EFFECTS OF HYDROSTATIC-PRESSURE ON THE DIELECTRIC-PROPERTIES OF HEXAGONAL BARIUM-TITANATE AROUND THE 222-K STRUCTURAL PHASE-TRANSITION

Temperature dependence of the dielectric constant of hexagonal BaTiO//3 has been studied at a variety of hydrostatic pressures up to about 10 kbar. The phase transition point T//0 equals 222 K decreases with pressure: dT//0/dp equals minus 3. 4 K kbar** minus **1. The hexagonal crystal deforms slightly and changes to orthorhombic form below T//0 (a prime (ortho. ) approx a(hex. ) approx 5. 72 A, b prime (ortho. ) approx ROOT 3a(hex. ) and c prime (ortho. ) approx c(hex. ) approx 13. 96 A). Temperature dependence of lattice parameters has been examined. No anomaly has been detected in c prime parameter at T//0; however thermal expansion coefficient alpha //a prime for a prime axis changes the sign below T//0. The difference of the volume thermal expansion coefficient above and below T//0 ( DELTA beta // upsilon equals 19 multiplied by 10** minus **6 K** minus **1) and the dT//0/dp are used in the Ehrenfest relation; the specific heat anomaly at T//0 is estimated as DELTA C//p equals 4. 7 J mol** minus **1 multiplied by (times) K** minus **1.