  Hydrogen and lithium, along with their compounds, are crucial materials for
nuclear fusion research. High-pressure studies have revealed intricate
structural transitions in all these materials. However, research on lithium
hydrides beyond LiH has mostly focused on the low-temperature regime. Here, we
use density functional theory and ab initio molecular dynamics simulations to
investigate the behavior of LiH2, a hydrogen-rich compound, near its melting
point. Our study is particularly relevant to the low-pressure region of the
compression pathway of lithium hydrides toward fusion. We discovered a
premelting superionic phase transition in LiH2 that has significant
implications for its mass transportation, elastic properties, and sound
velocity. The theoretical boundary for the superionic transition and melting
temperature was then determined. In contrast, we also found that the primary
compound of lithium hydrides, LiH, does not exhibit a superionic transition.
These findings have important implications for optimizing the compression path
to achieve the ignition condition in inertial confinement fusion research,
especially when lithium tritium-deuteride(LiTD) are used as the fuel.
