Abstract
A review is given of the deuteration effect on the crystal structure, in particular on the hydrogen-bond system, and its relation to the phase transition. The correlation between hydrogen-bond parameters and some examples of peculiar phase diagrams between deuterium content and temperature are discussed in the context of the geometric isotope effect. Similarity and difference between the deuteration effect and the negative pressure effect are discussed in the case of KH2PO4 and squaric acid (H2C4O4) . It is pointed out that the hydrogen bond is not necessarily the softest part of the crystal as usually assumed, and pressure effect is responded by rotation of the PO4 or C4O4 group as well. The case of (NH4) 3H (SO4) 2 is discussed as an example where a surprising similarity between deuteration and positive pressure effect is seen in their phase diagrams.
