Pb-based Relaxor Ferroelectrics Research Articles

Investigations on electric properties and domain structures of Nd-doped 0.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 relaxor ferroelectric ceramics with high piezoelectric properties

Although rare earth neodymium (Nd) doping is common in Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) single crystals, it is rarely reported in PMN–PT ceramics. To explore the effect of Nd doping on PMN–PT ceramics, PMN–30PT:xNd3+ (x = 0%, 1%, 2%, and 3%) relaxor ferroelectric ceramics were fabricated using a solid-state method via two-step sintering. An enhanced piezoelectric charge coefficient (d33) of ~870 pC/N and a high piezoelectric strain coefficient (d33⁎) of ~1025 pm/V were achieved for x = 2%. Through Rayleigh analysis of polarization–electric field (P–E) hysteresis loops under small electric fields, it was found that the dielectric property was mainly influenced by the intrinsic contribution (local lattice distortion). Furthermore, by investigating domain configurations, high piezoelectric properties were found to be associated with the domain size reduction and local structural heterogeneity. The results indicate that the PMN–30PT:xNd3+ ceramics is a promising material for electronic devices, and that rare earth Nd doping is an efficient strategy for improving the electronic performance of Pb-based relaxor ferroelectrics.

Electrical fatigue in 0.94Na0.5Bi0.5TiO3–0.06BaTiO3: Influence of the surface layer

Similar to several Pb-based relaxor ferroelectrics, 0.94Na0.5Bi0.5TiO3–0.06BaTiO3 is reported to have a micrometer-sized surface layer. We hypothesize that since this layer has structural properties distinct from the bulk, it would undergo a different property degradation than the bulk during cyclic electrical loading or fatigue. First, we show the existence of a surface layer by comparing X-ray diffraction patterns of the ceramic surface and powders. Then, we show that fatigue damage is mainly localized in the surface layers, and thus, property degradation due to fatigue can be recovered on removing the affected surface layer. We also show that ion migration may be occurring in the surface layer during fatigue experiments using secondary ion mass spectroscopy, where the ion sources may be the sample itself, the electrode layer or the insulating oil in which the experiment is performed. Finally, we show that permanent fatigue damage such as microcracks is dependent on the choice of electrodes. While permanent damage was observed for Pt electrodes, it was not present for oxide electrodes, suggesting that oxygen permeation and accumulation at the electrode/surface interface may play a role in the formation of observed microcracks. In summary, we have shown that fatigue is influenced by the surface layer, and surface layer damage can be controlled using the selection of electrodes.

Chemically induced renormalization phenomena in Pb-based relaxor ferroelectrics under high pressure

The pressure-induced phase transition sequence in PbSc0.5Ta0.5O3 (PST) and PbSc0.5Nb0.5O3 (PSN) heavily doped with homo- and heterovalent cations on the A- or B-site of the perovskite-type structure (ABO3) was analysed by in situ synchrotron x-ray diffraction and Raman spectroscopy up to pressures of 25 GPa. We focused on the structural phenomena occurring above the first pressure-induced phase transition at pc1 from a relaxor state to a non-polar rhombohedral phase with antiphase tilting of the BO6 octahedra. The samples studied were PST doped with Nb5+ and Sn4+ on the B-site, PST doped with Ba2+ and La3+ on the A-site and PSN doped with Sr2+ and La3+ on the A-site. All of them exhibit a second pressure-induced phase transition at pc2, similar to pure PST and PSN. The second transition involves the development of either order of antiparallel Pb2+ displacements and complementary a+b−b− octahedral tilts, or a−b−b− (0 ≤ a < b) tilting alone. As in pure PST and PSN, the second phase transition is preceded by the occurrence of unequal octahedral tilts on the local scale. The substitution of Nb5+ for Ta5+ as well as the coupled substitution of Sn4+ for Sc3+ + Ta5+ on the octahedral B sites increases the second critical pressure. The doping by Nb5+ also reduces the length of coherence of antipolar Pb2+ order developed at pc2. The isovalent substitution of the larger Ba2+ for Pb2+ on the A-site suppresses the antipolar Pb2+ order due to the induced local elastic stresses and thus significantly increases pc2. The substitution of smaller cations for Pb2+ on the A-site generally favours the development of long-range order of antiparallel Pb2+ displacements because of the chemically enhanced a−a−a− octahedral tilts. However, this ordering is less when the dopant is aliovalent, due to the charge imbalance on the A-site. For all of the relaxors studied here, the dynamic compressibility estimated from the pressure derivative of the wavenumber of the soft mode associated with the first phase transition is larger in the pressure interval between pc1 and pc2 than above pc2. The dynamic compressibility of the phase above pc2 decreases if the antipolar Pb2+ order is disturbed.

