Mechanical, electronic and thermodynamic properties of crystalline molecular hydrogen at high pressure

Solid molecular hydrogen exhibits a rich variety of unique properties, including insulator-to-metal transition. However, the specific structure of phase III remains unclear. Experimental evidence suggests high-pressure hydrogen shows a hexagonal close-packed lattice. Therefore, as a potential candidate for phase III, the structure, energetic, mechanical, electronic, and thermodynamic properties of P6122 structure are systemically studied. It is found phase III with P6122 space group is both mechanically and thermodynamically stable within the pressure range of 120-300 GPa. Besides, we observed significant enhancements in the bulk modulus, shear modulus, and Young's modulus. The band structures exhibit a transition from topological insulator to topological semiconductor and eventually to a Wely semimetal as the pressure increases. The electron density accumulates between pseudo H2 molecules in the insulating phase but it partly channels into the interlayer space in the semimetallic phase. Phase III with P6122 structure is barely explored previously in solid hydrogen, therefore, we believe our present investigation contributes valuable insights towards understanding the behavior of metallic hydrogen.& COPY; 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/).

Automated summary

Solid molecular hydrogen exhibits unique properties, such as the insulator-to-metal transition. However, the structure of phase III is still unclear. Based on experimental evidence, high-pressure hydrogen is observed to have a hexagonal close-packed lattice. In this study, the structure, energetic, mechanical, electronic, and thermodynamic properties of the P6122 structure as a potential candidate for phase III were systematically investigated. The findings indicate that phase III with P6122 space group is both mechanically and thermodynamically stable within the pressure range of 120-300 GPa, with enhanced bulk modulus, shear modulus, and Young's modulus. The band structures also show a transition from topological insulator to topological semiconductor to a Wely semimetal as pressure increases.