Periodic Reporting for period 3 - MetElOne (The Metallization Conditions of Element One)
Berichtszeitraum: 2023-11-01 bis 2025-04-30
Element number one, hydrogen, is the simplest and most abundant element in the universe. The relative abundance is reflected in the gas giant Jupiter, where under extreme pressures and temperatures, hydrogen exists in a dense metallic fluid state. It has been long predicted that such a metallic state could also be realised at considerably lower temperatures, whereby the molecular solid would dissociate under compression into an atomic metal. The metallic state of hydrogen is predicted to exhibit a whole host of fascinating properties at high pressure, from room temperature superconductivity, to novel states of matter. If quenchable to ambient conditions, metallic hydrogen could pave the way for a new generation of rocket propellants. As such, the pursuit of reaching these phenomena inspires many from interdisciplinary fields of science.
The overarching aim of MetElOne is to conclusively reach the metallic state of hydrogen and explore the fascinating properties predicted. However, this presents a monumental experimental challenge. The pressure conditions required to reach metallic hydrogen is thought to be in excess of 420 GPa (4.2 million times atmospheric pressure), which far exceeds the pressure at the centre the Earth (3.6 million times atmospheric pressure). To reach these conditions, we use a device called a diamond anvil cell, which harnesses the incredible strength of diamond to exert extreme pressures.
However, the pressures required for metallization are on the limit of conventional diamond anvil cells and hydrogen is light, diffusive and reactive resulting in containment issues and diamond embrittlement. The MetElOne project is exploring novel approaches to high pressure experimentation to overcome the challenges associated with hydrogen. Using a focussed-ion beam, diamonds will be milled into different geometries to optimize and extend current accessible pressure regimes. In parallel, rapid compression techniques are being developed that will be used to compress hydrogen to the metallic state on timescales that will avoid diffusion into the anvils. The combination of these techniques should put metallic hydrogen within reach to investigate with a broad suite of diagnostics. The advances alone will be transformative in the broader field of high pressure research, from recreating planetary interior conditions in the experimental laboratory to the synthesis of novel high-temperature superconductors.
The overarching aim of MetElOne is to conclusively reach the metallic state of hydrogen and explore the fascinating properties predicted. However, this presents a monumental experimental challenge. The pressure conditions required to reach metallic hydrogen is thought to be in excess of 420 GPa (4.2 million times atmospheric pressure), which far exceeds the pressure at the centre the Earth (3.6 million times atmospheric pressure). To reach these conditions, we use a device called a diamond anvil cell, which harnesses the incredible strength of diamond to exert extreme pressures.
However, the pressures required for metallization are on the limit of conventional diamond anvil cells and hydrogen is light, diffusive and reactive resulting in containment issues and diamond embrittlement. The MetElOne project is exploring novel approaches to high pressure experimentation to overcome the challenges associated with hydrogen. Using a focussed-ion beam, diamonds will be milled into different geometries to optimize and extend current accessible pressure regimes. In parallel, rapid compression techniques are being developed that will be used to compress hydrogen to the metallic state on timescales that will avoid diffusion into the anvils. The combination of these techniques should put metallic hydrogen within reach to investigate with a broad suite of diagnostics. The advances alone will be transformative in the broader field of high pressure research, from recreating planetary interior conditions in the experimental laboratory to the synthesis of novel high-temperature superconductors.
At the midpoint of MetElOne, good progress has been made in all aspects of the project. One of the main technique developments involved the optimization of diamond anvil geometry to access higher pressure regimes beyond current capabilities. Before making the leap to study hydrogen, arguably the most difficult element to study in under pressure, we have selected a variety of test materials that would be ‘safe’ to test each new geometry. Furthermore, the test subjects were selected such that new, exciting, results would be obtained.
One such test subject was the methane-hydrogen system. Hydrogen and methane are, besides water, the most prevalent small molecules in the outer solar system. At high pressure, they have a propensity to form inclusion compounds and their signatures have emerged in a variety of studies investigating hydrocarbons at deep earth conditions and in the synthesis of high temperature superconducting hydrides, but were never formally identified. In our work, we reported the high pressure formation of the most H2-rich compounds found to date, (CH4)3(H2)25, CH4(H2)2 and (CH4)2H2. With a completely unique composition, (CH4)3(H2)25 contains the most molecular hydrogen (over 50 wt%) of any known compound. This work was published in Physical Review Letters (https://doi.org/10.1103/PhysRevLett.128.215702 , https://physics.aps.org/articles/v15/s69 ).
