R. Farhi

A Raman scattering study of the ferroelectric ordering in rhombohedral and tetragonal La‐modified lead zirconate titanate ceramics

A Raman study of La‐doped lead zirconate titanate ceramics with La/Zr/Ti ratios of 9/65/35 and 12/40/60 has been undertaken in order to compare the onset of ferroelectric order in these two compositions. Some features, such as a splitting of a threefold degenerate T1u mode of the cubic structure into at least two lines or an enhancement of the T2u mode, can be considered as the signature of the occurrence of some ferroelectric order below Tc≂363 K in the 12/40/60 compound. Nevertheless, the Raman spectra obtained for the two ceramics are very similar, and no significant narrowing of the lines was observed in the ferroelectric phase of 12/40/60. It is thus suggested that the onset of long range order in this compound is not developed homogeneously in every direction of space, in agreement with previous transmission electron microscopy observations. The Raman scattering was dominately second order, with the broadening of the lines reflecting a distribution of the correlation function of the quasistatic pola...

Ferroelectric thin film properties—Depolarization field and renormalization of a “bulk” free energy coefficients

We suggest an approach to calculate the physical properties of ferroelectric thin films. Our approach takes into account the depolarization field and spatial inhomogeneity of the order parameter inherent to thin films. In or approach, we consider the thin film to be short circuited with the perfectly conducting electrodes and monodomain. The solution of the corresponding Euler–Lagrange equation by the direct variational method shows that the initial free-energy functional of a thin film can be reduced to some effective functional which does not include spatial derivatives of the order parameter. The coefficients of this functional, however, are renormalized as compared to those of the initial free-energy functional. We present the detailed calculation of the spontaneous polarization, pyrocoefficient, and dielectric susceptibility. We analyze in detail the spatial profile of the depolarization field with special attention to its properties near size-driven phase transition in a film.

Ferroelectric and glassy states in La-modified lead zirconate titanate ceramics: A general picture

Raman scattering, pyroelectric current, and dielectric susceptibility measurements have been carried out on a series of tetragonal and rhombohedral-structured Pb1−xLax(ZryTi1−y)1−x/4O3 (PLZT) ceramics, under zero-field heating after zero-field cooling (ZFH/ZFC) and zero-field heating after field-cooling (ZFH/FC) conditions. The results have been shown to be in good agreement with previous transmission electron microscopy observations. A dependence of the Raman spectra on light polarization (i.e., Raman selection rules or RSR), which are known to be a characteristic property of single crystals, has been observed in the PLZT compositions 9/65/35, 12/65/35, 6/40/60, and 12/40/60 illuminated with a 2 μm wide laser spot (i.e., micro-Raman). The texture which has been observed by x-ray diffraction on 6/40/60 is discussed in conjunction with the Raman results. The combined data of Raman scattering experiments and electrical measurements, together with electromechanical and ferroelectric properties, are explained...

A comparative Raman study of ferroelectric PbTiO3 single crystal and thin film prepared on MgO substrate

A Raman study of a highly oriented PbTiO3 thin film deposited on a MgO single crystal has been performed as a function of the temperature. The results have been compared with those obtained from a single crystal prepared from the same batch and those reported in the literature for polycrystalline thin films and single crystals. A shift in the transition temperature is observed from the soft mode behavior, together with more usual lower values of its frequency, which are both related to effective hydrostatic pressure due to the clamping of grains by their neighbors in the distorted tetragonal structure. The large soft mode linewidth and its integrated intensity, which do not vanish at the ferroelectric to paraelectric transition temperature, have been interpreted in terms of the coupling of this mode (and more generally of all polar phonons) with the static strain fields originating from the clamping effect and the epitaxial conditions.

Polarized Raman and electrical study of single crystalline titanium modified lead magnesio-niobate

Raman scattering experiments were performed on the relaxor ferroelectric lead magnesio-niobate (PMN), the ferroelectric 0.8 PMN-0.2 (PT) and the intermediate composition 0.9 PMN-0.1 PT. In order to understand in more detail the behaviour of this series, Raman scattering spectra were also collected under dc electric field. A ferroelectric phase could be induced in the latter composition for electric fields higher than a threshold value. The behaviour of this field-induced phase was compared with the (zero-field) ferroelectric phase in 0.8 PMN-0.2 PT. Barkhausen jumps were observed in the pyroelectric current measurements, and a strong dielectric anomaly was detected in zero field heating after field cooling experiments on 0.9 PMN-0.1 PT. These results are consistent with a description of the behaviour in this series based on a cancellation of 1:1 (Mg:Nb) domain random fields by titanium ions, favouring the onset of long range ferroelectric order.

