Ultrasonic Studies of Incommensurate Phase Transitions

The temperature dependencies of the ultrasonic velocities and attenuation coefficients are studied in the vicinity of the incommensurate phase transitions of BCCD and AHSe crystals. The largest contribution to the longitudinal ultrasonic anomalies is due to the coupling to the amplitude mode, whose relaxation time τ diverges as the transition temperature Ti is approached with the temperature dependence given by : τ=τoTi/T-Ti). The amplitudon relaxation time for AHSe crystal (τo=5.1.10-12s) was found to be considerably longer than for BCCD (τo=1.3.10-13s). From the measurements of shear ultrasonic wave velocity in incommensurate phase below Ti = 164 K the critical exponent that describes the order parameter temperature behaviour has been calculated for BCCD. The value β is consistent with three dimensional X - Y model. The substitution of Cl by Br in BCC1-xBrxD crystals with x<0.24 manifests itself in smearing of the elastic anomalies and shifting Ti to lower temperatures with increase of Br concentration

Dielectric Dispersion in Ferroelectric Glycine Phosphite

Dielectric dispersion in ferroelectric hydrogen bonded glicyne phosphite crystal was investigated in the frequency range 100 Hz - 27 GHz. Dielectric relaxation of Debye type observed in the paraelectric phase shows a critical slowing down of the polarization fluctuations. The relaxation frequency decreases with temperature according to f$\text{}_{s}$ = 0.305(T-T$\text{}_{0}$) GHz in the paraelectric phase. The activation energy for flipping dipole motion ΔU = 2.07kT$\text{}_{c}$ confirms order-disorder character of the phase transition. In the ferroelectric phase pronounced low frequency (100 Hz - 1 MHz) dispersion related to domain contribution to permittivity was found

Dielectric and magnetic properties of BaTiO3-NiFe2O4 multiferroic composites

Multiferroic composites with the general formula x BaTiO3-(1-x) NiFe2O4 (x=0.5, 0.7, 0.8) were prepared by mixing chemically obtained nickel ferrite and barium titanate powders. Optimum processing parameters for a reduction of chemical reactions at the interfaces between ferroelectric-ferrite phases were found. The presence of the nickel ferrite and barium titanate single phases was detected by X-ray measurements. Good quality of the homogenized ceramics containing both phases was confirmed by use of scanning electron microscopy. Dielectric measurements showed the superposition of the multiple conductivity mechanisms responsible for the dielectric dispersion at frequencies up to 1 MHz. Calculated activation energies for DC conductivity showed the contribution of oxygen vacancies and Maxwell-Wagner related phenomena. Magnetic measurements of the composite materials were performed showing the coercivity increase and saturation of the magnetization moment decrease with the increase of barium titanate content