Periodic Reporting for period 4 - HECATE (Hydrogen at Extreme Conditions: Applying Theory to Experiment for creation, verification and understanding)
Reporting period: 2021-04-01 to 2023-03-31
My team has been investigating how materials behave at conditions of extreme pressure - comparable to those found in the interiors of giant planets. Two major techniques are used: atomistic simulations and diamond anvil cell experiments.
In atomistic simulations, we track the motion of atoms using a method called "Molecular Dynamics" which can be thought of as three-dimensional billiards with millions of balls. By varying the density and energy of the simulated atoms, we can model extreme pressures and temperatures, or external stimuli such as shock waves. This lets us interpret signals from experimental probes such as scattering laser light or X-rays, and inderstand difficult-to-measure properties, for example whether the material is metallic, superconducting, viscous, or transparent. We can do calculations at pressures and temperatures not reached by experiment, to determine whether such experiments are likely to find anything interesting.
In experiments we create materials at high pressure by squeezing them between two diamonds. Physics and chemistry are very different at high pressure, as stability is dominated by the need for atoms and electrons to occupy as little space as possible. This can be of practical importance, because materials made at high pressure may be recovered for use under normal conditions
High pressure metallic hydrogen and other materials with high hydrogen content are candidates for high-temperature superconductors. In the past ten years, materials with ever-high critical superconducting temperatures: hydrides with lanthanum and yttrium currently hold the record. Theory has been crucial in predicting promising materials, and in helping reject implausible claims which have dogged the field.
Our overall objective is to use both theory and experiment to tackle these issues.
In atomistic simulations, we track the motion of atoms using a method called "Molecular Dynamics" which can be thought of as three-dimensional billiards with millions of balls. By varying the density and energy of the simulated atoms, we can model extreme pressures and temperatures, or external stimuli such as shock waves. This lets us interpret signals from experimental probes such as scattering laser light or X-rays, and inderstand difficult-to-measure properties, for example whether the material is metallic, superconducting, viscous, or transparent. We can do calculations at pressures and temperatures not reached by experiment, to determine whether such experiments are likely to find anything interesting.
In experiments we create materials at high pressure by squeezing them between two diamonds. Physics and chemistry are very different at high pressure, as stability is dominated by the need for atoms and electrons to occupy as little space as possible. This can be of practical importance, because materials made at high pressure may be recovered for use under normal conditions
High pressure metallic hydrogen and other materials with high hydrogen content are candidates for high-temperature superconductors. In the past ten years, materials with ever-high critical superconducting temperatures: hydrides with lanthanum and yttrium currently hold the record. Theory has been crucial in predicting promising materials, and in helping reject implausible claims which have dogged the field.
Our overall objective is to use both theory and experiment to tackle these issues.
Our work with Group I elements brought two remarkable discoveries. For the past 70 years, the lowest-energy crystal structure of lithium was believed to be a relatively complex one called the 9R structure. PWe calculated that the actual lowest-energy structure for lithium is the simple face-centred cubic (fcc) form. Previous workers had made their samples by cooling, which trapped the materials in a defective structure at temperatures too cold to rearrange: we made our samples under pressure, then cooled, and decompressing, producing the predicted fcc lithium.
In potassium, we predict a new phase of matter, in which approximately 70% of the atoms are solid and 30% liquid at the same time, all intermingled and able to interconvert.
This material has only been produced in tiny quantities within diamond anvil cells, so these properties have yet to be observed. Subsequently, we showed that rubiudium, sodium and cesium have similar phases.
The early stages of the project involved mapping out the pure hydrogen phase diagram, and interpreting the experimental signatures coming from experiments on hydrogen (H), deuterium (D) and mixtures of H-D. Standard methods struggle to simulate these light elements, while our calculations and experiments showed a spectacular agreement. The quantum rotation of molecules is crucial, and the twofold difference if mass between hydrogen and deuterium means that the normal theory of long-ranged correlated motion - "phonons" - breaks down.
Our work touched on two classic mathematical challenges:
The "Kepler conjecture" : we showed that the classic "Lennard-Jones potential" - no fewer than 50 different stability regimes were identified depending on the truncation range of the potential.
The "Kelvin problem" : We found that hydrogen is most stable when surrounded by four metal ions: so much so that metal atoms rearrange to maximise the number of tetrahedral interstices, we showed that Ba and La hydrides adopt structures where all interstitial sites are tetrahedra occupied by hydrogens.
We carried out a number of experimental studies on other diatomic molecules under pressure,. The overall picture which emerges is that with increasing pressure oxygen, nitrogen, chlorine, iodine etc. initially solidify in arrangements that optimise quadrupole interactions, then structures wherein the molecules are increasingly efficiently packed, then undergo a polymerization. We believe they ultimately metallize, but await experimental verification.
We discovered a new "form" of hydrogen, a supermolecule containing 13 rotating molecules, which in turn form a near-spherical cluster with one central molecule surrounded by 12 others. This supermolecule was created in an iodine hydride which, with a ratio of 27:1, has the largest number-fraction of hydrogens of any known compound.
In a shockwave, the high pressure region behind the shockfront occurs fast and only lasts for nanoseconds. Our new interatomic potentials were used to simulate the shock directly using molecular dynamics: it showed different phase behaviour under rapid compression. Only structures which can be accessed quickly are observed, and structures never seen under normal conditions can appear if the transformation is fast enough.
In potassium, we predict a new phase of matter, in which approximately 70% of the atoms are solid and 30% liquid at the same time, all intermingled and able to interconvert.
