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 T_c there exists ordering of the protons in one of these two minima. A large increase in T_c 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 T_c), the hydrogen-bond length decreases initially with increasing pressure, but then an abrupt and pronounced elongation occurs at a critical pressure P_c, corresponding to 2.7 and 4.2 GPa for KH₂PO₄ and KD₂PO₄ respectively. At P_c, 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 P_c. 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.