Thermodynamics and Dielectric Properties of the Nh_4h_2po_4 Type Antiferroelectrics

Abstract

Within the framework of the proton model we study thermodynamic and static dielectric properties of the ADP type antiferroelectrics with taking into account the tunnelling effects of protons on hydrogen bonds. In the fourparticle cluster approximation for the short-range interactions and in the mean field approximation for the long-range interactions we calculate the free energy, entropy, and components of static dielectric susceptibility of the crystals. We also get the system of equations for the sublattice proton order parameter and an equation for the phase transition temperature. We find the values of the theory parameters for the NH 4 H 2 PO 4 and NH 4 H 2 AsO 4 crystals and get a good agreement between the theoretical results and the corresponding experimental data for the temperature dependences of the spontaneous sublattice polarization, proton specific heat, longitudinal and transverse static dielectric permittivities of the crystals.

Thermodynamics and dynamical properties of the KH_{2}PO_{4} type ferroelectric compounds. A unified model

“…In KD 2 PO 4 the calculated difference is (T c − T 0 ) D = 1.159 K, which is rather close to the experimental value 1, 5 K [216]. In RbH 2 PO 4 the theory yields (T c − T 0 ) H = 0.032 K, which agrees with the value obtained with tunneling taken into account [121]. Using the value of T 0 = 147.57 K from [247] for RbH 2 PO 4 we can find that here T c − T 0 ∼ 0.04 K. In KH 2 AsO 4 the calculated difference is (T c − T 0 ) H = 1.281 K; taken tunneling being taken into account increases it up to (T c − T 0 ) H = 1.611 K [121], whereas the experiment gives 2.63 [248] [119].…”

“…Unfortunately, it is impossible to find the values of the model parameters that would yield a larger value of T c − T 0 and still provide a quantitative agreement with experiment for other characteristics of the crystal. Tunneling being taken into account increases the difference T c − T 0 up to 0.060 [121]. In KD 2 PO 4 the calculated difference is (T c − T 0 ) D = 1.159 K, which is rather close to the experimental value 1, 5 K [216].…”

“…Similar investigations of the proton ordering model for the NH 4 H 2 PO 4 type antiferroelectrics with short-range and long-range interactions and tunneling were performed in [122]. The temperature behavior of sublattice spontaneous polarization, proton specific heat, and dielectric permittivities of undeuterated antiferroelectrics of the NH 4 H 2 PO 4 are described; at the proper choice of the theory parameters the theoretical results well agree with experimental data.…”

“…A thorough investigation of the proton ordering model for the KH 2 PO 4 type ferroelectrics with short-range and long-range interactions and tunneling was made in [120,121]. A method for analytic calculations of the static dielectric susceptibility tensor components was proposed.…”

“…In [123][124][125][126] within the framework of proton ordering model with tunneling being taken into account and within four-particle cluster approximation, using the results of [120,121], the thermodynamic and dielectric characteristics of the partially deuterated Rb(H 1−x D x ) 2 PO 4 and K(H 1−x D x ) 2 PO 4 ferroelectrics were calculated in the mean crystal approximation. A good agreement of the obtained results with the corresponding experimental data was obtained.…”