Dielectric, optical, and domain properties of the improper ferroelectric (NH4)2 Cd2(SO4)3

The temperature dependences of single-domain permittivity, spontaneous polarization, and spontaneous birefringence of CAS have been measured. The decrease of ϵ at Ttr and the temperature independence of Ps show that an interaction P2 q2 of polarization with the transition parameters q must be considered in the thermodynamic potential of this material. The birefringence of CAS cannot be explained by electrooptical effects and is mostly due to the improper nature of the ferroelectric transition. The optical distinguishability of domains is discussed and results of a detailed twinning study are presented. Antiparallel domains differ in extinction angles by 18°; they are separated by {100} walls. Boundaries between domains with mutually perpendicular Ps-vectors have the orientations {570}. They are shown to be stressed, not fulfilling the mechanical compatibility requirements. Finally, results of coercive field measurements and of switching mechanism observations are described.