This material has only been produced in tiny quantities within diamond anvil cells, so these properties have yet to be observed. Subsequently, we showed that rubiudium, sodium and cesium have similar phases.
The early stages of the project involved mapping out the pure hydrogen phase diagram, and interpreting the experimental signatures coming from experiments on hydrogen (H), deuterium (D) and mixtures of H-D. Standard methods struggle to simulate these light elements, while our calculations and experiments showed a spectacular agreement. The quantum rotation of molecules is crucial, and the twofold difference if mass between hydrogen and deuterium means that the normal theory of long-ranged correlated motion - "phonons" - breaks down.
Our work touched on two classic mathematical challenges:
The "Kepler conjecture" : we showed that the classic "Lennard-Jones potential" - no fewer than 50 different stability regimes were identified depending on the truncation range of the potential.
The "Kelvin problem" : We found that hydrogen is most stable when surrounded by four metal ions: so much so that metal atoms rearrange to maximise the number of tetrahedral interstices, we showed that Ba and La hydrides adopt structures where all interstitial sites are tetrahedra occupied by hydrogens.
We carried out a number of experimental studies on other diatomic molecules under pressure,. The overall picture which emerges is that with increasing pressure oxygen, nitrogen, chlorine, iodine etc. initially solidify in arrangements that optimise quadrupole interactions, then structures wherein the molecules are increasingly efficiently packed, then undergo a polymerization. We believe they ultimately metallize, but await experimental verification.
We discovered a new "form" of hydrogen, a supermolecule containing 13 rotating molecules, which in turn form a near-spherical cluster with one central molecule surrounded by 12 others. This supermolecule was created in an iodine hydride which, with a ratio of 27:1, has the largest number-fraction of hydrogens of any known compound.
In a shockwave, the high pressure region behind the shockfront occurs fast and only lasts for nanoseconds. Our new interatomic potentials were used to simulate the shock directly using molecular dynamics: it showed different phase behaviour under rapid compression. Only structures which can be accessed quickly are observed, and structures never seen under normal conditions can appear if the transformation is fast enough.
In related spin-off work with industrial applications, we applied our methods to semiconductors (AlN) aerospace alloys (ti oxidation) and nickelbase alloys".
We released two open-source codes enabling new simulational methodologies monteswitch and MIST. Our new method for calculating spectroscopic signal from calculations of the overall polarisability, proved essential in understanding the spectroscopic signal from non-harmonic materials, such as rotating molecules and localised vibrations.
We pioneered calculations for hydrogen systems in which the chemical bond is broken, notably showing how deuterium is harder to metalise than hydrogen. This is due to quantum zero-point energy, which weakens the bond in hydrogen. Especially remarkable was the discovery of quantum effects many thousands of Kelvins. Also, quantum effects mean hydrogen is a rotor that can never settle, even at absolute zero, because to orient itself would cost zero point energy. By contrast deuterium, with its lower zero-point energy, can orient at low temperatures. This is consistent with experimental data, though it had never previously been interpreted this way. Finally, we applied these methods to superconducting hydrides: superconductivity is in competition with covalent bond formation, and the zero-point energy is critical augmenting pressure to destabilise H2.
We developed improved methods for interpreting the neutron scattering signal from fluids, building also on our work showing solubility of methane in water at pressure. This led to a new project led by Pruteanu to fully combine DFT and neutron scattering results in a single package to get the best possible understanding on solubility at high pressure. There are strong suggestions from the calculations that solubility in supercritical soild atomic hydrogen may be very high. This has major implications for the structure of gas-giant planets, as it seems likely that any material postulated as forming a core will simply be dissolved in the metallic hydrogen regions. fellowship.
We pioneered new methods to build fast, machine-learned potentials. The work initiated on hydrogen has had spin-offs for many other materials including titanium, iron, tantalum, nitrogen, krypton and methane.
We released two open-source codes enabling new simulational methodologies monteswitch and MIST. Our new method for calculating spectroscopic signal from calculations of the overall polarisability, proved essential in understanding the spectroscopic signal from non-harmonic materials, such as rotating molecules and localised vibrations.
We pioneered calculations for hydrogen systems in which the chemical bond is broken, notably showing how deuterium is harder to metalise than hydrogen. This is due to quantum zero-point energy, which weakens the bond in hydrogen. Especially remarkable was the discovery of quantum effects many thousands of Kelvins. Also, quantum effects mean hydrogen is a rotor that can never settle, even at absolute zero, because to orient itself would cost zero point energy. By contrast deuterium, with its lower zero-point energy, can orient at low temperatures. This is consistent with experimental data, though it had never previously been interpreted this way. Finally, we applied these methods to superconducting hydrides: superconductivity is in competition with covalent bond formation, and the zero-point energy is critical augmenting pressure to destabilise H2.
We developed improved methods for interpreting the neutron scattering signal from fluids, building also on our work showing solubility of methane in water at pressure. This led to a new project led by Pruteanu to fully combine DFT and neutron scattering results in a single package to get the best possible understanding on solubility at high pressure. There are strong suggestions from the calculations that solubility in supercritical soild atomic hydrogen may be very high. This has major implications for the structure of gas-giant planets, as it seems likely that any material postulated as forming a core will simply be dissolved in the metallic hydrogen regions. fellowship.
We pioneered new methods to build fast, machine-learned potentials. The work initiated on hydrogen has had spin-offs for many other materials including titanium, iron, tantalum, nitrogen, krypton and methane.