STRUCTURE AND DYNAMICS OF SELENIUM CHAIN MELTS - A MOLECULAR-DYNAMICS STUDY

A molecular dynamics (MD) study of liquid selenium modeled by 16 linear chains of 40 monomers each is presented. The simulated thermodynamic state corresponds to the experimental density of 3570 Kg m-3 at 873 K. The structural and force constant data of the chains were obtained from previous studies of neutron diffraction experiments, lattice dynamics, and first principles calculations. The computed structural properties show a good agreement with available neutron scattering data. The flexibility of the chains and the high temperature thermodynamic state of the liquid enabled the observation of fast torsional motions and different spatiotemporal dynamic ranges, which can be described by the Rouse model for dense polymer solutions. We identify the crossover from an atomic to an intermediate or ''universal'' chain regime, and subsequently to global chain behaviors. The dynamics of the system is discussed in terms of time and space-dependent transport coefficients. The generated MD trajectory thus provides information on the single particle motions, the collective dynamics of one chain, and the dynamics of the global system. This separation is useful for understanding the low frequency collective motions which can be measured by inelastic neutron scattering. The spectra are interpreted in terms of existent dynamical models, which imply a degree of trapping of the atoms in some spatial regions of the liquid (''chain cages'') defined by atomic crosslinks, plus a slow diffusive process which modifies the shape of the cage according the renewal of the atomic crosslinks.