The kinetic of decomposition in the quasi-binary ionic system AgCl-NaCl has been studied by means of time-resolved neutron diffraction and smallangle neutron scattering. The coherent critical point is estimated as 447 ± 3 K, 24 K below the upper critical binodal temperature. The chemical decomposition is almost completed in the first 200 s irrespective of the temperature. The mechanical relaxation of the lattice, however, takes place on a much longer timescale and is dominated by coherency strains leading to metastable intermediate states.

The combination of stroboscopic techniques and neutron diffraction can be used to study the kinetics of structural changes in condensed matter on a microscopic level. Transient states may be identified and characterized on time-scales down to the microsecond regime. Hence, valuable information about the underlying mechanisms can be obtained from time-resolved experiments. Particularly interesting subjects for this type of investigation are spatially modulated systems which undergo phase transitions or phase separation. The potential of stroboscopic neutron diffraction is demonstrated using three different examples in which samples are periodically perturbed by the variation of temperature, mechanical stress or electric field and their structural response is characterized by time-resolved diffraction. Spinodal decomposition in ionic crystals of the silver-alkali halide type is shown to be dominated by two different processes on different time-scales. The stress-induced phase transition into the incommensurate phase of quartz involves relaxation processes which are reflected by different kinetic behaviours of Bragg peaks and satellite reflections, respectively. Finally, metastable transient states are observed during the field induced lock-in transition in ferroelectric Rb

The dynamics of mixed single crystals of AgxNa1−xCl has been investigated by inelastic neutron scattering before and after chemical demixing. In the homogeneous phase, the concentration dependence of acoustic phonons reveals that doping of NaCl with silver chloride leads to a considerable softening of the lattice, while the elastic properties of AgCl are almost independent of sodium chloride additives. After quenching into the miscibility gap, the phase separation is associated with a well-defined splitting of acoustic phonons which provide the most direct information about the underlying mechanism. In contrast to the local dynamical properties, the lattice structure is essentially determined by coherency strains which hinder the relaxation of lattice parameters. Thus diffraction and inelastic scattering yield independent and complementary information about demixing processes in ionic solids.

Real-time inelastic neutron scattering has been used to study the kinetics of the demixing process in AgCl–NaCl mixed single crystals. The variation of transverse acoustic phonon spectra provides the most direct information about the microscopic non-equilibrium behaviour. It is shown that a well defined splitting of phonon peaks corresponding to the formation of the product phases takes place on a timescale of seconds. Along with the observation of satellite reflections these findings clearly demonstrate that the phase separation is governed by the mechanism of spinodal decomposition in this ionic system. During the entire process the lattice parameter keeps almost unchanged, thus leading to a metastable state characterized by large coherency strains.

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