Hydrogen is particularly useful for studying the distribution of matter in the universe as well as the behavior of giant gas planets. However, due to the difficulty of conducting experiments with high-pressure hydrogen, the crystal structures of solid hydrogen that form under high pressure remain controversial.
Hydrogen has particularly large fluctuations, making it even more difficult to predict its crystal phases, and this makes the structural pattern subject to a delicate balance of factors including electric forces on electrons and quantum mechanical fluctuations.
Using a combination of sophisticated supercomputer simulations and data science with different crystal structures for hydrogen at low temperatures near 0 K and high pressures, an international team of researchers led by Professor Ryo Maezono and Associate Professor Kenta Hongo from the Japan Advanced Institute of Science and Technology took on this problem.
As a result, the researchers used a technique known as “particle swarm optimization” and density functional theory (DFT) calculations to search for various possible structures that can form with 2 to 70 hydrogen atoms at high pressures of 400 to 600 gigapascals (GPa). They then estimated their relative stability using the quantum Monte Carlo method based on first principles and DFT zero-point energy corrections.
One mixed structure, Pbam-8, which is composed of alternating atoms and molecular crystal layers, was found to have 10 possible crystal structures that had never been found experimentally. However, they discovered structural dynamic instability in all ten structures. Stability was achieved by relaxing Pbam-8 towards instability in order to create P21/c/8, a new dynamically stable structure.
One of the six structural phases required for solid hydrogen at high pressure is H 2 -PRE, and the new structure proved to be more stable than Cmca-12.
