Time-evolution of phonon spectra during decomposition in AgxNa1-xBr ionic crystals

Real-time inelastic neutron scattering has been used to characterise the mechanism of decomposition in AgxNa1-xBr mixed single crystals well below the critical temperature on a microscopic scale. The chemical demixing process is associated with a splitting of acoustic phonon branches, which is observed to take place within some 100 s. Interestingly, the Bragg reflections exhibit a completely different time behaviour: the adaptation of the equilibrium lattice parameters occurs on a much longer time scale. Hence, decomposition is governed by large coherency strains which persist over a long period of time. Only in a second stage does the crystal lattice relax into the final equilibrium state.