The role of defects for the lock-in transition and the formation of polar nanodomains in A2BX4-compounds

High-resolution gamma-ray diffraction in combination with in situ impedance spectroscopy has been used to characterize the lock-in transition in high-quality (K1-xRbx)(2)ZnCl4 mixed crystals with different doping levels x (0 < x < 7.5%). It is shown that the phase transition becomes extremely sluggish if the defect concentration is increased. While the incommensurate phase is destroyed quite rapidly, the adaption of the stable and well-ordered commensurate structure takes several days. In between there is a strongly disordered phase characterized by residual discommensurations which are pinned by the impurity defects. By careful preparation of the samples and consideration of the kinetic behavior, the lock-in transition and the interesting regime just above T-c where polar nanodomains exist can be shifted towards room temperature.