Megabar Neutron Diffraction for Hydrogen, Ices and Superconductor Research MENHIR

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The pressure of around one million atmospheres is an important milestone for systems composed of molecules. Pressures around this value are required to break the strong covalent bonds which hold molecules together and hence transform molecular systems into non-molecular systems. This change from molecular to non-molecular leads to profound changes in properties. Hydrogen has been reported to become a metal and this metallic form may be the cause of Jupiter's magnetic field. The hydrogen atoms in water and ice detach from the oxygen atoms and become mobile so that they conduct electricity in a way similar to the way that lithium ions in the batteries of laptops and phones conduct. Hydrogen sulphide forms a metallic compound which superconducts at a temperature of only -70 C. A study of these and similar phenomena yields information that is of both fundamental and technological importance. For example, the behaviour of hydrogen atoms provides stringent tests for our understanding of quantum behaviour, the way hydrogen-bonded solids like ice behave provides insight into the way biochemical processes work, and novel superconductors offer new routes to a potential room temperature superconductor which would transform power distribution medical imaging and could make a 'Back To The Future II' hoverboard a reality. However, our understanding of these and other important hydrogen-rich systems at million atmosphere pressures is limited by the fact that the most fundamental information -- the position of the hydrogen atoms in the crystal structures -- is currently not known. The reason for this lack of information is that neutron diffraction which is the only technique able to measure hydrogen atom positions directly was until recently restricted to pressures below 300,000 atmospheres and so information on the positions of the hydrogen atoms had to be obtained indirectly or from computational modelling. For the past seven years we have been developing the use of suitable technology for neutron diffraction studies using the SNAP instrument and the Spallation Neutron Source at Oak Ridge National Laboratory in the United States. We have now reached the stage where structures can be successfully determined at pressures in excess of one million atmospheres using both powder and single crystal diffraction techniques.This project aims to use a so called diamond anvil cell where the sample is compressed between two large gem quality diamonds to study the structures of hydrogen, ice and related ices (ammonia hemihydrate and hydrogen chloride), and very high Tc superconductors up to million atmosphere pressures.