Phase transition of the novel hydrogen bonded ferroelectric (NH4)2H2P2O6 studied by specific heat measurements

The first precise measurements of specific heat changes have been performed for a novel ferroelectric material, (NH4)2H2P2O6 by means of an ac calorimeter. The paraelectric–ferroelectric phase transition at about 177.5 K of second order type is revealed and described by the Landau model using specific heat and linear birefringence data. Thermal parameters (such as the excess enthalpy (ΔH) and the excess entropy (ΔS)) of the continuous phase transition were estimated and discussed. The calorimetric measurements indicate a complex mechanism of the ferroelectric transition, both a displacive and order–disorder contributions are suggested.

► Pure continuous phase transition in hydrogen bonded ferroelectric (NH4)2P2O6.
► Order–disorder and displacive phase transition mechanism confirmed by the transition excess entropy value (∼1/3R ln 2).
► The ferroelectric phase transition is far from the tricritical one.