Dynamic structure in a molten binary alloy by ab initio Molecular Dynamics: Crossover from Hydrodynamics to the Microscopic Regime

TL;DR

This study uses ab initio molecular dynamics to analyze the dynamic structure factor of a molten Li-Na alloy, revealing a crossover from hydrodynamic to microscopic regimes and predicting observable modes in X-ray scattering.

Contribution

First ab initio simulation of a binary alloy showing the transition from hydrodynamic to microscopic collective modes at specific wavevectors.

Findings

Identification of hydrodynamic sound dispersion at low q values.

Observation of two distinct collective modes at higher q.

Prediction that inelastic X-ray scattering can detect the slow mode.

Abstract

The dynamic structure factor of the 7Li0.61Na0.39 liquid alloy at T=590 K has been calculated by ab initio molecular dynamics simulations using 2000 particles. For small wavevectors, 0.15 <= q/A-1 <= 1.6, we find clear side peaks in the partial dynamic structure factors. Whereas for q <= 0.25 A-1 the peak frequencies correspond to the hydrodynamic sound dispersion of the binary alloy, for greater q values we obtain two modes with phase velocities above and below the hydrodynamic sound. A smooth transition between hydrodynamic sound and the two collective modes is shown to take place in the range 0.25 <= q/A-1 <= 0.35. The mass ratio in this system, mNa/mLi = 3, is the smallest one so far for which the fast mode is observed. We also predict that inelastic X-ray scattering experiments would be able to detect the slow mode, and explain why the inelastic neutron scattering experiments [P.R. Gartyrell-Mills et al, Physica B 154, 1 (1988)] do not show any of these modes.