The polarization field relationship in the improper ferroelectric lock-in phase of purified Rb 2 ZnCl 4 crystals is analyzed under quasistatic conditions. Domain walls are shown to interact in the same way as an ensemble of phase solitons. This justifies the application of soliton theory to derive information about the ratio n ϭw/x 0 between wall thickness and wall distance. Above TЈϷ140 K, n is found to increase with temperature and thermal hysteresis between cooling and heating data is observed. Apparently, x 0 increases via annihilation of antistripples on cooling until an equilibrium distance x 0 (T) is achieved, whereas it remains constant on heating. In addition, w(T) seems to be an unambiguous increasing function of temperature. However, a completely different behavior is observed below TЈ where n is constant, indicating a freeze in of both x 0 and w. We suggest that this may indicate a transformation from rough domain walls above TЈ to flat walls at low temperatures.