Effect of the Hydrostatic Pressure on the Thermodynamics of Structural Phase Transition in Quasi-One-Dimensional Ferroelectrics Pb(H/D)PO 4 : Quantum Chemical Analysis

The change in the structural phase transition thermodynamics of lead hydrogen phosphate (LHP) and lead deuterium phosphate (LDP) depending on the external hydrostatic pressure was studied using the Ising pseudo-spin model taking account of tunneling and long-range effects. The same simplified version of the quantum chemical approach that we proposed for studying these systems at atmospheric pressure was used. The critical transition temperatures T c were estimated for both systems and the considerable decrease in T c observed experimentally was explained in each case. The calculations gave an estimate for the width of the barrier in LHP (≈0.35 Å); no experimental neutron diffraction value for this barrier is available from the literature.