#	Pagina
attuale pagina	/open-h2020/projects/203289/results.html

Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - HECATE (Hydrogen at Extreme Conditions: Applying Theory to Experiment for creation, verification and understanding)

Teaser

Summary

\"This research is directed at what is arguably the most fundamental problem in condensed matter physics: what is the equilibrium state of a system containing protons and electrons?

Specifically, 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 use a computer to solve the laws of Quantum Mechanics to work out the forces on atoms. Then we track the motion of the simulated 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. The calculations also tell us how the material will respond to experimental probes such as scattering laser light or X-rays. This enables us to compare the simulations with experiment and verify whether the simulation is producing an accurate model of reality. Simulations can also determine difficult-to-measure properties, for example whether the material is metallic, superconducting, viscous, or transparent. Once one has confidence in the method, we can do calculations at pressures and temperatures not reached by experiment, to determine whether such experiments are likely to find anything interesting.

In diamond anvil cell experiments we create materials at high pressure by squeezing them between two diamonds. Diamond anvil cells exploit the relationship that pressure is force divided by area: a modest force applied by an Allen key to a very small (micron-sized) area can generate immense pressure. Such experiments require high precision manufacture of the gasket which holds the sample: any imperfection or misalignment can cause the diamonds to shatter, allowing the high-pressure sample to escape. Loading the sample into the cell requires great experience and patience, especially for gaseous samples such as hydrogen. Once loaded, the transparent diamonds allow us to shine highly-focussed lasers or X-rays onto the sample, to measure its properties.

Understanding how materials behave is crucial to the design of any product. High pressure metallic hydrogen is believed to be a room-temperature superconductor. Although this has yet to be demonstrated experimentally, it gave a clue that other materials with high hydrogen content would also superconduct. In 2014, the record for the highest-temperature superconductor passed from layered CuO materials to high pressure hydrogen compounds, specifically hydrogen sulphide superconducts above 200K. Since then we, and other groups, have predicted and synthesized materials with ever-high critical superconducting temperatures: hydrides with lanthanum and yttrium currently hold the record. For practical usage, the issue is that these materials exist only at extremely high pressures, around 200GPa. The ongoing challenge is to find materials with superconductivity at higher temperatures and lower pressures.

Hydrogen storage is another possible application: if we can find a lightweight compound at modest pressures rich in molecular hydrogen which decomposes to release the hydrogen on depressurisation it could replace bulky hydrogen tank systems in vehicles.

Our overall object is to ally theory and experiment in this area. In particular, there is a layer of \"\"theory\"\" which is used to interpret experiments based on some very strong approximations. In many cases, it is now possible to circumvent these assumptions and use the theory to directly calculated the experimental output. In the traditional approach, two independent methods, experiment and theory, are used to make models of where the electrons and atoms are, and how they move. Those two models are then compared and if they agree, both methods are va\"

Work performed

\"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.

In a key 2017 paper \"\"Infrared Peak Splitting from Phonon Localization in Solid Hydrogen\"\" we reanalysed spectroscopic data from the Harvard group on H-D mixtures. This data had a very unconventional appearance which led rival groups to dispute its veracity.
Using the conventional analysis method of identifying spectroscopic peaks and assigning them to possible vibration the only plausible conclusion from the data was that H-D mixtures exhibit and entirely different set of crystal structures to hydrogen and deuterium - and at the pressures considered, H and D are identical. No theoretical explanation for this could be found. We took an alternative approach. We modelled H-D samples assuming the same crystal structure as known for hydrogen and deuterium, but calculated directly the entire spectroscopic response of the material, rather than proceding peak-by-peak. Our results showed a spectacular agreement with the experiment, both conforming our methods and the veracity of their data. What we found was that the disorder in nuclear mass causes any vibration to be localised to a few molecules in HD, whereas in pure H or pure D an excitation can travel unscattered throughout the material. The many possible local arrangements of H2, D2 and HD molecules leads to a range of possible vibrational frequencies, and in turn to a complex manifold of scattering which cannot be interpreted as a series of discrete peaks.

More technical work included \"\"The role of van der Waals and exchange interactions in high-pressure solid hydrogen\"\" where we showed how different levels of theory give significantly different structure, in particular a molecular-metal (previously, it had been assumed that metallization is the same process as breaking molecules).

A huge number of density-functional-theory simulation papers from other groups also appeared since 2016 reporting structures at various temperatures and pressures. Many of these, unfortunately, contained technical errors and were not carried out correctly. We too came close to making a serious error when poorly sampled statistics suggested a structure with chains of H atoms. Prior to publication, we identified the specific error causing this, and we describe our mistake in a conference paper \"\"Charge density wave in hydrogen at high pressure\"\", wherein we also identify a number of published papers which have the same error but got through peer review undetected.

Also in 2017, in \"\"Simple thermodynamic model for the hydrogen phase diagram\"\" we provided a unifying qualitative explanation for all features of the hydrogen phase diagram, in terms of three objects: free-rotating H2 molecules, fixed orientation H2 molecules and dissociated H atoms. Surprisingly, these objects can be used to build seven solid phases and a liquid more compressible that the solid. The key insight is that two H atoms occupy less volume than a molecule, and therefore dissociation is favoured at high pressure. the various phases then arise from a trade-off between bonding energy, disorder entropy and packing efficiency. Notions of packing led us to look at the famous \"\"Kepler conjecture\"\" as applied to the \"\"Lennard Jones\"\" potential beloved of introductory physics courses: In \"\"Stacking Characteristics of Close Packed Materials\"\" we showed an incredible dependence of the crystal structure on the range of the potential - no fewer than 50 different stability regimes were identified depending on the truncation range of the potential, a practical consideration previously regard as so unimportant that it is not even discussed in many older studies.

In hydrogen-like monovalent metals we wrote four papers on Lithium and Potassium (see above for details)

We carried out a number of experimental studies on other diat\"

Final results

\"We released two open-source codes enabling new simulational methodologies \"\"beyond the state of the art\"\" when the project began: monteswitch and MIST. We had also invented method for directly calculating the spectroscopic signal from calculations of the overall polarisability, rather than identifying individual vibrations and assigning peak strengths. These proved essential in understanding the spectroscopic signal from non-harmonic materials, such a the lowest pressure solid hydrogen phase (rotating molecules) and hydrogen-deuterium mixtures (localised vibrations).\"