Zuo-Guang Ye

Role of random electric fields in relaxors

Significance Relaxors are characterized by a frequency-dependent peak in the dielectric permittivity and are critical to modern technological applications because they exhibit large dielectric constants and unparalleled piezoelectric coefficients. Despite decades of study a fundamental understanding of the origin of relaxor behavior is lacking. Here we compare the structural, dynamical, dielectric, and piezoelectric properties of two highly similar piezoelectric lead-oxide materials: ferroelectric PbZr1–xTixO3 and relaxor Pb(Mg1/3Nb2/3)1–xTixO3. Random electric fields are implicated as the genesis of relaxor behavior, and the diffuse scattering associated with short-range polar order is identified as the order parameter. The piezoelectric response is found to be greatly amplified in crystals that display this diffuse scattering. PbZr1–xTixO3 (PZT) and Pb(Mg1/3Nb2/3)1–xTixO3 (PMN-xPT) are complex lead-oxide perovskites that display exceptional piezoelectric properties for pseudorhombohedral compositions near a tetragonal phase boundary. In PZT these compositions are ferroelectrics, but in PMN-xPT they are relaxors because the dielectric permittivity is frequency dependent and exhibits non-Arrhenius behavior. We show that the nanoscale structure unique to PMN-xPT and other lead-oxide perovskite relaxors is absent in PZT and correlates with a greater than 100% enhancement of the longitudinal piezoelectric coefficient in PMN-xPT relative to that in PZT. By comparing dielectric, structural, lattice dynamical, and piezoelectric measurements on PZT and PMN-xPT, two nearly identical compounds that represent weak and strong random electric field limits, we show that quenched (static) random fields establish the relaxor phase and identify the order parameter.

Structural Heterogeneity and Diffuse Scattering in Morphotropic Lead Zirconate-Titanate Single Crystals

Complementary diffuse and inelastic synchrotron x-ray scattering measurements of lead zirconate-titanate single crystals with composition near the morphotropic phase boundary (x=0.475) are reported. In the temperature range 293 K<T<400 K a highly anisotropic quasielastic diffuse scattering is observed. Above 400 K this scattering disappears. Its main features can be reproduced by the model of inhomogeneous lattice deformations caused by inclusions of a tetragonal phase into a rhombohedral or monoclinic phase. This observation supports the idea that PZT at its morphotropic phase boundary is essentially structurally inhomogeneous.

Phase transition behaviors of PbZr1−xTixO3 single crystals as revealed by elastic anomalies and central peaks

Two PbZr1−xTixO3 (PZT) single crystals with x≈ 0.45 and 0.42 near the morphotropic phase boundary were studied by Brillouin light scattering, giving rise to the first single-crystal elastic data which were discussed in terms of the correlation between the acoustic anomalies and the phase transition sequence. The ferroelectric phase transition of PZT with x≈ 0.45 was second order (or very close to the second order) which accompanied very large polarization fluctuations resulting in the formation of strong central peaks. A revised phase diagram of PZT was suggested including refinement of the tilt phase boundary and a possible existence of tricritical point.

Growth and piezo-/ferroelectric properties of PIN-PMN-PT single crystals

Ternary solid solution crystals of 0.19Pb(In1/2Nb1/2)O3− 0.46Pb(Mg1/3Nb2/3)O3− 0.35PbTiO3 [PIMNT(19/46/35)] with dimensions of 35 × 38 × 15 mm3 were grown by the top-seeded solution growth (TSSG) method. The dielectric, piezo- and ferroelectric properties of the grown crystals were characterized. The ternary piezocrystals exhibit a Curie temperature TC = 190 °C and a tetragonal-rhombohedral phase transition temperature TR-T = 130 °C, which are increased significantly compared with TC ≈ 155 °C and TR-T ≈ 80 °C of PMN-PT crystals. The dielectric constant (ɛ′) and dielectric loss tangent (tan δ) are 4300 and 0.40 at room temperature. The piezoelectric coefficient d33 is found to be 2380 pC/N. The longitudinal electromechanical coupling factor k33 reaches 90% at room temperature. A peak-to-peak bipolar strain value of 0.13% is obtained at E ≈ ± 14 kV/cm. The coercive field Ec and remanent polarization Pr are 5.50 kV/cm and 27.10 μC/cm2, respectively, which are also improved from the vales of PMN-PT crystals.

Enhanced dielectric, ferroelectric, and electrostrictive properties of Pb(Mg1/3Nb2/3)0.9Ti0.1O3 ceramics by ZnO modification

The effects of ZnO modification on the dielectric, ferroelectric, and electrostrictive properties of 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 (PMNT) ceramics are systematically studied in this work. The PMNT/xZnO (with x = 0, 0.4, 2.0, 4.0, and 11.0 mol. %) ceramics of complex perovskite structure were prepared by solid state reaction and sintering process. It is found that the maximum value of the dielectric constant decreases with increasing ZnO amount up to x = 4.0 mol. %, and then significantly increases with x = 11.0 mol. %. The temperature of maximum dielectric constant tends to increase, while the diffuseness of the dielectric peak is reduced, with increasing ZnO content. The remanent polarization and the coercive field increase with increasing ZnO concentration. The induced strain and the electrostrictive coefficient reach the maximum values of 0.10% (at E = 10 kV/cm) and 12.94 × 10−16 m2/V2, respectively, with x = 2.0 mol. % ZnO. The ceramic doped with a high ZnO content (11.0 mol. %) exhibits a macroscopic...