MetElOne also required significant development of spectroscopy techniques to probe hydrogen in an ultra-dense state. In the development of this, we used a range of metal hydride systems as test subjects that also have implications for the goals of MetElOne. An alternative approach to achieving metallic hydrogen, is the insertion of hydrogen into metallic systems, whereby hydrogen is chemically pre-compressed into an atomic state. This method has been exploited extensively in the past decade, producing hydrogen-rich compounds with novel properties such as high temperature superconductivity. We discovered a series of alkaline-earth tetrahydrides CaH4, SrH4 and BaH4, all of which contain both atomic and quasi-molecular hydrogen. We found that the hydrogen intermolecular bond lengths within the compounds at 20 GPa are comparable to ultradense hydrogen above 200 GPa, demonstrating the feasibility of chemical precompression. This work was published in The Journal of Physical Chemistry Letters (https://doi.org/10.1021/acs.jpclett.2c02157(öffnet in neuem Fenster)).
One such test subject was the methane-hydrogen system. Hydrogen and methane are, besides water, the most prevalent small molecules in the outer solar system. At high pressure, they have a propensity to form inclusion compounds and their signatures have emerged in a variety of studies investigating hydrocarbons at deep earth conditions and in the synthesis of high temperature superconducting hydrides, but were never formally identified. In our work, we reported the high pressure formation of the most H2-rich compounds found to date, (CH4)3(H2)25, CH4(H2)2 and (CH4)2H2. With a completely unique composition, (CH4)3(H2)25 contains the most molecular hydrogen (over 50 wt%) of any known compound. This work was published in Physical Review Letters (https://doi.org/10.1103/PhysRevLett.128.215702 , https://physics.aps.org/articles/v15/s69 ).
MetElOne also required significant development of spectroscopy techniques to probe hydrogen in an ultra-dense state. In the development of this, we used a range of metal hydride systems as test subjects that also have implications for the goals of MetElOne. An alternative approach to achieving metallic hydrogen, is the insertion of hydrogen into metallic systems, whereby hydrogen is chemically pre-compressed into an atomic state. This method has been exploited extensively in the past decade, producing hydrogen-rich compounds with novel properties such as high temperature superconductivity. We discovered a series of alkaline-earth tetrahydrides CaH4, SrH4 and BaH4, all of which contain both atomic and quasi-molecular hydrogen. We found that the hydrogen intermolecular bond lengths within the compounds at 20 GPa are comparable to ultradense hydrogen above 200 GPa, demonstrating the feasibility of chemical precompression. This work was published in The Journal of Physical Chemistry Letters (https://doi.org/10.1021/acs.jpclett.2c02157(öffnet in neuem Fenster)).
As part of the MetElOne project a new type of diamond anvil cell has been designed and is in the process of being tested. This new cell will be utilized extensively throughout the project and beyond.
Now having reached the midpoint of the project, the MetElOne team are now in a position to implement new anvil designs to the study of hydrogen (and its isotope deuterium), with the primary aim to measure spectroscopic signatures above 450 GPa (4.5 million times atmospheric pressure). Preliminary test results on other molecular systems have been promising, yielding results which will be disseminated in their own stand-alone peer reviewed publications.
To reach high-temperature conditions, a laser heating system (capable of achieving temperatures in excess of 5000 K) has been integrated into a time resolved Raman spectroscopy system. By measuring spectroscopic signatures of the hot sample on a timescale of nanoseconds, we avoid thermal emission that would otherwise drown out the signal. In parallel, testing is underway for combining this system with a dynamic diamond anvil cell, whereby we will subject samples to high compression rates to avoid hydrogen diffusion. To put this into perspective, conventional diamond anvil cell experiments compress samples on the scale of 0.1 GPa per second, with data acquisition lasting days, whilst dynamic diamond anvil cell experiments can compress samples on the scale of TPa per second, with data acquisition complete within sub-second timescales. In addition to hydrogen (and deuterium), results on test samples will be published in due course in peer-reviewed journals.
Now having reached the midpoint of the project, the MetElOne team are now in a position to implement new anvil designs to the study of hydrogen (and its isotope deuterium), with the primary aim to measure spectroscopic signatures above 450 GPa (4.5 million times atmospheric pressure). Preliminary test results on other molecular systems have been promising, yielding results which will be disseminated in their own stand-alone peer reviewed publications.
To reach high-temperature conditions, a laser heating system (capable of achieving temperatures in excess of 5000 K) has been integrated into a time resolved Raman spectroscopy system. By measuring spectroscopic signatures of the hot sample on a timescale of nanoseconds, we avoid thermal emission that would otherwise drown out the signal. In parallel, testing is underway for combining this system with a dynamic diamond anvil cell, whereby we will subject samples to high compression rates to avoid hydrogen diffusion. To put this into perspective, conventional diamond anvil cell experiments compress samples on the scale of 0.1 GPa per second, with data acquisition lasting days, whilst dynamic diamond anvil cell experiments can compress samples on the scale of TPa per second, with data acquisition complete within sub-second timescales. In addition to hydrogen (and deuterium), results on test samples will be published in due course in peer-reviewed journals.