Melting and High P−T Transitions of Hydrogen up to 300 GPa (original) (raw)

Quantum phase transition in solid hydrogen at high pressure

Physical Review B, 2019

Extensive experimental and theoretical studies have been devoted to determining the high-pressure phase diagram of hydrogen. We present evidence of a phase at a higher pressure than phase III and below the pressure of the recently observed phase of metallic hydrogen (495 GPa). This phase was determined from infrared (IR) spectroscopy of hydrogen samples at static pressures above 360 ± 15 GPa in a diamond anvil cell, and has been observed in three separate experiments. Whereas earlier studies found new high-pressure phases that only occurred at elevated temperatures, this phase transition occurs at the lowest temperatures investigated, ∼5 K, and the steep phase line indicates that it is a quantum phase transition. This phase is characterized by two distinct IR absorption bands (2950 and 3335 cm -1 at 365 GPa). Above the transition pressure we observe strong darkening of the sample in the visible spectrum as pressure is increased. Observations are compatible with the cmca-12 crystal structure.

Raman Spectroscopy of Hot Dense Hydrogen

Physical Review Letters, 2003

High P-T Raman measurements of solid and fluid hydrogen to above 1100 K at 70 GPa and to above 650 K in 150 GPa range, conditions previously inaccessible by static compression experiments, provide new insight into the behavior of the material under extreme conditions. The data give a direct measure of the melting curve that extends previous optical investigations by up to a factor of 4 in pressure. The magnitude of the vibron frequency temperature derivative d=dT P increases by a factor of 30 over the measured pressure range, indicating an increase in intrinsic anharmonicity and weakening of the molecular bond.

The Melting Line of Molecular Hydrogen at High Pressure

Bulletin of the American Physical Society, 2008

The insulator to metal transition in solid hydrogen was predicted over 70 years ago but the demonstration of this transition remains a scientific challenge. In this regard, a peak in the temperature versus pressure melting line of hydrogen may be a possible precursor for metallization. However, previous measurements of the fusion curve of hydrogen have been limited in pressure and temperature by diffusion of hydrogen into the gasket or diamonds. To overcome this limitation we have used an innovative technique of pulsed laser heating of the sample and find a peak in the melting line at P 64:7 4 GPa and T 1055 20 K.

High-pressure melting curve of hydrogen

The Journal of Chemical Physics, 2008

The melting curve of hydrogen was computed for pressures up to 200 GPa, using molecular dynamics. The inter-and intramolecular interactions were described by the reactive force field ͑ReaxFF͒ model. The model describes the pressure-volume equation of state solid hydrogen in good agreement with experiment up to pressures over 150 GPa, however the corresponding equation of state for liquid deviates considerably from density functional theory calculations. Due to this, the computed melting curve, although shares most of the known features, yields considerably lower melting temperatures compared to extrapolations of the available diamond anvil cell data. This failure of the ReaxFF model, which can reproduce many physical and chemical properties ͑including chemical reactions in hydrocarbons͒ of solid hydrogen, hints at an important change in the mechanism of interaction of hydrogen molecules in the liquid state.

In situ Raman spectroscopy of low-temperature/high-pressure transformations of H2O

The Journal of Chemical Physics, 2007

In situ Raman spectra of transformations of H2O as functions of pressure and temperature have been measured starting from high-density amorphous ice (HDA). Changes above Tx, the crystallization temperature of HDA, were observed. The spectra provide evidence for an abrupt, first-order-like, structural change that appears to be distinct from those associated with the transformation between low-density amorphous ice (LDA) and HDA. In separate experiments, in situ Raman spectra of ice XII transformed from HDA have been measured at various P-T regions, in order to improve the understanding of the stability limits of ice XII. The spectra of ices VI and XII differ in shape, but the vibrational frequencies are very close in the same P-T regimes. A metastable phase of ice found to form within the stability field of ice VI appears to be distinct from ice XII.

Low-frequency Raman spectroscopy of deuterium to megabar pressures at 77-295 K

Physical Review B, 1993

Rotational and lattice phonon excitations in the Raman spectrum of solid molecular deuterium have been measured from 1.8 GPa to-200 GPa at 77-295 K to study pressure-induced changes in structural and dynamical properties of the dense solid. Continuous and discontinuous changes in three distinct pressure ranges are observed. At lower pressures ((30 GPa), there is a gradual increase in the linewidth of the So(0) band, together with gradually decreasing resolution of the higher-energy So(1) and So(2) features. At intermediate pressures (60-100 GPa), a change in the pressure dependence and linewidth of So(0), and a linewidth and intensity decrease in the E2~p honon band occur. This is interpreted as evidence for a phase transformation in the molecular solid beginning at-65 GPa (77 K). At the highest pressures (-160 GPa), abrupt changes in the low-frequency excitations suggest either an expansion of the molecular bond or a change in ordering at the vibron discontinuity. Evidence from the lowfrequency spectra for interaction between the deuterium sample and diamond anvil is also examined. ' ' In addition, a distinct isotope effect is observed in the pressure of the low-temperature 150-GPa

The Behavior of Solid Hydrogen at 342GPa

THE REVIEW OF HIGH PRESSURE SCIENCE AND TECHNOLOGY, 1998

Theoretical calculations indicate that the hydrogen molecule should depair at high pressures and eventually form a new alkali metal. The resultant alkali metal sample in the diamond anvil cell would not be transparent in the visible. Experimentally, we find that hydrogen is still transparent at 342 GPa. This represents over a 50% increase in the static pressure at which scientific studies have been made by other groups. [estimated metallic transition pressure, optical transmission at pressure, refractive index at pressure, molar refraction at pressure]

Melting line of hydrogen at high pressures

Physical review letters, 2008

The insulator to metal transition in solid hydrogen was predicted over 70 years ago but the demonstration of this transition remains a scientific challenge. In this regard, a peak in the temperature vs. pressure melting line of hydrogen may be a possible precursor for metallization. However, previous measurements of the fusion curve of hydrogen have been limited in pressure by diffusion of hydrogen into the gasket or diamonds. To overcome this limitation we have used an innovative technique of pulsed laser heating of the sample and final peak in the melting line at P= 64.7 ± 4 GPa and T=1055 ± 20 K.