Structural state of relaxor ferroelectrics PbSc0.5Ta0.5O3and PbSc0.5Nb0.5O3at high pressures up to 30 GPa

The pressure-induced structural changes in perovskite-type (ABO${}_{3}$) Pb-based relaxor ferroelectrics are studied on the basis of in situ single-crystal synchrotron x-ray diffraction and Raman scattering experiments on PbSc${}_{0.5}$Ta${}_{0.5}$O${}_{3}$ and PbSc${}_{0.5}$Nb${}_{0.5}$O${}_{3}$ conducted under hydrostatic conditions up to 30 GPa. Complementary density functional theory calculations have been performed to compare the stability of various atomic configurations for both compounds at high pressures. By combining the experimental and theoretical results, the following sequence of structural transformations is proposed. At a characteristic pressure ${p}_{1}^{\ensuremath{\star}}$ the mesoscopic polar order is violated and a local antipolar order of Pb atoms as well as quasidynamical long-range order of antiphase octahedral tilts is developed. These structural changes facilitate the occurrence of a continuous phase transition at ${p}_{c1}g{p}_{1}^{\ensuremath{\star}}$ from cubic to a nonpolar rhombohedral structure comprising antiphase octahedral tilts of equal magnitude (${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{a}^{\ensuremath{-}}$). At a characteristic pressure ${p}_{2}^{\ensuremath{\star}}g{p}_{c1}$ the octahedral tilts around the cubic [100], [010], and [001] directions become different from each other on the mesoscopic scale. The latter precedes a second phase transition at ${p}_{c2}$, which involves long-range order of Pb antipolar displacements along cubic $[uv0]$ directions and a compatible mixed tilt system (${a}^{+}{b}^{\ensuremath{-}}{b}^{\ensuremath{-}}$) or long-range ordered antiphase tilts with unequal magnitudes (${a}^{0}{b}^{\ensuremath{-}}{b}^{\ensuremath{-}}$) without Pb displacement ordering. The phase-transition pattern at ${p}_{c2}$ depends on the fine-scale degree of chemical B-site order in the structure.

Temperature dependence of dielectric and piezoelectric properties of (1−x)(BiScO3–0.64PbTiO3)–xLiNbO3 high-temperature relaxor ferroelectric ceramics

(1−x)(0.36BiScO3–0.64PbTiO3)–xLiNbO3 (BSPT64–xLN) high-temperature relaxor ferroelectric ceramics near the morphotropic phase boundary (MPB) composition were investigated. X-ray diffraction showed a change in symmetry from MPB phase to rhombohedral phase with LiNbO3 content increasing. A change from normal ferroelectric features to relaxor ferroelectric features was observed with LiNbO3 substitution up to x = 0.06, while high-temperature dielectric relaxation was exhibited at T max ~230 to 383°C for 0.02 ≤ x ≤ 0.06. The BSPT64–xLN ceramics with x = 0.02 LiNbO3 exhibited good piezoelectric properties compared with BS–0.64PT ceramics: piezoelectric coefficients d 33 = 505pC/N, planar electromechanical coupling factors k p = 0.47, and remnant polarization P r = 40 μC/cm2, respectively. The annealing temperature dependence of piezoelectric response for BSPT64–xLN ceramics was measured and it was found that the piezoelectric properties decreased slightly before 210°C for x = 0.02 and 0.04, showing excellent thermal stability, which indicated that BSPT64–xLN relaxor ferroelectric ceramics can be used in the range of high temperature compared to Pb-based relaxor ferroelectrics.

