New study reveals novel crystal structure for hydrogen under high pressure


Atomic arrangement pattern of the P21/c-8 structure of the H2-PRE phase of solid hydrogen. Caption: A new crystal structure (atomic arrangement pattern) called P21/c-8 type, which is expected to be achieved under very high pressures, such as deep inside the Earth. Photo credit: Ryo Maezono, JAIST

Elements on the periodic table can take several forms. Carbon, for example, is present as diamond or graphite depending on the environmental conditions at the time of its formation. Crystal structures formed in ultra-high pressure environments are particularly important as they provide clues to planet formation. However, recreating such environments in a laboratory is difficult, and materials scientists often rely on simulation predictions to identify the existence of such structures.

Hydrogen is particularly important for analyzing the distribution of matter in the universe and the behavior of gas giant planets. However, the crystal structures of solid hydrogen formed under high pressure are still controversial due to the difficulty of conducting experiments with high pressure hydrogen. Furthermore, the structural pattern is determined by a delicate balance of factors including electrical forces on the electrons and fluctuations imposed by quantum mechanics, and for hydrogen the fluctuations are particularly large, making predictions of its crystal phases even more difficult.

Recently, in a joint study published in Physical Check B, a global research team including Professor Ryo Maezono and Associate Professor Kenta Hongo from the Japan Advanced Institute of Science and Technology tackled this problem using a sophisticated combination of supercomputer simulations and data science, and approximated different crystal structures for hydrogen at low temperatures 0 K revealed high pressures.

“For crystal structures under high pressure, we were able to generate several candidate patterns using a recent data science method known as genetic algorithms, etc. But whether these candidates are really the phases that survive under high pressure can only be determined by high pressure. dissolution simulations,” explains Prof. Maezono.

Accordingly, the team searched for various possible structures that can be formed with two to 70 hydrogen atoms at high pressures of 400 to 600 gigapascals (GPa) using a technique called particle swarm optimization and density functional theory (DFT) calculations, and underestimated their relative stability Using the first principles quantum Monte Carlo method and DFT zero-point energy corrections.

The search revealed 10 possible crystal structures not previously found through experiments, including nine molecular crystals and one mixed structure, where Pbam-8 comprises alternating atomic and molecular crystal layers. However, they found that all 10 structures exhibited structural dynamic instabilities. To obtain a stable structure, the team relaxed Pbam-8 in the direction of instability to form a new dynamically stable structure called P21/c-8. “The new structure is a promising candidate for the solid hydrogen phase, which is realized under high-pressure conditions, such as those found deep inside the Earth,” says Dr. Hongo.

The new structure was found to be more stable than Cmca-12, a structure previously considered a valid candidate in H2-PRE phase, one of the six structural phases identified for solid hydrogen at high pressure (360 to 495 GPa) and stable near 0 K. The team further validated their results by comparing the infrared spectra of the two structures, which revealed a similar pattern typical of H2-PRE phase.

Although this is an interesting finding, Prof. Maezono explains the importance of their results: “The hydrogen crystal problem is one of the most challenging and intractable problems in materials science. Depending on the type of approximation used, the predictions can vary widely and avoidance. Approximations pose a typical challenge. With our now verified result, we can continue our research on other structure prediction problems, such as for silicon and magnesium compounds, which have a significant impact on the Earth – and have planetary science.”

New crystal structure for hydrogen compounds for high-temperature superconductivity

More information:
Tom Ichibha et al., Candidate Structure for the H2-PRE Phase of Solid Hydrogen, Physical Check B (2021). DOI: 10.1103/PhysRevB.104.214111

Provided by Japan Advanced Institute of Science and Technology

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