Vanishing of Transition Temperature in Hydrogen-Bond Crystals Under High Pressure

Abstract

Some of hydrogen-bond crystals are well known to undergo order-disorder type phase transitions at ambient pressure. After recent experimental studies, however, transition temperatures T c of these crystals vanish at respective critical pressures p c contrary to the third law of thermodynamics. The Curie constants in KDP (KH 2 PO 4 ) and DKDP(KD 2 PO 4 ) were shown to hold finite values in contrast to the vanishing of T c for p > p c . This fact has been explained as the crossover of phase transition mechanism from the order-disorder type to the displacive one under high pressure. The saturation of the divergence in the dielectric constant k c near T = 0 in both crystals has been fitted with the Barrett formula given in terms of three parameters used to discuss the quantum paraelectricity. The pressure dependences of the three parameters have been evaluated for p > p c . The mechanism of phase transition in hydrogen-bond crystals under high pressure has been discussed on the basis of these experimental res...