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Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures.

Drummond ND, Monserrat B, Lloyd-Williams JH, López Ríos P, Pickard CJ, Needs RJ - Nat Commun (2015)

Bottom Line: By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase.Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment.This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Lancaster University, Lancaster LA1 4YB, UK.

ABSTRACT
Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead, our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energetically favoured at a broad range of pressures up to 400 GPa, where it is known experimentally that hydrogen is non-metallic. Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment. This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.

No MeSH data available.


Theoretical temperature–pressure phase diagram for H.The solid black lines show the phase transitions calculated in this work, that is, the set of points at which the relative Gibbs free energy of two phases is zero. The dotted lines show the set of points at which the relative Gibbs free energy is one error bar from zero, and hence indicate the uncertainty in the phase boundaries. At pressures in excess of 350–375 GPa the Gibbs free energies of the C2/c-24 and Pc-48 structures are within error bars of each other. The grey region indicates the temperature–pressure conditions under which phase I is found to exist in experiments.
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f5: Theoretical temperature–pressure phase diagram for H.The solid black lines show the phase transitions calculated in this work, that is, the set of points at which the relative Gibbs free energy of two phases is zero. The dotted lines show the set of points at which the relative Gibbs free energy is one error bar from zero, and hence indicate the uncertainty in the phase boundaries. At pressures in excess of 350–375 GPa the Gibbs free energies of the C2/c-24 and Pc-48 structures are within error bars of each other. The grey region indicates the temperature–pressure conditions under which phase I is found to exist in experiments.

Mentions: Figure 4 shows the two structural phase transitions that we have determined in this work, and our theoretical temperature–pressure phase diagram for solid molecular H is shown in Fig. 5. At 0 K, we find a transition from P21/c-24 to C2/c-24 at around 235±10 GPa. The corresponding transition pressure for D is 13 GPa higher (note that the difference between H and D is purely due to the DFT vibrational free energy and hence the difference in transition pressures between H and D is relatively precise). Our transition pressure is around 75 GPa greater than those observed experimentally for the transition between phases II and III, but the 13 GPa difference between the transition pressures for H and D agrees well with the experimentally measured value23. We note that the theoretical transition pressures between H and D would only differ by around 6 GPa without the inclusion of anharmonic effects.


Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures.

Drummond ND, Monserrat B, Lloyd-Williams JH, López Ríos P, Pickard CJ, Needs RJ - Nat Commun (2015)

Theoretical temperature–pressure phase diagram for H.The solid black lines show the phase transitions calculated in this work, that is, the set of points at which the relative Gibbs free energy of two phases is zero. The dotted lines show the set of points at which the relative Gibbs free energy is one error bar from zero, and hence indicate the uncertainty in the phase boundaries. At pressures in excess of 350–375 GPa the Gibbs free energies of the C2/c-24 and Pc-48 structures are within error bars of each other. The grey region indicates the temperature–pressure conditions under which phase I is found to exist in experiments.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525154&req=5

f5: Theoretical temperature–pressure phase diagram for H.The solid black lines show the phase transitions calculated in this work, that is, the set of points at which the relative Gibbs free energy of two phases is zero. The dotted lines show the set of points at which the relative Gibbs free energy is one error bar from zero, and hence indicate the uncertainty in the phase boundaries. At pressures in excess of 350–375 GPa the Gibbs free energies of the C2/c-24 and Pc-48 structures are within error bars of each other. The grey region indicates the temperature–pressure conditions under which phase I is found to exist in experiments.
Mentions: Figure 4 shows the two structural phase transitions that we have determined in this work, and our theoretical temperature–pressure phase diagram for solid molecular H is shown in Fig. 5. At 0 K, we find a transition from P21/c-24 to C2/c-24 at around 235±10 GPa. The corresponding transition pressure for D is 13 GPa higher (note that the difference between H and D is purely due to the DFT vibrational free energy and hence the difference in transition pressures between H and D is relatively precise). Our transition pressure is around 75 GPa greater than those observed experimentally for the transition between phases II and III, but the 13 GPa difference between the transition pressures for H and D agrees well with the experimentally measured value23. We note that the theoretical transition pressures between H and D would only differ by around 6 GPa without the inclusion of anharmonic effects.

Bottom Line: By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase.Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment.This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Lancaster University, Lancaster LA1 4YB, UK.

ABSTRACT
Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead, our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energetically favoured at a broad range of pressures up to 400 GPa, where it is known experimentally that hydrogen is non-metallic. Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment. This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.

No MeSH data available.