Pressure-induced transition of the hydrogen bond in the ferroelectric compounds KH2P04 and KD2P04

Abstract

THE prototypical ferroelectric compound KH2PO4 contains hydro-gen bonds that exhibit interesting properties. The proton lies in one of two equivalent positions between oxygen atoms of two PO4tetrahedra. Below the ferroelectric transition temperature Tc there exists ordering of the protons in one of these two minima. A large increase in Tc is observed when the proton is substituted by a deuteron. Here we report results which show that in both KH2PO4and its deuterated form KD2PO4 at room temperature (that is, above Tc), the hydrogen-bond length decreases initially with increasing pressure, but then an abrupt and pronounced elongation occurs at a critical pressure Pc, corresponding to 2.7 and 4.2 GPa for KH2PO4 and KD2PO4 respectively. At Pc, the bond lengths are nearly equal to those predicted if the proton (deuteron) were to be located in a single minimum at the midpoint of the two oxygens. The P–O bond lengths in the PO4 tetrahedra also begin to decrease above Pc. These observations suggest that there may be a fairly complex interrelation of the various bond lengths involved in the ferroelectric transition, and reveal a hitherto unsus-pected property of the hydrogen bond.