New study shows novel crystal structure for hydrogen under high pressure


ISHIKAWA, JAPAN — The elements of the periodic table can take many forms. Carbon, for example, exists as diamond or graphite depending on the environmental conditions at the time of formation. Crystal structures that formed in ultra-high pressure environments are particularly important because they provide clues to how planets formed. However, recreating such environments in a laboratory is difficult, and materials scientists often rely on simulation predictions to identify the existence of such structures.

In this regard, 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 involving hydrogen at high pressure. Moreover, the structural model is governed by a delicate balance of factors, including the electrical forces on the electrons and the fluctuations imposed by quantum mechanics, and for hydrogen the fluctuations are particularly large, making the predictions of its even more difficult crystal phases.

Recently, in a collaboration study published in Physical examination B, a global team of researchers involving Professor Ryo Maezono and Associate Professor Kenta Hongo of Japan Advanced Institute of Science and Technology have tackled this problem using an ingenious combination of supercomputer simulations and data science, revealing various crystal structures for the hydrogen at low temperature close to 0 K and high pressures.

“For the crystal structures under high pressure, we were able to generate several candidate models using recent method of known data science, like genetic algorithms, etc. But if these candidates are really the phases that survive under high pressure, this can only be determined by high resolution simulations,” explains Maezono.

As a result, the team searched for various possible structures that could 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 calculations of density functional theory (DFT) and estimated their relative stability using first-principles quantum Monte Carlo and DFT zero-point energy corrections.

The search yielded 10 possible crystal structures not previously found by experiments, including nine molecular crystals and a mixed structure, Pbam8 comprising alternately appearing 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 realized under high pressure conditions such as those found deep within the Earth,” says Hongo.

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

Although an interesting discovery, Maezono explains the significance of their results: “The problem of hydrogen crystals is one of the most difficult 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 result now verified, we can continue our research into other structural prediction problems, such as that of silicon and magnesium compounds, which have a significant impact on Earth and planetary science.

– This press release was provided by the Japan Advanced Institute of Science and Technology


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