Yung Park

Low-frequency-dispersion of Pb(Fe1/2Nb1/2)O3 single crystal in the region of its paraelectric ferroelectric phase transition

Abstract An empirical equation at low frequency regime is derived for a part of the spectrum of the complex dielectric constant ϵ ∗ =ϵ′− i ϵ″ of Pb(Fe1/2Nb1/2)O3 single crystal with a linear relationship between real dielectric constant ϵ′ and imaginary dielectric constant ϵ″. This equation is used to find the relaxation time and other relaxation parameters and to investigate their temperature dependences in the phase transition region. The low frequency dispersion in the polar phase can be understood in terms of irreversible motion of domain walls in polydomain crystal. The reasons for the inconsistency between the observed dispersion of ϵ∗ and the dielectric dispersion of the Debye type are discussed.

Particle-size effect on the ferroelectric phase transition in PbSc1/2Ta1/2O3 ceramics

The particle-size dependence on the ferroelectric phase transition in nanocrystalline PbSc1/2Ta1/2O3 ceramics has been investigated for particles in the 0.01–0.16 μm size range. The phase transition was monitored through measurement of the dielectric constant as a function of temperature, variable temperature x-ray diffraction, and differential scanning calorimetry. The c/a tetragonality ratio decreased monotonically with particle size, reducing to unity at 0 °C for particles ⩽0.053 μm in size. The ferroelectric transition temperature Tc decreased gradually and the phase transition became more diffuse in nature as the particle size decreased. Below a mean particle size of 0.02 μm there was no peak in either the dielectric constant or the differential scanning calorimetry curve as a function of temperature, even though variable temperature x-ray diffraction could detect tetragonality indicative of ferroelectric distortions at sufficiently low temperatures down to the smallest crystallite size of 0.01 μm.

Successive phase transitions in the highly ordered complex perovskite PbLu1/2Nb1/2O3

Abstract The structural phase transitions and the electrical behaviour of the complex perovskite PbLu1/2Nb1/2O3 have been investigated using X-ray powder diffraction, dielectric constant measurements, differential scanning calorimetry and measurement of the polarisation as a function of applied electric field. The high-temperature paraelectric phase is highly ordered. A first-order paraelectric-antiferroelectric phase transition occurs at 270°C and an antiferroelectric-ferroelectric phase transition, characterised by dispersion in the curves of dielectric constant as a function of temperature, occurs at ≈ 30°C. The antiferroelectric phase is isostructural with the orthorhombic form of PbYb1/2Nb1/2O3. The low-temperature ferroelectric phase also has an orthorhombic crystal structure.

Glass-like behaviour in the relaxor ferroelectric 0.6 Pb( - 0.4 Pb(

Studies have been conducted on the ferroelectric properties of -, a solid solution of the antiferroelectric oxides and . X-ray diffraction shows that at room temperature this solid solution has a tetragonal perovskite crystal structure. Evidence for a diffuse phase transition in this material is provided from measurements of the dielectric and piezoelectric properties and the form of the P-E hysteresis loop. A Frohlich-type model for the relaxation time spectrum is shown to model the complex dielectric constant as a function of temperature satisfactorily. Cole-Cole plots of the experimental real and imaginary parts of the dielectric constant, and , as a function of temperature and curves of polarization as a function of temperature suggest a freezing temperature of 152 K and 173 K respectively, in contrast to Vogel-Fulcher fits of the temperature dependence of the maxima in and which suggest a freezing temperature of 310 K and 308 K respectively. The nature of this diffuse phase transition has been interpreted in terms of a glass-like behaviour.

Surface effect and phase transitions in ferroelectric thin films

Abstract A phenomenological approach based on the assumptions of Landaus theory of phase transitions is used to treat the transition from the ferroelectric state to the paraelectric state in a thin film. The effect of conditions on the surface is investigated. Conditions are formulated for the occurrence of a surface energy at temperatures exceeding the ferroelectric–paraelectric temperature of a bulk sample.

GLASSY DYNAMICS OF THE INCOMMENSURATE-COMMENSURATE PHASE TRANSITION IN ZR0.98HF0.02TIO4 CERAMICS

The complex dielectric constants of Zr0.98Hf0.02TiO4 ceramics are reported in the frequency range 10−3–1010 Hz for the temperature range 830 °C⩽T⩽860 °C, just above the commensurate-incommensurate phase transition temperature, Tc. The disappearance of x-ray satellite reflections at 825 °C was used to determine Tc experimentally. The dielectric measurements show that a relatively narrow distribution of relaxation times is present in the ceramics, which broadens as the temperature approaches Tc from above. By analyzing the temperature at which the dielectric constant is a maximum as a function of frequency using the Vogel-Fulcher relationship, an activation energy of 550±15 kb,where kb is the Boltzmann constant, and an effective freezing temperature, Tf, of 825 °C have been determined, confirming that, to within experimental error, Tf coincided with Tc. The experimental results for Zr0.98Hf0.02TiO4 are interpreted in terms of a possible dipole-glass-like phase that has been proposed for relaxor systems.

X-ray studies in ZrTiO4 ceramics under a high electric field

Abstract The existence of several new field-induced regions of stability is shown for the commensurate deviation δ corresponding to the high commensurability orders 2 15 , 4 29 and 6 43 . It is suggested that this result can be accounted for by sequences of antiphase domains of the type 〈7〉n〈8〉 (n = 1,3 and 5) where 〈n〉 represents a domain of n consecutive aligned layers. A high field hysteresis effect is also observed.

Crystallographic and Dielectric Studies in the Solid Solution System PbCo1/2W1/2O3–PbYb1/2Ta1/2O3

The crystal structures and dielectric properties of (1-x)PbCo1/2W1/2O3–xPbYb1/2Ta1/2O3 perovskites have been investigated as a function of temperature. A continuous series of solid solutions was found and the phase diagram was determined. Ordered end compositions with 0.0 ≤x < 0.2 and 0.8 < x ≤1.0 showed successive paraelectric-antiferroelectric-ferroelectric phase transitions with decreasing temperature. Diffuse paraelectric-ferroelectric phase transitions occurred for 0.2 < x < 0.8, with the low temperature ferroelectric phase having disorder on the B-sites occupied by Yb3+, Ta5+, Co2+ and W6+. Plots of polarisation as a function of applied electric field confirmed the nature of each crystallographic phase.

Tunnel hopping conductivity of an amorphous film

An expression is proposed for the conductivity of an amorphous film in the range of thicknesses in which the flow of the current is governed by tunnel hopping of electrons between isolated chains of impurities joining metal contacts. Comparison with a numerical estimate confirms the basic principles of the model.

Incommensurate phase of Zr0.98Sn0.02TiO4 ceramics in an applied electric field

Abstract The effect of a d.c. electric field on the dielectric constant and phase stability of Zr0.98Sn0.02TiO4 ceramics between 800°C and 1200°C has been investigated. The results show a non-linear variation of the transition temperature between the incommensurate and the commensurate phase and a decrease in the dielectric constant at this transition temperature as a function of the applied electric field. The incommensurate phase disappears in fields greater than the Lifshitz field EL of ≈100 kV cm−1.