Octahedral tilting in Pb-based relaxor ferroelectrics at high pressure

We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc(0.5)Ta(0.5)O(3) (PST) and Pb(0.78)Ba(0.22)Sc(0.5)Ta(0.5)O(3) (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is Fm3m as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h = 2n + 1, does not change with pressure. This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the 111 cubic directions. Rietveld refinement to the neutron powder data shows that the high-pressure phase has either R3c or R3 symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a(-)a(-)a(-) that continuously evolve with pressure. The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane. For PST the anisotropy of the Pb displacement parameters decreases at approximately 3 GPa above the phase-transition pressure. For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 7.35 GPa.

Structural investigations of the co-fired interface of Pb-based relaxor ferroelectrics and Ag–Pd electrode

With the miniaturization trend of modern electronic devices and increasing demand for multilayer ceramic chip components, a full understanding of the role of the co-fired ceramic/electrode interface in the performance and long-term reliability of the components becomes very important. In the present work, the inter-diffusion and structure of the co-fired interface of Pb-based relaxor ferroelectrics and Ag–Pd metal electrode were investigated by scanning electron microscopy (SEM) and transmission electron microscopy–energy dispersive analysis of X-rays (TEM–EDAX). No strong structural distortions were observed at the co-fired ceramic/electrode interface except for a thin interfacial layer of 1.5 nm thick on each side of the interface. The ceramic near the interface retains the perovskite lattice structure. It is suggested that the diffused electrode compositions could be incorporated into the perovskite lattice.

Interpretation of the diffuse scattering in Pb-based relaxor ferroelectrics in terms of three-dimensional nanodomains of the ⟨110⟩-directed relative interdomain atomic shifts

Polar nanoregions in relaxor ferroelectrics were modeled by the Monte Carlo method. To account for correlations between atomic shifts, Potts-like models were applied with a random field used to mimic the chemical disorder impact on the polar ordering. The diffuse scattering effects for generated structures were calculated and compared with the neutron and x-ray experimental data known from the literature. An alternative explanation of diffraction effects contrary to the polar ``pancakes'' concept reported in several recent papers is given. Generated three-dimensional polar domains with the walls parallel to {110}-type crystallographic planes result in one-dimensional diffuse scattering rods fully compatible with the experimental results. Moreover, special extinction conditions for diffuse lines clearly indicate that the relative shifts of the atoms in the neighboring domains have to be parallel to ⟨110⟩-type directions. This can be achieved by various combinations of atomic shifts. Therefore, it is impossible to elucidate their actual directions based only on the analysis of the diffuse scattering effects. We show that the intensities of diffuse scattering are sensitive to the relative magnitudes of the atomic displacements and can be used as a starting point for their determination.

Nanoscale phase transformations in relaxor-ferroelectric lead scandium tantalate and lead scandium niobate

Abstract Structural transformations that take place on cooling down in Pb-based relaxor ferroelectrics are investigated on the basis of polarized Raman spectroscopy and far infrared ellipsometry applied on single-crystal samples of PbSc0.5Ta0.5O3 (PST) and PbSc0.5Nb0.5O3 (PSN). The temperature evolution of phonon anomalies reveals different preferred ferroic species in PST and PSN. Near the dielectric constant maximum cooperative shifts of Pb atoms in respect to the oxygen sheets perpendicular to the cubic body diagonal occur in larger spatial regions for PST than for PSN. On cooling down this structural modification becomes preferential for PST, thus giving rise to highly anisotropic ferroelectric domains. In PSN the temperature decrease favours B-cation deviations from the BO6-octahedral centres, which leads to formation of small-sized polar clusters distributed in the isotropic matrix. Near 180 K PST undergoes an additional phase transition that involves reduction of the rotation symmetry. The nanoscale phase transformations in PST are affected by the presence of point defects rather than by the degree of long-range B-site ordering. The spectroscopic data give some evidence that in PSN additional multiplication of the unit cell takes place, besides the common perovskite-structure doubling Pm3̅m – Fm3̅m.

