THERMODYNAMICS AND DIELECTRIC PROPERTIES OF KH_2PO_4, RbH_2PO_4, KH_2AsO_4, RbH_2AsO_4 FERROELECTRICS

Abstract

Within the proton ordering model with taking into account the short-range and long-range interactions as well as tunnelling effects, in the framework of the cluster approximation, we study the thermodynamic and static dielectric properties of the KH 2 PO 4 type ferroelectrics. Theoretical results are compared with the corresponding experimental data and with the results of theoretical calculations of other sources. We show that under the proper choice of the theory parameters, a very good quantitative description of the available experimental data is obtained for the temperature dependence of spontaneous polarization, specific heat, longitudinal and transverse static dielectric permittivities, as well for the Curie-Weiss temperature and constant. Suitability of the model for the description of experimental data for the crystals KH 2 PO 4 , RbH 2 PO 4 , KH 2 AsO 4 , RbH 2 AsO 4 without taking into account striction, fluctuations, and other effects is discussed.

Thermodynamics and Dielectric Properties of the Nh_4h_2po_4 Type Antiferroelectrics

“…We also obtain the unphysical behaviour for the physical characteristics of the proton ordering model for NH 4 H 2 PO 4 type crystals. The character of this behaviour is practically the same as that described in [9,21,22,48,49]. It should be only mentioned that the temperature where this behaviour emerges is about 80 K for the sets 1, 2 and 110 K for the set 3.…”

“…HereĤ is given by (2.1), whereasV describes the interaction of protons with electric field E; this interaction in the present case has a similar form as that for the KH 2 PO 4 type crystals [22,27]…”

“…As has been mentioned in [9,21,22,48,49], the cluster approximation for a Hamiltonian with non-commuting operators yields an unphysical behaviour of the physical characteristics in a certain region of low temperatures. We also obtain the unphysical behaviour for the physical characteristics of the proton ordering model for NH 4 H 2 PO 4 type crystals.…”

“…Our goal is a systematic study of the dependence of the above characteristics on the theory parameters in order to set the optimal values of the parameters which would provide a good quantitative agreement between the theory and the experiment for two crystals: NH 4 H 2 PO 4 and NH 4 H 2 AsO 4 . The mentioned study is carried out using the proposed in [21,22] Thus, the theoretical formulas for thermodynamic and static dielectric characteristics of the ADP type antiferroelectrics contain five independent theory parameters Ω,ε,w,w 1 , and ν a . Since the energyw 1 of the least probable proton configurations with four or with none protons near a given tetrahedron is very large, it is often put equal to infinityw 1 → ∞ (see [14,[27][28][29]).…”

“…Numerical analysis will be carried out using the fitting procedure [21,22] proposed earlier for the ferroelectric KDP type crystals, modified to the antiferroelectric ADP type crystals. The obtained results will be compared with the experimental data.…”