Structural origin of tricritical point in KDP-type ferroelectrics

At ambient pressure the phase transition in hydrogen-bonded ferroelectric crystals of the (KDP stands for KH2PO4) is a combined effect of atomic displacements and of proton disordering in hydrogen bonds. It is shown, that the occurrence of tricritical point in these materials result from the competition between the structural features connected with the hydrogen bonds, favouring angular displacements of structural units from the positions required by the paraelectric symmetry, and van der Waals and electrostatic interactions favouring the symmetric arrangement. The displacements, contributing to the first order character of the phase transition, are reduced at elevated pressures. The tricritical point of the phase transition in KDP had been evaluated from structural data.