Isotope effect in hydrogen-bonded crystals with asymmetric O-H-O type bond

The relation between the H atom position in the O-H-O type hydrogen bond and a large increase of the phase transition temperature by deuteration is studied on the basis of the crystal structure data. We find a clear tendency that the symmetric hydrogen bond promotes this isotope effect, while the asymmetric hydrogen bond suppresses this effect as far as the hydrogen bond which combines the tetrahedral groups is concerned.