Hydrogen’s Unique Crystal Structure Revealed Under High Pressure

Hydrogen, the first element to form, provides clues to the distribution of matter in the universe.

A new crystal structure (atomic arrangement pattern) called P21/c-8 type expected to be reached under very high pressure, such as deep inside the Earth. (Image credit: Ryo Maezono of the Japan Advanced Institute of Science and Technology).

Generally a gas, hydrogen occurs as a solid under ultra-high pressure conditions typically found in the cores of massive gaseous planets. However, the structures of solid hydrogen have not been determined due to challenges in duplicating such conditions in the laboratory.

A new study is currently providing insights into this aspect with the help of simulations and data science approaches. Elements on the periodic table can come in many forms. Carbon, for example, is present as diamond or graphite depending on the environmental conditions at the time of its formation.

Crystal structures developed 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 detect the presence of such structures.

Incidentally, hydrogen is of particular importance for studying the distribution of matter in the universe and the behavior of massive gas planets. However, the crystal structures of solid hydrogen developed under high pressure are still controversial due to the difficulty of conducting experiments with high pressure hydrogen.

Furthermore, the structural pattern is governed by a delicate balance of factors including electrical forces on the electrons and fluctuations enforced by quantum mechanics, and for hydrogen the fluctuations are predominantly large, making predictions of its crystal phases even more difficult.

Recently in a joint study published in Physical Check B, an international team of scientists consisting of Professor Ryo Maezono and Associate Professor Kenta Hongo Japan Advanced Institute of Science and Technology (JAIST) addressed this problem with a clever combination of supercomputer simulations and data science, revealing several crystal structures for hydrogen at low temperatures near 0 K and 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.

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

Accordingly, using a method known as “particle swarm optimization” and density functional theory (DFT) calculations, the researchers examined several possible structures that can be developed with 2 to 70 hydrogen atoms at high pressures (400-600 GPa) and projected theirs relative stability using the first principles quantum Monte Carlo technique and DFT zero-point energy corrections.

The search yielded 10 possible crystal structures not previously found by experiments, including nine molecular crystals and one mixed structure, Pbam-8, comprising alternating atomic and molecular crystal layers.

However, they learned that all 10 structures exhibited structural dynamic instabilities. To obtain a stable structure, the scientists relaxed Pbam-8 towards instability to develop a new dynamically stable structure called P21/c-8.

The new structure is a promising candidate for the solid hydrogen phase realized under high-pressure conditions, such as those found deep in the Earth’s interior.

Dr. Kenta Hongo, Außerordentlicher Professor, Japan Advanced Institute of Science and Technology

The new structure was confirmed to be more stable than Cmca-12, a structure previously considered a valid option in the H2-PRE phase, one of the six structural phases demonstrated for solid hydrogen at high pressure (360–495 GPa) and stable near 0 K. The scientists further confirmed their findings by comparing the infrared range of the two structures, which showed a similar pattern normally seen in H2-PRE-Phase.

The hydrogen crystal problem is one of the most challenging and intractable problems in materials science. Depending on the type of approximation used, predictions can vary widely, and avoiding approximations is a typical challenge. With our now verified result, we can continue our research on other structure prediction problems, such as silicon and magnesium compounds, which have a significant impact on earth and planetary science.

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

Based on the study results, more such exciting discoveries could be possible in the future.

Magazine reference:

Ichibha, T., et al. (2021) Candidate structure for the H2-PRE phase of solid hydrogen. Physical Check B. doi.org/10.1103/PhysRevB.104.214111.

Those: https://www.jaist.ac.jp/english


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