Measuring the Melt curve of ¬7LiH at High Pressures

Lithium Hydride (LiH) has often been viewed as an ideal test case of theories on simple ionically bonded molecules (Fm-3m, a=4.075Ǻ). The small atomic mass of both elements makes LiH particularly susceptible to isotopic shifts in properties. Isotopic substitution of Li or H in LiH results in changes in the associated equations of state (EoS) [1], the location of phase boundaries [2], and the melting curve [3] as the isotopic composition is altered. While LiH and LiD have been studied at ambient temperatures at high pressure using both x-ray [1, 4] and neutron [5] diffraction, to date there have been no published experimental studies to confirm theoretical prediction of the high-pressure high temperature (HPHT) properties of Lithium Hydride or the isotopically enriched variants (6LiH, 6LiD, 7LiH, 7LiD). Further to an earlier experiment on the more neutronically favourable variant 7LiD, it is proposed that the melt curve of 7LiH is examined with neutron diffraction. Specifically looking at the structural changes upon approaching melting and determination of some experimentally determined data points on the melting curve, which has so far only been determined at ambient pressure [1, 4]. Additionally, by looking at the structure approaching melting we will be able to look for dissociation of the Li and H species or changes to the stoichiometry that would give rise to new structural phases. All these measurements would add detail to the growing volume of theoretical work