Examining the behavior of alkali metals under extreme conditions

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Physics and Astronomy

Abstract

The research focuses on examining the dynamics of alkali metals under extreme temperatures and pressures. This will involve the design of diamond anvil cells (DACs) by extending existing designs and using computer simulations. Following the subsequent manufacture of these designs, materials will be loaded and experiments conducted at high-energy x-ray sources around Europe. We anticipate that our efforts will enable studies of hitherto inaccessible regions on P-T space using the improved DAC design.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509644/1 01/10/2016 30/09/2021
1941902 Studentship EP/N509644/1 01/09/2017 31/08/2021 Christian Storm
 
Description We have conducted various experiments examining the properties of pure elements at ultra-high pressures. In particular, our work has focussed on two main areas: developing tools to achieved pressures exceeding the typical limit of a diamond anvil cell of 350 GPa, and using these methods to explore the behaviour of alkali metals at these conditions.

Regarding our efforts to extend the accessible pressure range, we have successfully conducted experiments on elements such as tungsten (W) up to 390 GPa. For reference, the pressure at the centre of the earth if about 330 GPa. This result therefore constitutes an extension of the known phase diagram of more than 150 GPa, and has enabled us to formulate an equation of state describing the compressibility of tungsten at these extreme conditions.

As for our efforts to study alkali metals at extreme conditions, these present a challenge in the sample preparation stage and in the execution of the experiment itself, because of their high reactivity with air and moisture. However, we have successfully studied samples of potassium and rubidium to pressures of 320 GPa and 260 GPa, which is 200 GPa and 160 GPa higher than has ever been measured before. At these pressures, the high compressibility of these elements result in a 8-fold volume decrease, causing electron-bands to blend into each other and forcing core electrons into the valence bands.
Exploitation Route As tungsten is used in a multitude of industrial settings our improved equation of state will enable engineers to examine its utility for extreme conditions, such as high-stress points in aircraft, transportation or the energy generation sector. We hope that this will enable more durable and reliable machinery to be built.

Our results for the alkali metals primarily have an impact on further computation research; by understanding how electron bands behave at extreme conditions, theoretical models of band structure and atomic behaviour can be calibrated. This in turn is invaluable to predicting the behaviour of more complex systems like molecules or proteins, the research of which results in anything from chemical synthesis to drug development.
Sectors Aerospace, Defence and Marine,Chemicals,Construction,Energy,Pharmaceuticals and Medical Biotechnology,Transport,Other
 
Title In-House Focussed Ion Beam Milling in Diamond 
Description In order to create toroidal diamond anvil cells, we have developed techniques to modify a diamond culet's diameter using focussed ion beam (FIB) milling. A beam of ions, usually Ga, is incident on the diamond surface. If the energy absorbed by the surface atoms during this bombardment exceeds the surface binding energy, the atoms are sputtered and ejected from the diamond. The transfer of ion momentum and energy to the surface occurs either through ion-electron interactions which results in ionized atomic cores and the ejection of electrons and radiation from the surface, or ion-atom interaction which can displace or sputter atomic cores. By scanning the ion beam across a solid surface, intricate patterns can be milled including circular tores. There are several parameters influencing the milling process such as incidence angle, scan pattern, dwell time, and beam current to name a few. 
Type Of Material Technology assay or reagent 
Year Produced 2019 
Provided To Others? Yes  
Impact The development of in-house FIB milling facilities has greatly accelerated our toroidal DAC programme, and enabled us to deploy these diamond anvils at synchrotron beam lines in Germany and the USA. 
 
Title Toroidal Diamond Anvil Cells 
Description Ever since its inception, the diamond anvil cell (DAC) has been an important tool for research at high pressures. At its core, the DAC consists of two opposing diamond anvils mounted on seats containing an aperture to allow an X-ray to penetrate the apparatus. The material under consideration is placed between the anvils along with a pressure transmitting medium a pressure calibrant before the entire pressure chamber is compressed by applying a force on the seats. A central gasket placed between the anvils provides lateral support to the pressure chamber and prevents the sample from extruding between the anvils. When placed in front of an X-ray beam the resulting diffraction pattern depends intimately on the material's crystal structure, allowing for analysis of the unit cell's response to pressure and revealing pressure-induced phase changes. We have modified the standard DAC by sculpting a circular tore into the diamond culet using focussed ion beam milling. This has several effects: it decreases the surface area of the culet, increasing the pressure for a constant applied force; it increases radial support for the gasket and sample, mitigating sample extrusion past the culet and containing it at ultra-high pressures; and it decreases the effect that 'cupping' has, the phenomenon of culet edges bending towards each other as the culet becomes increasingly concave under pressure, by milling away part of the culet edge. 
Type Of Material Technology assay or reagent 
Year Produced 2018 
Provided To Others? Yes  
Impact Presently, this development has enabled us to extend the pressure range accessible to our group from 320 GPa to 390 GPa. We are working to extend it further, at least until the compression stress limit of diamond at 420 GPa.