Abstract
We present extensive molecular dynamics (MD) simulations investigating numerous candidate crystal structures for hydrogen in conditions around the present experimental frontier (400 GPa). Spontaneous phase transitions in the simulations reveal a new structure candidate comprising twofold coordinated chains of hydrogen atoms. We explain the electronic structure of this phase in terms of a charge density wave and calculate its experimental signature. In detailed tests of the accuracy of our calculation, we find that k-point sampling is far more important in MD than in static calculations, because of the freedom it give the atoms to rearrange themselves optimally for the given sampling.
Highlights
Hydrogen is typically located atop group 1A in the periodic table, and while the H2 molecule has no analogue among the alkali metals, one might expect similarities to appear if a non-molecular phase is obtained at extreme conditions
For analysis we use the radial distribution function (RDF), angular distribution function (ADF), projected mean square displacement (MSD), phonon and vibrational mode projection; details of which are given in the SM
For Chains and Cmca − 4 calculated from both molecular dynamics (MD) and PIMD at 400GPa and temperatures as labeled
Summary
Hydrogen is typically located atop group 1A in the periodic table, and while the H2 molecule has no analogue among the alkali metals, one might expect similarities to appear if a non-molecular phase is obtained at extreme conditions. Hydrogen is typically located atop group 1A, and while the H2 molecule has no analogue among the alkali metals, one might expect similarities to appear if a non-molecular phase is obtained at extreme conditions. For the alkali metals in this pressure range, complex phases are observed in which the electronic structure can be assigned to two classes. Static experiments using diamond anvil cells are reaching equivalent pressures in hydrogen, where the work done in compression is greater than the chemical binding energy. Measurements at these pressures are restricted to spectroscopy and conductivity, so density functional calculations are used to determine the structure. The practical role of calculations is to propose candidate structures for test against experiment
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