The dynamical structure factor S (Q,ω) of solid α-N2

Abstract

We report the first results of a classical computer simulation (molecular dynamics) study of the dynamical structure factor S (Q,ω) for a simple model of a homonuclear diatomic solid in the α-N2 structure. For a particular wave vector Q the peaks in S (Q,ω) can be identified with phonons and their associated widths as inverse lifetimes. The model consists of 250 molecules arranged with periodic boundary conditions in the cubic Pa3 structure and interacting via a Mie–Lennard-Jones (12–6) potential between their ends. Two runs were carried out with parameters such that for solid α-N2 they corresponded to the conditions (T=17 K, a=5.65 Å) and (T=35 K, a=5.684 Å). In neither case do the phonon peaks agree well with the predictions of quasiharmonic lattice dynamics. Moreover, at the higher temperature many of the S (Q,ω) peaks show a large quasielastic peak which we interpret as being because of ’’rotational diffusion’’ of the molecules.