Coupling of displacive and order-disorder transformations in hydrogen-bonded ferroelectrics

Bistable hydrogen bonds are shown to induce systematic displacements in arrangement of molecules or ions in the structures of ferroelectric crystals: methods of calculating the magnitudes of the displacements, and characteristic structural transformations on disordering of the H atom above Tc are described. The displacements are correlated with Tc, and their contribution to the isotope effect on Tc, and to the differences in Tc between isostructural ferroelectric crystals are discussed.