GEOMETRIC EFFECTS OF DEUTERATION ON HYDROGEN-ORDERING PHASE-TRANSITIONS

IT has long been known that the substitution of deuterium for hydrogen in hydrogen-bonded materials can lead to a change in the geometry of the hydrogen bonds - the so-called Ubbelohde effect1. There can also be significant accompanying changes in the physical properties of the material. Among the most striking examples is the increase in the transition temperature, Tc, of hydrogen-ordering systems of the KH2PO4 type by >100K on full deuteration. In all of these systems, Tc decreases under pressure, making it possible in principle to compare protonated and deuterated forms at the same Tc by applying pressure to the latter. Here we report the results of a high-pressure neutron diffraction study of the deuterated H(D)-ordering material PbDPO4. We find that much, and possibly all, of the increase in Tc on deuteration is attributable to the accompanying changes in the hydrogen-bond dimensions. This suggests that purely mass-dependent effects on the quantum-mechanical tunnelling between the equivalent H(D) potential minima - an explanation that has been favoured previously - have little or no direct influence on Tc. Substantial revisions to the theory of this type of ordering transition are apparently required. © 1990 Nature Publishing Group.