Vanishing ofTcand appearance of quantum paraelectricity in KD2PO4and KH2PO4under high pressure

The temperature dependences of the dielectric constants of the hydrogen-bond ferroelectrics KH2PO4 (KDP) and KD2PO4 (DKDP) were measured under high hydrostatic pressure. Their ferroelectric transition temperatures Tc monotonically decreased with increasing pressure and the ferroelectric state vanished at pc: 1.7 GPa for KDP and 6.1 GPa for DKDP. On the other hand, the Curie constant remained finite at pc, which indicates that the ferroelectric phase transition at high pressure is of displacive type. At pressures around pc, quantum paraelectricity was observed in KDP and DKDP.