Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb+for the ammonium cations on the ferroelectric phase transition temperature in NH4H(ClH2CCOO)2 have been studied by electric permittivity measurements. The transition temperature (Tc) decreases with increasing pressure up to 800 MPa and the pressure coefficient dTc/dp=−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 Tc. 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.