CONICET | Buscador de Institutos y Recursos Humanos

In 1939 Robertson and Ubbelohde observed that Deuterium substitution for Hydrogen may lead to structural changes in H-bonded compounds.1 H-bonded ferroelectrics, whose transition happens when Hydrogens order off-centred in the bridges, show a dramatic isotope effect on Tc upon deuteration. Typical is the KDP (KH2PO4) family of materials. In 1960 Blinc et al.2 explained this effect in KDP based on the supposition that H tunnels between two equivalent off-centred positions in the bridge between PO4 tetrahedra. However, the Ubbelohde effect was not taken into account. In fact, the lattice constants of KDP grow significatively upon deuteration. Since 1978, X-ray and high-resolution neutron measurements in H-bonded ferroelectrics showed linear correlation between Tc and several structural parameters, particularly the O-H···O bridge length.3 Thus the tunneling model required to be revised. By means of ab-initio DFT calculations we found that the observed bistable H distribution in the bridges can only be explained if correlated motions of H clusters are considered, the required cluster size being extremely large if relaxation of the heavier atoms is not allowed.4 Thus, the mass change alone, due to deuteration, is not enough to explain the huge isotope effect in Tc. It is necessary to consider the structural changes arising from nucleae localization grow in the H-bridges upon deuteration, which is accompanied by electronic localization. This weakens the H-bond, which becomes then longer and leads to further localization, and so long, in a self-consistent form. This process was qualitatively simulated through a model we had proposed.4 By means of analogous calculations we explain the origin of antiferroelectricity in the isomorphous compound ADP, where A (NH4) substitutes K in KDP. ADP shows also a huge isotope effect upon deuteration. The feedback effect between tunneling and structural modifications observed in the H-bonded ferroelectrics is a phenomenon of wider implications, particularly important in many biological process.