Lithium hydride is the lightest metal hydride, formed through reacting metallic lithium with hydrogen gas at elevated temperatures. Wider academic interest in this material stems from the search for simple metallic hydrides used in reversible hydrogen storage, and in applications as a shielding material in environments under high neutron irradiation. These conditions are unforgiving, typically operating at elevated temperatures and pressures. The sample is highly water sensitive, which presents a number of challenges for studying the material under non-ambient conditions. Dr Craig Wilson, Atomic Weapons Establishment, UK (AWE), in collaboration with high-pressure researcher and PEARL Instrument Scientist Dr Christopher Ridley, ISIS, are looking to map the melt-line of lithium hydride under these harsh conditions.
Craig is seeking to add to the currently limited experimental data in the literature on molten lithium hydride, for wider comparison against computational theory. This is absent from the literature partly because lithium hydride is such an aggressive material; once it is liquified, it is very difficult to contain without it reacting, or escaping its confines.
To surmount these problems, Craig worked with the ISIS Neutron and Muon Source to develop a glovebox, especially designed for use with the high-pressure equipment on the PEARL instrument (the Paris-Edinburgh press). This allows the use of a micro-TIG welding system in a low moisture and low oxygen environment, to seal very small metallic capsules containing the sample. These capsules are then loaded into the pressure cell, preventing the sample from reacting with moisture in the air. When the sample is molten, the capsules prevent the liquid lithium hydride from escaping the pressure cell.
“We couldn't use the welding system in a regular glovebox", explained Craig, “so we designed our own system. It makes it significantly easier to load moisture-sensitive samples into this type of high-temperature/high-pressure setup. Here at ISIS, one of the advantages of neutrons is that we probe a larger volume of material, giving a highly representative measurement of the sample under these conditions. Neutrons also offer a better contrast for the lighter elements than we can measure with X-rays. We can heat reliably up to 1300 K which should be enough to see it melt a few times at different pressures."
Theoreticians are interested in these samples because they are chemically simple, so you can perform a lot of complex calculations to compare against experimental data. If Craig and Christopher are able to obtain good data, this will almost certainly be revisited by theoreticians to try and match what they see but the trick is obtaining an accurate measurement where it melts, which sits in about a 100 degree window.
PEARL Instrument Scientist Christopher Ridley adds, “It's an unusual experiment, combining some of the most difficult experimental conditions we work with on the instrument. Users who come to PEARL don't need a working knowledge of high-pressure, but Craig's background in the field made him well suited to developing this equipment with us, which is now ready for future users. It's been a fantastic collaboration between ISIS, AWE and with Holger Kohlmann at University of Leipzig. Lithium hydride is challenging to study with neutrons, as it used extensively to shield against them! It was essential to isotopically enrich the sample with 7Li. The sample is difficult to use in applications due to its reactivity, but it is an interesting model system."
