New Study Challenges Previous Predictions on Hydrogen's Cold Phase Diagram
A team led by Stefano Racioppi and Eva Zurek, including Racioppi's affiliation with the University of Cambridge, has reanalyzed hydrogen's cold phase diagram between 400 and 700 GPa. Their findings challenge previous predictions and bring computational results closer to experimental observations.
The behaviour of hydrogen under extreme pressure has long puzzled condensed matter physicists due to varying predictions from computational methods. The team addressed this challenge by employing Density Functional Theory (DFT) calculations using meta-GGA functionals like R2SCAN and SCAN0. These functionals preserved a more localized molecular character, providing a more realistic description of bonding in hydrogen at extreme pressures.
Previously, commonly used methods like PBE overestimated the importance of quantum effects, leading to inaccurate predictions of hydrogen's structure at high pressure. At 507 GPa, PBE predicted the atomic I41/amd phase to have the lowest energy, whereas meta-GGA calculations demonstrated that molecular phases remained stable at higher pressures. The molecular phases Cmca-4, Cmca-12, and C2/c were stabilized to significantly higher pressures than previously predicted by PBE, aligning computational results with experimental observations.
Phonon spectra calculated using R2SCAN revealed that dynamical instabilities and anharmonic signatures previously predicted with PBE vanished. This suggests that these effects stemmed from functional deficiencies rather than genuine quantum effects. Accurately modelling the potential energy surface of hydrogen, particularly its curvature, is crucial for correctly assessing both its dynamic behaviour and bonding properties under extreme conditions.
The reanalysis by Racioppi, Zurek, and their team has significantly improved our understanding of hydrogen's behaviour under extreme pressure. By employing meta-GGA functionals, they have provided a more realistic description of hydrogen's bonding and stability at high pressures. These findings bring theoretical predictions closer to experimental observations and highlight the importance of accurate computational methods in studying hydrogen's behaviour under extreme conditions.
