Abstract
By means of accurate measurements of birefringence, phase diagrams in the (E,T) plane have been constructed around the lock-in type of transition between the incommensurate and the commensurate phase of K2SeO4 and Rb2ZnCl4. While the features characterising the first-order nature of the transition are insensitive to E up to 40 kV cm-1, Tc follows the law Tc(E)=Tc(0)-A mod E mod ln(B mod E mod ). Under an applied field, the permittivity anomaly at Tc gradually disappears. This can be understood by assuming that the soliton lattice response becomes clamped. The behaviour of permittivity along complex trajectories in the (E,T) plane has been measured and is discussed on the basis of a generalised phase diagram.