title: Proton vibrational dynamics in lithium imide investigated through incoherent inelastic and Compton neutron scattering.
abstract: 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 from 10 K up to room temperature. From the former technique the H-projected density of phonon states up 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 the lowest 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 complemented by ab initio lattice dynamics simulations, both harmonic and, at room temperature, carried out in the framework of the so-called "quantum colored noise thermostat" method. The single-particle mean kinetic energies in lithium imide as a function of temperature show a quite peculiar behavior at the moment 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 be precisely explained in terms of standard phonon calculations.
authors.affiliation: Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milan, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Pietropaolo
authors.forename: D
authors.initials: D
authors.lastname: Colognesi
authors.forename: M
authors.initials: M
authors.lastname: Catti
authors.forename: A-C
authors.initials: AC
authors.lastname: Nale
authors.forename: M A
authors.initials: MA
authors.lastname: Adams
authors.forename: A J
authors.initials: AJ
authors.lastname: Ramirez-Cuesta
authors.forename: J
authors.initials: J
authors.lastname: Mayers