Vanishing of order-disorder type phase transition in DKDP at high pressure

Temperature dependences of the dielectric constant along the ferroelectric c-axis have been measured at various pressures up to 7.9 GPa in DKDP. The vanishing of the ferroelectric state has been observed at 6.3 GPa. The Curie constant C has been saturated to about three fourths of that at atmospheric pressure above 4 GPa even if the transition temperature Tc becomes 0. The vanishing of Tc with finite C is theoretically derived from the temperature dependence of electric polarizability in displacive type ferroelectrics. The crossover of the order-disorder type to the displacive one under high pressure is discussed on the mechanism of phase transition to explain these experimental results in DKDP.