Dynamic structure in a molten binary alloy by ab initio molecular dynamics: Crossover from hydrodynamics to the microscopic regime

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 wave vectors, 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 measurements (GARTRELL-MILLS P. R. et al., Physica B, 154 (1988) 1) do not show any of these modes.