VANISHING OF THE PHASE-TRANSITIONS IN FERROELECTRIC HEXAGONAL BATIO3 AT HIGH-PRESSURE

The dielectric constant epsilon(c) along the c-axis of hexagonal BaTiO 3 has been measured from 5 K to 300 K as a function of hydrostatic pre ssure p up to 4 GPa. The hexagonal to orthorhombic transition point T0 (222 K at 10(5) Pa) decreases at a rate of 34 K/GPa and vanishes at p ressures above p(c) = 3.4 GPa; the T0 vs p curve is well represented b y an equation of T0 = a(p(c)-p)1/2 over the whole pressure range. Whil e, the ferroelectric phase transition point T(C) (77 K at 10(5) Pa) in itially increases and then decreases only slightly with increasing pre ssure; interestingly, the dielectric anomaly at T(C) disappears at pre ssures above 1 GPa. Another lattice instability like a quantum paraele ctricity seems to exist at low temperatures. Some discussions are pres ented for the mechanism of the phase transitions.