Predicting superconductor crystal structures with computer simulations

Isikawa, Japan -- Superconductivity refers to the loss of electrical resistance of a material and requires extremely low temperatures (< -200°C) in order to persist. This low temperature, otherwise known as the transition temperature, has been a limiting factor in the application of superconductors and finding materials that can display superconductivity at higher temperatures has become an important research objective worldwide.

A breakthrough has been made with hydrogen-rich compounds (called hydrides) containing rare earth or alkaline metals, which display room temperature superconductivity at high pressures (100-200 GPa). These metal hydrides have cage-like structures of hydrogen atoms stacked on top of each other enabling them to withstand the high pressures required for the superconductivity phenomenon.  Many of the crystal structures and compositions for these hydrides have been predicted by combining various binary metal hydrides together.

Now, in a study published in The Journal of Physical Chemistry on January 26, 2022, a group of researchers led by Professor Ryo Maezono from Japan Advanced Institute of Science and Technology (JAIST) has used a supercomputer to make similar predictions for viable high-temperature ternary metal hydride superconductors containing Magnesium (Mg), an alkaline metal, and Scandium (Sc), a rare earth element. “MgH₂ and ScH₂ are known to be stable phases at ambient pressure. Therefore, MgH₂ and ScH₂ can be used to chemically synthesize the ternary Mg−Sc−H compounds”, explains Prof. Maezono.

For their search, the researchers initially started out with certain Mg-Sc-H compounds (MgSc₃H_x, MgSc₂H_x, MgScH_x, Mg₂ScH_x, and Mg₃ScH_x, where x = 2−12, 14, 16, and 18). Starting with random initial structures, they used the supercomputer to determine possible combinations and crystal structures that would result in a valid superconductor in a pressure range of 100-200 GPa.

In order to achieve superconductivity, the predicted compound must meet certain conditions: it must be thermodynamically stable, i.e., it cannot degrade into its elementary components, have a high transition temperature, have a valid synthesis route, and possess a structure capable of withstanding high pressures where the phenomenon takes place. In the simulations, four hydrogen-rich structures were found to meet the criteria: R3̅m-MgScH₆, C2/m-Mg₂ScH₁₀, Immm-MgSc₂H₉, and Pm3̅m-Mg-(ScH₄)₃.

Out of the crystal structures, R3̅m-MgScH₆ was found to have the highest transition temperature of (23.3 K) at 200 GPa and 41 K at 100 GPa. The compound was found to possess a hexagonal crystal structure, in which each Mg and Sc atom is surrounded by 14 H atoms (Figure 1). The transition temperature was, however, much lower than that of the binary halide counterparts (LH₁₀ and YH₁₀) and this low temperature was attributed to the low density of states at the Fermi level due to the lower hydrogen content.

Among the metal hydrides, ternary metal hydrides that contain hydrogen bonded to two other metals are promising candidates for low-pressure, room-temperature superconductivity. It was, however, a challenging and time-consuming process to predict the appropriate elements and crystal structure that resulted in a superconducting ternary hydride due to the large number of possible combinations with metals. With the help of supercomputers, researchers are now able to quickly determine potential superconducting candidates. The discovery of the Mg-Sc-H compounds as valid superconductors is the third such prediction for ternary hydrides made by the research group using the computer simulations. “This is the third news with 'Mg/Sc' compounds following the preceding findings with 'La/Y' in December 2021 and 'Y/Mg' in January 2022. New findings are being launched one after another,” says Prof. Maezono.

Despite having low transition temperatures, the predicted Mg-Sc-H compounds remain stable at pressures that are lower than those normally observed for high-temperature superconductors. Simulations like these are enabling researchers to understand the contributions of each element towards the superconductivity phenomenon, accelerating the development of high-temperature superconductors.

 

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Reference

Title of original paper:	High-Pressure Mg−Sc−H Phase Diagram and Its Superconductivity from First-Principles Calculations
Journal:	The Journal of Physical Chemistry
DOI:	10.1021/acs.jpcc.1c08743

 

 

About Japan Advanced Institute of Science and Technology, Japan

Founded in 1990 in Ishikawa prefecture, the Japan Advanced Institute of Science and Technology (JAIST) was the first independent national graduate school in Japan. Now, after 30 years of steady progress, JAIST has become one of Japan’s top-ranking universities. JAIST counts with multiple satellite campuses and strives to foster capable leaders with a state-of-the-art education system where diversity is key; about 40% of its alumni are international students. The university has a unique style of graduate education based on a carefully designed coursework-oriented curriculum to ensure that its students have a solid foundation on which to carry out cutting-edge research. JAIST also works closely both with local and overseas communities by promoting industry–academia collaborative research.  

 

About Professor Ryo Maezono from Japan Advanced Institute of Science and Technology, Japan

Dr. Ryo Maezono has been a Professor at the School of Information Science at the Japan Advanced Institute of Science and Technology (JAIST) since 2017. He earned his PhD from the University of Tokyo in 2000 and worked as a researcher at the National Institute for Materials Science in Ibaraki, Japan from 2001 to 2007. His research areas include material informatics and condensed matter theory using high-performance computing. He is a senior researcher with 107 publications to his name.

 

Funding information

The computations in this work have been performed using the facilities of Research Center for Advanced Computing Infrastructure (RCACI) at JAIST.

Dr. Ryo Maezono is grateful for financial support from MEXTKAKENHI (19H04692 and 16KK0097), FLAGSHIP2020 (Project Nos. hp190169 and hp190167 at K-computer), Toyota Motor Corporation, I−O DATA Foundation, the Air Force Office of Scientific Research (AFOSR-AOARD/ FA2386-17-1-4049; FA2386-19-1-4015), and JSPS Bilateral Joint Projects (with India DST).

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