Nature Of Ferroelectric Phase Transition Research Articles

Lanthanum induced diffuse phase transition in high energy mechanochemically processed and poled PLZT 8/60/40 ceramics

La+3 substitution based piezoelectric material, (Pb0.92La0.08) (Zr0.60Ti0.40)O3 also known as PLZT 8/60/40, was prepared by high energy mechano-chemical ball milling. The highly reactive nature of the nano size (∼15–40 nm) milled PLZT powders enable the partial perovskite phase (confirmed by X-ray diffraction) at room temperature itself. Compaction of these fine scale nano powders by the cold isostatic pressing technique leads to a higher density >98% with a close packed dense microstructure that can be seen in SEM images for sintered ceramics. Dielectric measurements for the electrically poled PLZT 8/60/40 ceramics suggest that the lanthanum substitution affects the nature of ferroelectric phase transition of PZT showing a deviation from normal to the diffuse type phase transition (DPT). The parameters of this DPT, such as degree of deviation from Curie–Weiss law (ΔTm), diffuseness empirical parameters γ and ΔTdiff were calculated at various frequencies from 1 kHz to 500 kHz. The large value of γ (1 < γ < 2) and ΔTdiff at said frequencies confirms the deviation from normal phase transition and the high degree of disorderliness in the material. To support this study, polarization vs. electric field and electromechanical coupling factor (kp) measurements were also performed on PLZT 8/60/40 ceramics within the temperature range of 30 °C–170 °C and 30 °C–210 °C respectively. The effect of temperature on the ferroelectric properties was also studied.

The Nature of the Ferroelectric Phase Transition in the Modified SbSI Ceramics

The modified SbSI (ceramic) has been grown by Bridgman method. The dielectric permitivity of modified SbSI along c(z)-axis has been measured using LCR meter as a function of temperature at fixed frequency 1 KHz. The nature of ferroelectric phase transition in the modified SbSI has been discussed using ab initio calculations. Theoretical investigations reveal how ferroelectricity and phase transition in the modified SbSI are closely related to deformation of the unit cell and the phonon interaction. Dielectric measurements reveal that substitution of the individual (I−) with (Cl−) ions creates the deformation of the unit cell and increase the temperature of the ferroelectric phase transition TC. Deformations of the unit cell change the interaction between phonons and the anharmonicity of soft B1u normal mode.

Enhanced polarization and dielectric properties of Pb(Zr1−xTix)O3 thin films

We report the fabrication of PbZr0.57Ti0.43O3 (PZT) thin films with preferential growth along (111) and random crystalline orientation on the platinized silicon substrates using pulsed laser deposition technique. X-ray diffraction patterns and surface morphology indicate increase in grain size and nucleation, which support better perovskite matrix with increase in annealing temperature. We observed large dielectric constant (∼4000) and enhanced remanent polarization 70 μC/cm2 at room temperature attributed to grain growth and intermetallic Pt–Pb transient phase. Frequency dependent polarization showed minor reduction in polarization above 10 kHz frequencies. Normalized fatigue characteristic of PZT thin films showed minimal 25% degradation in remanent polarization after 109 cycles, which may be useful for memory devices. ac conductivity spectra illustrated that anomaly near the phase transition temperature with activation energy (Ea∼0.60–0.75 eV) supports the intrinsic nature of ferroelectric phase transition.

Relaxor in KF-doped BaTi2O5 ceramics by spark plasma sintering

Dense BaTi2O5 ceramics with KF doping up to 5at.% were synthesized by spark plasma sintering (SPS). The SPS method enabled us to overcome the densifying difficulty in conventional sintering which is caused by the limit of the decomposing temperature of this metastable phase. We observed drastic KF-doping effect on the ferroelectric (FE) properties of BaTi2O5. Although there is only very small change in lattice parameters upon KF doping, the peak temperature in dielectric constant strongly decreases in an exponential way. The diffuseness of phase transition increases and FE relaxor state sets in for KF content as low as 1at.%. Possible structural reasons for the effect and the implication for the nature of FE phase transition in BaTi2O5 are discussed.

Direct observation of dynamic property of domain configuration in ferroelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals with synchrotron topography

In situ observation of the ferroelectric phase transition in Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZNT91/9) single crystals has been carried out by white-beam synchrotron radiation X-ray topography. Sequent phase transitions are confirmed by the dynamic behavior of the domain configuration in the PZNT crystal upon heating, which exhibits good agreement with dielectric measurement of the same sample. Such ferroelectric transitions exhibit a first-order-transition character observed by the dynamic property of the domain structure combined with dielectric measurement. Abnormal fluctuation of domain walls is observed around temperatures for which phase transitions occur, which cannot be interpreted by the nature of the ferroelectric phase transition. Such interesting phenomena are supposed to be related to nucleation and growth of domains and lattice distortion induced by thermal stress. An interpretation is put forward regarding the influence of temperature on the domain-wall energy and the potential barrier surmounted for domain reversal combined with a thermal stress-induced domain effect. This experiment furnishes an understanding of the nature of a first-order ferroelectric phase transition with some diffused character. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Order-disorder nature of ferroelectric phase transition in stoichiometric LiNbO3 Crystals

In order to study the nature of a ferroelectric phase transition of LiNbO3, the A1(z) symmetry optical modes were investigated by Raman spectroscopy. The marked difference among stoichiometric and off-stoichiometric crystals was observed in their damping factors at 90 K. The asymmetric phonon peaks in the spectra were analyzed by taking account the anharmonicity of the interatomic potential. The temperature dependence of phonon frequency and damping was determined using the fitting by a superposition of multi-Lorentzians. The lowest mode frequency does not display the marked softening, while the damping factor increases gradually upon heating, in stoichiometric crystals. The validity of the order-disorder nature was examined for the ferroelectric phase transition

Method of bifurcation of nonlinear integral equations in the theory of ferroelectricity

The bifurcation method of a nonlinear equation system determining the crystal state has been effective in theoretical investigations of the nature of ferroelectric phase transition. Hereafter the method is developed relevant to ferroelectrics of the perovskite type.The cubic barium titanate in the self-consistent field approximation (SCF) is described by means of five single-particle distribution functions, being solutions of a system of nonlinear engaged integral equations. Since the system is nonlinear, bifurcation of solutions appears. It is shown that the latter may be correlated to phase transition (PT) in the crystal state, namely the set of branching solutions characterizes the phase conditions of the crystal near PT temperature, being the bifurcation temperature at the same time. A method is defined by which the bifurcation temperature can be found. The latter appears to be a function of density and an integral characteristic of interaction forces between the structural elements of the crystal. So...