Proton vibrational dynamics in lithium imide investigated through incoherent inelastic and Compton neutron scattering

In the present study we report neutron spectroscopic measurements on polycrystalline lithium imide,namely, incoherent inelastic neutron scattering at 20 K, and neutron Compton scattering from10 K up to room temperature. From the former technique the H-projected density of phonon statesup to 100 meV is derived, while the latter works out the spherically averaged single-particle (i.e.,H, Li, and N) momentum distributions and, from this, the mean kinetic energies. Only for H at thelowest investigated temperature, non-Gaussian components of its momentum distribution are detected. However, these components do not seem directly connected to the system anharmonicity,being fully compatible with the simple N-H bond anisotropy. Neutron data are also complementedby ab initio lattice dynamics simulations, both harmonic and, at room temperature, carried out in theframework of the so-called "quantum colored noise thermostat" method. The single-particle meankinetic energies in lithium imide as a function of temperature show a quite peculiar behavior at themoment not reproduced by ab initio lattice dynamics methods, at least as far as H and Li are concerned. As matter of fact, neither their low temperature values nor their temperature trends can beprecisely explained in terms of standard phonon calculations.