Theory of related to shear strain u6 physical properties of ferroelectrics and antiferroelectrics of the KH2PO4 family

Abstract

We propose an extension of the proton tunneling model in order to study related to the shear strain u6 dielectric, piezoelectric, and elastic properties of ferro- and antiferroelectrics of the KH2PO 4 family This strain is introduced into the proton Hamiltonian via the linear contributions into the configurational energies of protons surrounding an oxygen tetrahedron. In the four-particle cluster approximation for the short-range interactions and the mean field approximation for the long-range interaction we explore the phase transition and piezoelectric effect in the KH2PO4 ferroelectrics. In the paraelectric phase the dielectric, piezoelectric, and elastic characteristics of the NH 4H2PO4 antiferroelectrics are calculated. At the proper choice of the theory parameters a good agreement between theoretical and experimental results is obtained. Taking into account of the piezoelectric coupling with the shear strain u6 allows one to calculate the static dielectric permittivities of free and clamped crystals as well as to obtain a good description of ferroelectric spontaneous shear strain u6. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.