We verify whether the previously developed model of a KD2PO4 crystal, with the shear strain ε6 taken into account, is able to describe the longitudinal electric field E3 influence on the KH2PO4 family ferroelectrics. Major effects of the strain ε6 are splitting of the Slater energies of the shortrange correlations and the effective field created by piezoelectric coupling. Calculated TC − E3 phase diagrams, field dependences of polarization, susceptibility, and elastic constant of deuterated KD2PO4 and pure KH2PO4 well accord with the available experimental data. For a consistent description of all dielectric and piezoelectric characteristics of the crystals, phonon degrees of freedom must be taken into account.
The ground state and thermodynamics of distorted Ising-Hubbard chains are studied with on-site Coulomb repulsion taken into account. A decoration-iteration transformation method is used to obtain exact results for the free energy, entropy, specific heat, magnetization of the Ising and Hubbard subsystems, and magnetic susceptibility. The effect of Coulomb repulsion on the ground state, field and temperature dependences of the magnetization, magnetic susceptibility, and specific heat is studied for the case of a geometrically frustrated system. Strong repulsion leads to formation of an additional high-temperature peak in the specific heat. Independently of any repulsion, the temperature dependence of the specific heat can have two low-temperature peaks.
We propose an extension of the proton tunneling model in order to study related to the shear strain u 6 dielectric, piezoelectric, and elastic properties of ferro-and antiferroelectrics of the KH 2 PO 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 KH 2 PO 4 ferroelectrics. In the paraelectric phase the dielectric, piezoelectric, and elastic characteristics of the NH 4 H 2 PO 4 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 u 6 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 u 6 .
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