Improper Ferroelectric Phase Transitions

Recent progress in the understanding of the improper ferroelectric phase transitions is reviewed. The phase transition in ammonium Rochelle salt is discussed from the symmetry point of view. The phase transitions in some improper ferroelectrics, such as langbeinite, boracite and dicalcium strontium propionate, are discussed and several problems on these materials, which are still controversial, are pointed out. In connection with the isomorphous phase transition recently discovered in dicalcium strontium propionate, the role of identity representation in the theory of phase transitions is briefly discussed.