COMPRESSIBILITY OF HYDROGEN BONDS. Andrzej Katrusiak, Department of Crystal Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan.

Compressibility, one of the fundamental properties of any substance, with reference to hydrogen bonded compounds, abundant in nature and industrial or pharmaceutical products, has received a relatively little coverage in literature. General features of the pressure dependence of hydrogen bonded structures - ionic or molecular crystals (e.g. ices) - will be discussed. Despite considerable differences in the composition and structure between such crystals, compressibility of the OH--O bonds is remarkably similar. A simple model can be applied to understand the mechanism of the hydrogen-bond transformations in the microscopic scale. High pressures change the balance between the hydrogen bonds and other cohesion forces, leading to phase transitions. The thermodynamic character of the phase transitions and the transformation of the hydrogen bond are interdependent: the first-order phase transitions are connected with breaking/formation of hydrogen bonds or proton transfer between the hydrogen bonded groups, while the second order phase transitions often involve proton disordering. Characteristic changes in the hydrogen bond dimensions accompanying these phase transitions will be systematized [A.Katrusiak, J.Mol.Struct. 269(1992)329; Phys.Rev. B48(1993) 2992, Phys. Rev. B51(1995)589; Cryst.Rev. - in press], and a series of physical phenomena resulting from these changes will be presented: for example, the structural origin of the tricritical point in the KDP-type ferroelectrics (KDP denotes the potassium dihydrogen phosphate) will be shown to be combined with the hydrogen-bond features, which allows one to evaluate the critical pressure or the Curie temperature from structural data.