Why is there an isotope effect on Tc in hydrogen-bonded ferroelectrics upon deuteration but absent upon replacing 16O by 18O?

Isotope effects in ferroelectrics have not attracted much attention in the past, with the exception of the large effect on the transition temperature T-c in hydrogen-bonded ferroelectrics upon deuteration. Reinvestigating this class of compounds experimentally reveals that the exchange of oxygen by its isotope does merely not affect T-c of hydrogen-bonded KH2PO4 and squaric acid, in contrast to the recently observed isotope induced ferroelectricity in SrTiO3. The new findings in hydrogen-bonded systems are in agreement with theoretical predictions of a vanishing isotope effect in the classical regime and substantially support our earlier conclusions on the coexistence of order-disorder and displacive dynamics in these compounds.