Study displays novel crystal structure for hydrogen under high pressure

Using data science and supercomputer simulations, researchers have identified a potential crystal phase for hydrogen solidified at extreme pressures.

Elements in the periodic table can appear in multiple forms. For example, Carbon, can exist as diamond or graphite depending on the environmental conditions at the time of formation. Crystal structures that have been formed in high-pressure environments are particularly crucial as they provide clues to the formation of planets.

However, recreating such environments in a laboratory is difficult, and material scientists often rely on simulation predictions to identify the existence of such structures. In this regard, hydrogen is especially important for analysing the distribution of matter in the universe and the behaviour of giant gas planets.

High-pressure hydrogen

However, the crystal structures of solid hydrogen formed under high pressure are still under contention, due to the difficulty of conducting experiments involving high-pressure hydrogen. Moreover, the structural pattern is governed by a delicate balance of factors including electric forces on the electrons and fluctuations imposed by quantum mechanics, and for hydrogen, the fluctuations are particularly large, making the predictions of its crystal phases even more difficult.

Recently, in a collaborative study published in Physical Review B, an international team of researchers, including Professor Ryo Maezono and Associate Professor Kenta Hongo from Japan’s Advanced Institute of Science and Technology, undertook this problem using an ingenious combination of supercomputer simulations and data science. Their work revealed various crystal structures for hydrogen at low temperatures near 0 K and high pressures.

“For crystal structures under high pressure, we have been able to generate several candidate patterns using a recent data science method known such as genetic algorithms,” explained Professor Maezono. “But whether these candidates are truly the phases that survive under high pressure can only be determined by high-resolution simulations”.

Crystal structures

Therefore, 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). The team then utilised a technique called ‘particle swarm optimisation’ and density functional theory (DFT) calculations to estimate their relative stability using first-principles quantum Monte Carlo method and DFT zero-point energy corrections.

The search produced 10 possible crystal structures that were previously not found by experiments, including nine molecular crystals and one mixed structure, Pbam-8 comprising atomic and molecular crystal layers appearing alternatively. However, they found that all the 10 structures showed 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 that found deep within the Earth,” says Dr. Hongo.

The results

The new structure was discovered to be more stable than Cmca-12, a structure that was previously found 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) that is stable at near 0 K. The team then further confirmed their results by comparing the infrared spectrum of the two structures, revealing a similar pattern typically observed for the H2-PRE phase.

While this is an interesting discovery, the results are significant, explained Professor Maezono: “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 greatly and avoiding approximations is a typical challenge. With our result now verified, we can continue our research on other structure prediction problems, such as that for silicon and magnesium compounds, which have a significant impact on earth and planetary science.”

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