Decoupling of the order–disorder and displacive contributions to the phase transition in NH 4H(ClH 2CCOO) 2

Abstract

The effects of hydrostatic pressure and substitution of Rb +for the ammonium cations on the ferroelectric phase transition temperature in NH 4H(ClH 2CCOO) 2 have been studied by electric permittivity measurements. The transition temperature ( T c) decreases with increasing pressure up to 800 MPa and the pressure coefficient d T c/d p=−1.4×10 −2 [K/MPa] has been experimentally determined. The substitution of Rb + for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.