Attempt of Quantum Chemical Modeling of the Structural Phase Transition in the Quasi-One-Dimensional H-Bonded Ferroelectric Crystal of CsH 2 PO 4 and Its Deuterated Analog CsD 2 PO 4

Thermodynamic features of the structural phase transition in an H-bonded ferroelectric material—cesium dihydrophosphate—and its deuterated analog were studied within the pseudospin Ising model with taking into account tunneling and long-range interaction. All the parameters of the pseudospin Hamiltonian that are necessary for analyzing the phase transition—the Slater parameters and the tunneling integrals—were found according to a procedure based on an independent quantum chemical method of determining them. A simplified scheme for choosing a model cluster was used, which allowed one to calculate the parameters using high-level methods (CCSD/6-311+G**). This scheme was proposed earlier to study similar quasi-one-dimensional ferroelectric materials Pb(H/D)PO 4 . Because of the extremely low reliability of the neutron diffraction data, the structural parameters of the para phase, including those of H-bonds, were widely varied, which enabled one to make reasonable estimates of the critical transition temperatures for both systems in the Bethe cluster approximation.