Hexagonal to Monoclinic Phase Transition in Dense Hydrogen Phase III Detected by High-Pressure NMR
Abstract
Conclusive crystal structure determination of the high pressure phases of hydrogen remains elusive due to lack of core electrons and vanishing wave vectors, rendering standard high-pressure experimental methods moot. Ab-initio DFT calculations have shown that structural polymorphism might be solely resolvable using high-resolution nuclear magnetic resonance (NMR) spectroscopy at mega-bar pressures, however technical challenges have precluded such experiments thus far. Here, we present in-situ high-pressure high-resolution NMR experiments in hydrogen phase III between 181 GPa and 208 GPa at room temperature. Our spectra suggest that at lower pressures phase III adopts a hexagonal P6122 crystal structure, transitioning into a monoclinic C2/c phase at about 197 GPa. The high resolution spectra are in excellent agreement with earlier structural and spectral predictions and underline the possibility of a subtle P6122 to C2/c phase transition in hydrogen phase III. These experiments show the importance of a combination of ab-initio calculations and low-Z sensitive spectral probes in high-pressure science in elucidating the structural complexity of the most abundant element in our universe.
Cite
@article{arxiv.2407.19368,
title = {Hexagonal to Monoclinic Phase Transition in Dense Hydrogen Phase III Detected by High-Pressure NMR},
author = {Meng Yang and Yishan Zhou and Rajesh Jana and Takeshi Nakagawa and Yunhua Fu and Thomas Meier},
journal= {arXiv preprint arXiv:2407.19368},
year = {2024}
}