LAUSR

Thermodynamic features of the structural phase transition (SPT) in the H-bonded ferroelectric material PbHPO4 (LHP) have been considered using a pseudo-spin Ising model with inclusion of tunneling and long-range effects. To determine all pseudo-spin Hamiltonian (PSH) parameters necessary for analysis of the SPT—Slater parameters and tunneling integrals, a technique based on an independent quantum-chemical method of their finding was applied. A simplified scheme has been suggested for selecting a model cluster, which makes it possible to use higher-level methods (CCSD and QCISD with the 6-311+G** basis set) in calculations of double-well potential profiles and PSH parameters. The computation results have been discussed in the framework of two statistical models—in the molecular field approximation and using the Bethe cluster method. The critical temperature of the transition of LHP has been evaluated and it has been demonstrated that experimental data can be semiquantitatively reproduced only in the statistical cluster approximation with inclusion of tunneling and long-range effects.