LaH10 structure. However, if approximately one in every four lanthanum atoms is replaced with cerium, the structure reorganizes into the arrangement seen in CeH9. In this sense, the addition of cerium alters the structure that the pure material would naturally assume. Moreover, this additive contributes to stability. While LaH10 requires a pressure of 1.5 million atmospheres, the lanthanum-cerium polyhydride remains stable at just 1 million atmospheres. Additionally, while cerium polyhydrides exhibit superconductivity only below -158°C, the newly synthesized lanthanum-cerium polyhydride demonstrates superconductivity at -97°C. This compromise not only demonstrates the researchers’ sound reasoning but also provides valuable insights into superconductivity, bringing us closer to achieving the ultimate goal of room temperature and atmospheric pressure superconductors with other materials.
Professor Artem R. Oganov from Skoltech, a co-author of the study, emphasizes the importance of testing and refining the principles that enable the discovery and improvement of superconductors in a reliable and systematic manner. Although he believes that polyhydrides, in general, are unlikely to achieve superconductivity at atmospheric pressure for large-scale applications like maglev trains or lossless power grids, he notes that their study offers valuable insights and techniques that can be applied to other materials in the pursuit of this ultimate goal.
Oganov describes polyhydrides as a treasure trove for fundamental research on superconductors under pressure, and the synthesis of the new compound not only tests and refines the tools and methods used in this research but also provides a convenient material for further studies.
The researchers also highlight two key findings of their work. First, they demonstrate the possible anisotropy of the upper critical field in hydrides, indicating that the critical temperature depends on the direction of the magnetic field. Second, they show that as pressure decreases, polyhydrides exhibit a pseudogap phase, similar to cuprate superconductors. This reveals intriguing similarities between polyhydrides and cuprates, despite their different superconductivity mechanisms.
When asked about other promising compounds emerging from current polyhydride research, the researchers suggest that hydrides and borohydrides of calcium, yttrium, lanthanum, and magnesium are deserving of further research attention.