Based only on conventional BCS theory, metallic hydrogen is assumed to be a room-temperature superconductor. However, the pressure needed to get pure metallic hydrogen is now unachievable. Therefore, researchers resorted to polyhydrides, where a hydrogen framework may be metalized at a moderate or relatively low-pressure thanks to the chemical precompression given by other elements.
Several polyhydrides, including LaH10, have been experimentally reported to be superconductors since the experimental finding of superconductivity in sulfur hydrides. Although superconductivity in alkali earth superhydrides is theoretically anticipated, superconductivity has not yet been experimentally seen.
Alkaline earth elements, as the neighboring group where hydrogen is located on the periodic table, offer one of best recipes for forming superconducting polyhydrides that to a great extent resemble to metallic hydrogen. They are chemically similar in terms of valence electron configurations.
Changqing Jin, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences
In terms of material processing under extreme circumstances, it is quite difficult. Before evaluating its superconductivity at high pressures, calcium polyhydrides must first be created at extremely high pressure and temperature. The finding was made possible by the cutting-edge methods that the Jin team developed to merge in situ high-pressure, high-temperature synthesis with severe condition characterizations.
The scientists used a diamond anvil cell together with in situ laser heating to manufacture the materials at 160~190 GPa and ~2000 K. In situ high-pressure electric conductance measurements for the sample that had been quenched from a high temperature and kept at a high-pressure provided proof of the superconductivity.
With the crucial transition temperature reaching 210 K at 160 GPa, the calcium polyhydrides become superconductive. In addition to sulfur hydride and lanthanum hydride, it is the third polyhydride superconductor with Tc exceeding 200 K. The Ginzburg Landau coherent length was determined to be ~11 Å, while the maximum critical field Hc2 was predicted to be ~268 T.
High Tc and high Hc2 show the superconducting properties of calcium polyhydrides and their potential for use. The results of the in-situ synchrotron x-ray diffraction showed that the produced calcium hydrides are made of CaH6.
But there may also exist other superconducting calcium hydrides with different hydrogen contents in the synthesized samples as indicated from the multistep superconducting transitions.
Xin He, Study Co-Author, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences
“We are trying to stabilize the high Tc phases at lower pressure or near ambient condition by introducing chemical pressure,” stated coauthor Prof. Xiancheng Wang at IOPCAS.
The research was published in Nature Communications 13, 2863 (2022). The National Science Foundation, China’s Ministry of Science and Technology, and the Chinese Academy of Sciences all provided funding for the study.
Li, Z., et al. (2022) Superconductivity above 200 K discovered in superhydrides of calcium. Nature Communications. doi.org/10.1038/s41467-022-30454-w.