TRICRITICAL POINT IN HYDROGEN BONDED CRYSTALS

Andrzej Katrusiak

Department of Crystal Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland

Hydrogen bonds OH- -O often introduce strains into crystal structures and intermolecular or interionic interactions. The different electronic structures of the hydroxyl and carbonyl oxygen atoms favour slightly different orientations of the hydrogen-bonded molecules or ions, with respect to the O...O direction. The different orientations of the donor and acceptor groups result in orientational displacement of the molecules/ions which, along with the off-centre H-site, may be the main distortions breaking the crystal symmetry [1]. The crystals with the structures of this type often exhibit ferroelectric or antiferroelectric properties, and undergo phase transitions to paraelectric phases when the H-atoms become disordered in the hydrogen bonds [2]. Such ferroelectrics are often referred to as KDP-type ferroelectrics, after the prototypic structure of potassium dihydrogen phosphate, KH₂PO₄. The H-disordering is accompanied by reorientations of molecules to the positions consistent with the high symmetry of the paraelectric phase [3]. The phase transitions at ambient pressures have always some contribution of the first-order transformations, which decreases with increasing pressure and disappears at the tricritical point. It can be shown that this first-order contribution is due to the orientational displacements of the molecules induces by the strains of the hydrogen bonds [4]. This subtle effect illustrates a relation between structure and macroscopic physical properties of the crystals.

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