Kinetics of the lock-in phase transition in Rb2ZnCl4

High-resolution X-ray scattering experiments have been performed on single crystals of the incommensurate system Rb2ZnCl4 with emphasis on the kinetics of the lock-in phase transition. The nearly perfect single crystal (mosaic spread approximately=0.6 min) displayed a large hysteresis of 20 K. As the sample was cooled quickly from the paraelectric phase into the commensurate phase, via the incommensurate phase, the time evolution of the disappearance of the incommensurate peak intensity was monitored. The time becomes longer when the sample is quenched to lower than the lock-in temperature, which is contrary to the expected kinetic behaviour of nucleation and growth. The characteristic time of decay exhibits an Arrhenius-like behaviour with an energetic barrier Eb to overcome of the order of 0.37 eV. The origin of the energetic barrier is interpreted in terms of an order-disorder process of the ZnCl4 tetrahedra network.