Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(C1H2CCOO)2

The effects of hydrostatic pressure and substitution of Rb+ for the ammonium cations on the ferroelectric phase transition temperature in NH4H(CIH2CCOO)(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 dT(c)/dp = -1.4 X 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. (c) 2005 Elsevier Ltd. All rights reserved.