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 22 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.