The influence of defects on the phase transition behavior of A2BX4-compounds was studied by diffraction methods, impedance spectroscopy and atomic force microscopy. It was found that chemical point defects have a strong effect on the transition kinetics, whereas the thermodynamic stability of different modulated phases is only slightly affected by such doping with alkali ions. The lock-in-transition from incommensurate to commensurate modulated is slowed down by several orders of magnitude. The transition time hereby ranges from minutes to months, depending on impurity concentration. Due to this, metastable incommensurate phases can be obtained at room temperature. These undercooled phases exhibit remarkable structural and dielectric properties, such as polar nanodomains. The metastable phases undergo additional phase transitions into newly discovered commensurate phases. A unique property of those phases is a memory effect, which allows the incommensurate phase to be re-formed upon subsequent heating. A detailed microscopic model is discussed, which describes all aspects of the observed behavior. This model is based on the formation of a defect density wave and considers its interaction with discommensurations in the incommensurate phase. The thermodynamic properties of all observed phases can be deduced from this model.