A polarized Raman study of the relaxor and ferroelectric states of La-modified lead zirconate titanate ceramics

Lanthanum modified lead zirconate titanate ceramics with La/Zr/Ti ratios of 6/40/60, 12/40/60, 5/65/35, and 9/65/35 have been studied by polarized Raman scattering. In the two former (tetragonal-type) compositions, selection rules were rather well defined and allowed an assignment of the Raman lines at lower temperature. These selection rules disappeared for 6/40/60 but remained for 12/40/60 even at higher temperatures. The behavior of the two latter (rhombohedral-type compositions) was quite different. No assignment of lines could be done. In addition, the spectra recorded in parallel and crossed polarizations were almost identical for 5/65/35 regardless of temperature. A very smeared paraelectric to ferroelectric transition was evidenced by Raman intensity measurements for this composition. On the contrary, selection rules were always present for the relaxor composition 9/65/35 which were similar to those for 12/40/60 and 6/40/60. This suggests that both tetragonal and rhombohedral-type local polar orde...

On the nature of the glassy state in 9/65/35 PLZT ceramics

Abstract Dielectric susceptibility under electric field, pyroelectric current and Raman scattering measurements have been performed on 9/65/35 PLZT ceramics, which are considered as ferroelectric relaxors. All the results evidence a freezing of kinetic processes at a temperature T 0 ≌ 253 K, which characterizes a macrodomain to microdomain state transition for the experiments under field. Taking into account our previous Cole-Cole measurements inferring a superparaelectric behaviour at high temperature, it is suggested that a crossover from superparaelectric to microdomain state occurs within a temperature range which includes the temperature TL m at which the real part of the dielectric susceptibility displays a maximum.

Theory of phase transitions in disordered ferroelectrics allowing for nonlinear and spatial correlation effects

The order parameter, ferroelectric phase transition temperature, and critical concentrations of random-site electric dipoles, point charges and dilatational centres in disordered ferroelectrics are calculated. The calculations are carried out by the random-field method with a random-electric-field distribution function, allowing for nonlinear and spatial correlation effects. Essential differences from the linear case are revealed. First, the aforementioned effects lead to the transformation of the second-order phase transition into one of first order and vice versa for and respectively, where is the third-order nonlinearity constant. Second, for the phase transition temperature has a maximum as a function of the random-field-source concentration; its maximal value appears to be larger than that in the mean-field approximation. At the same time, for the maximum is absent and the -value is smaller than that in the linear case (i.e. at ). This means that nonlinear and spatial correlation effects enhance the long-range order in disordered ferroelectrics for and inhibit it for . It is shown that the critical concentrations of electric dipoles, point charges and dilatational centres are the same as in the linear case. The application of the theory developed to various disordered ferroelectrics is discussed.

A dielectric study of the X/65/35 lanthanum-modified lead zirconate-titanate series

Abstract The occurrence (or not) of a ferroelectric phase transition and the field induced ferroelectric to relaxor states transition has been studied through dielectric susceptibility and pyroelectric current experiments in the x/65/35 PLZT series (x=5 to 12). The results are qualitatively interpreted using random field theory principles.

A Raman and dielectric susceptibility study of superparaelectric PLZT ceramics

Raman and dielectric susceptibility experiments have been performed on 9/65/35 PLZT ceramics. Pseudo-first-order Raman scattering is interpreted as resulting from the coupling of hard modes with the quasi-static polarization and/or distortion. An increase of the spatial correlation function of the polarization as the temperature decreases is thus inferred from the behaviour of the intensity of polar modes. Built-in stresses are also suspected to contribute to the intensity of all modes, even at high temperatures. The relaxation of the polarization has been studied between 360 and 480 K using dielectric susceptibility measurements. The corresponding relaxation times are large and most likely related to the motion of ferroelectric nanodomains. When the PLZT ceramics are cooled from high temperature to room temperature, the correlation length of the polarization extends but the simultaneous drastic increase of the relaxation times of the polarization prevents any onset of a ferroelectric long-range order.