The B-site order-disorder transformation in Pb(Sc1/2Ta1/2)O3triggered by mechanical activation

Having pioneered mechanical activation on Pb-based relaxor ferroelectrics, we describe a unique order-disorder transformation phenomenon brought about by mechanical activation in Pb-based relaxor ferroelectrics of complex perovskite. B-site disorder in Pb(Sc1/2Ta1/2)O3 (PST) which is traditionally tailored by thermal annealing after sintering at elevated temperatures, can be triggered by mechanical activation. The B-site disorder in Pb(Sc1/2Ta1/2)O3 triggered by mechanical activation can well be retained in sintered Pb(Sc1/2Ta1/2)O3, leading to an unique ferroelectric transition behavior (e.g., from a normal ferroelectric to relaxor) and a shift in the Curie temperature.

Mechanical activation-induced sequential combination, morphotric segregation and order–disorder transformation in Pb-based relaxors

Having explored mechanical activation for synthesis and fabrication of Pb-based relaxor ferroelectrics, we observed several unique phenomena in association with mechanical activation of these electroceramic materials, including the unique nanocrystalline structure, phase transition, order–disorder transformation and activation-induced morphotropic behavior. In the study of sequential combination of Pb(Fe 1/2Nb 1/2)O 3 (PFN), nanocrystallite perovskite PFN phase is successfully derived from mechanical activation of both mixed oxides and Columbite precursor. However, the two nanocrystalline PFN phases are considerably different in phase stability. Mechanical activation can lead to morphotropic behavior in complex perosvkites, such as in Pb(Ni 0.5W 0.5)O 3–PbTiO 3 (PNW–PT), where an unique temperature-stable dielectric behavior is demonstrated over the wide temperature range of −120 to 40 °C. Both short range disordering and long range disordering occur in Pb(Mg 1/3Nb 2/3)O 3–Pb(Mg 1/2W 1/2)O 3 (PMN–PMW) upon mechanical activation. A Monte-Carlo simulation algorithm, based on the competition between the thermal activation and the mechanical activation-triggered shear process, is established to model the order–disorder transformation in Pb-based relaxor ferroelectrics.

Interfacial reaction of Ag/Pd metals with Pb-based relaxor ferroelectrics including additives

In order to effectively control the interfacial reactions during co-firing a multilayer ceramic device (MLCD), different effects of silver/palladium on Pb-based relaxor ferroelectric ceramics were investigated by means of a series of powder X-ray diffraction (XRD) spectra and chemical reaction thermodynamics. It is found that the reaction kinetics of palladium and Pb-based relaxor ceramics lies in the activity of palladium and the structural stability of relaxor ferroelectric ceramics. In this respect, silver can decrease the activity of palladium so that a less serious interfacial problem exists in a MLCD with 30Pd/70Ag electrodes. Moreover, BaTiO 3 and SrTiO 3 ceramic additives were found to effectively suppress Pd-PNN reactions because of increasing the structural stability of PNN. It is pointed out that the addition of BaTiO 3 or SrTiO 3 ceramic powders into the dielectric layers and particularly into the inner electrode pastes may play dual roles, including modifying the densification mismatch and preventing the interfacial reactions between the electrodes and the dielectric layers.

Mechanically Activating Nucleation and Growth of Complex Perovskites

Mechanical activation of the constituent oxides at room temperature led to the formation of nanocrystalline Pb-based relaxor ferroelectrics and piezoelectrics of complex perovskite structure. The activation-triggered occurrence of complex perovskites, such as Pb(Mg1/3Nb2/3)O3, 0.9[0.4Pb(Mg1/3Nb2/3)O3– 0.6Pb(Zn1/3Nb2/3)O3]–0.1PbTiO3, and Pb(Zr0.52Ti0.48)O3, in the mixed oxide compositions and in an amorphous precursor derived from coprecipitation proceeds via a process involving nucleation and subsequent growth of perovskite crystallites with increasing degree of mechanical activation. This is fundamentally different from the one or more interfacial reactions and diffusions responsible for a thermally activated solid state reaction occurring at an elevated temperature. Nucleation occurs in a highly activated state, where a degree of amorphization has taken place, and the subsequent growth proceeds as a result of the constant collisions and rearrangement of perovskite nuclei.