Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

Abstract

We present a technique for extracting Raman intensities from ab initio molecular dynamics (MD) simulations at high temperature. The method is applied to the highly anharmonic case of dense hydrogen up to 500 K for pressures ranging from 180 to 300 GPa. On heating or pressurizing we find first-order phase transitions under the experimental conditions of the phase III-IV boundary. At even higher pressures, close to 350 GPa, we find a second phase transformation to the previously proposed Cmca-4. Our method enables, for the first time, a direct comparison of Raman vibrons between theory and experiment at finite temperature. This turns out to provide excellent discrimination between subtly different structures found in MD. We find candidate structures whose Raman spectra are in good agreement with experiment. The new phase obtained in hightemperature simulations adopts a dynamic, simple hexagonal structure with three layer types: freely rotating hydrogen molecules, static hexagonal trimers, and rotating hexagonal trimers. We show that previously calculated structures for phase IV are inconsistent with experiment, and their appearance in simulation is due to finite-size effects.

Raman measurements of phase transitions in dense solid hydrogen and deuterium to 325 GPa

“…As described above, vibrational spectroscopic data, together with density functional theory (DFT) computations, suggest that phase IV is a mixed-layer structure, with honeycomb or graphene-structured sheets separated by molecular H 2 layers (14). First-principles classical molecular dynamics indicates that the molecules in the H 2 layers are quantum rotators (17), and even the graphene layers may be dynamic (18). As discussed below, the structures may differ according to changes in rotational ordering, weak structural distortions, and/or changes in electronic structure (e.g., degree of band overlap).…”

“…The thermodynamically stable phases must be ordered, but they might have a longer repeat unit. In fact, recent theoretical calculations predict stability of a doubled repeat unit having two types of distorted graphenestructured layers (18). Even longer sequences are possible, but given the low driving force near the transition, the molecular sheets may occur initially as stacking faults.…”

“…Important information about these transitions has been provided by recent theoretical calculations (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). Notably, the type of structure constrained by vibrational spectroscopy for phase IV was among the structures predicted by theory before the experiments (10).…”

“…These subsequent calculations predict that the structure consists of graphene-structured, honeycomb sheets and molecular interlayers. The molecular layers exhibit higherfrequency vibrons from the stronger molecular bonding, and the graphene sheets are predicted to have less strong bonding, therefore corresponding to the lower-frequency vibron in the vibrational spectra (10,(12)(13)(14)(15)(16)(17)(18). As for the question of metallization, the infrared transparency at photon energies down to 0.1 eV (5) rule out a transition to a fully metallic phase; however, phase IV might be a very poor metal, with absorption below 0.1 eV (20,21), perhaps above a particular range of P-T conditions.…”

The role of van der Waals and exchange interactions in high-pressure solid hydrogen

“…The phase diagram of high-pressure solid hydrogen has mainly been investigated using density functional theory (DFT) with local and semi-local exchange-correlation (XC) functionals [20][21][22][23][24][25][26][27][28][29]38 . In particular, DFT with generalized gradient approximation (GGA) functionals has been widely applied to search for candidate low-energy crystal structures and to calculate their vibrational properties.…”