A statistical model approximation for perovskite solid-solutions: A Raman study of lead-zirconate-titanate single crystal

Lead titanate (PbTiO3) is a classical example of a ferroelectric perovskite oxide illustrating a displacive phase transition accompanied by softening of a symmetry-breaking mode. The underlying assumption justifying the soft-mode theory is that the crystal is macroscopically sufficiently uniform that a meaningful free energy function can be formed. In contrast to PbTiO3, experimental studies show that the phase transition behaviour of lead-zirconate-titanate solid solution (PZT) is far more subtle. Most of the studies on the PZT system have been dedicated to ceramic or powder samples, in which case an unambiguous soft-mode study is not possible, as modes with different symmetries appear together. Our Raman scattering study on titanium-rich PZT single crystal shows that the phase transitions in PZT cannot be described by a simple soft-mode theory. In strong contrast to PbTiO3, splitting of transverse E-symmetry modes reveals that there are different locally ordered regions. The role of crystal defects, ran...

Low-temperature remote plasma-enhanced atomic layer deposition of graphene and characterization of its atomic-level structure

Graphene has attracted a great deal of research interest owing to its unique properties and many potential applications. Chemical vapor deposition has shown some potential for the growth of large-scale and uniform graphene films; however, a high temperature (over 800 °C) is usually required for such growth. A whole new method for the synthesis of graphene at low temperatures by means of remote plasma-enhanced atomic layer deposition is developed in this work. Liquid benzene was used as a carbon source. Large graphene sheets with excellent quality were prepared at a growth temperature as low as 400 °C. The atomic structure of the graphene was characterized by means of aberration-corrected transmission electron microscopy. Hexagonal carbon rings and carbon atoms were observed, indicating a highly crystalline structure of the graphene. These results point to a new technique for the growth of high-quality graphene for potential device applications.

Structure and multiferroic properties of Bi(1-x)DyxFe0.90Mg0.05Ti0.05O3 solid solution

Chemical modification is proven to be an effective way to improve the properties of perovskite BiFeO3 (BFO). In this paper, we studied the effects of the A-site Dy3+ ion substitution on the structure and multiferroic properties of BFO in which the Fe3+ ion on the B-site is partially co-substituted for by Mg 2+ and Ti4+ ions. The solid solution compounds of Bi(1-x)DyxFe0.90 Mg 0.05Ti0.05O3 (x = 0 – 1) were synthesized by a tartaric acid-assisted solution process, followed by thermal treatments at 850 °C. The structural transition and the ferroelectric and magnetic properties as a function of composition were investigated. It is found that the structure transforms from a rhombohedral (R3c) to an orthorhombic phase (Pn2 1 a) when x is increased to ≥ 0.15. The ferroelectric measurements indicate that the sample with x = 0.15 possesses the best ferroelectric property, and the remnant polarization of the samples with orthorhombic symmetry decreases with the increase of Dy3+ concentration. The compounds of Bi(1-x)DyxFe0.90 Mg 0.05Ti0.05O3 (x = 0.05 – 1.00) exhibit weakly ferromagnetic properties at low temperatures, but antiferromagnetic behavior at high temperatures. The magnetization of the solid solution increases linearly with the increase of the Dy3+ amount.

Role of dynamic polar nanoregions in heterovalent perovskite relaxor: Inelastic light scattering study of ferroelectric Ti rich Pb(Zn1/3Nb2/3)O3-PbTiO3

The role of dynamic polar nanoregions (PNRs) of (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-100xPT) single crystals with the PT content x = 0.15 (i.e., higher than above the morphotropic phase boundary composition x ∼ 0.09) has been investigated using inelastic light scattering. The remarkable anomaly of the longitudinal acoustic mode is clearly observed in the vicinity of the cubic-tetragonal phase transition temperature at the TC-T = 481 K. A broad central peak (CP) appears below the Burns temperature TB ∼ 700 K. Upon cooling, the relaxation time determined from the CP width clearly shows a critical slowing down when approaching TC-T in contrast with the suppressed slowing down previously observed in PZN-7PT [S. Tsukada and S. Kojima, Phys. Rev. B 78, 144106 (2008)]. These facts are due to the existence of dynamic PNRs and the disappearance of chemically ordered regions resulting from a higher PT content in the solid solution. In the cubic phase, the local symmetry breaking caused by the PNRs was observed by Ram...

Fano resonance and dipolar relaxation in lead-free relaxors

Fano resonance is a phenomenon in which a discrete state interferes with a continuum of states and has been observed in many areas of science. Here, we report on the prediction of a Fano resonance in ferroelectric relaxors, whose properties are poorly understood: an ab initio molecular dynamic scheme reveals such resonance between the bare optical phonon mode of the Zr sublattice (the discrete state) and the bare optical phonon mode of the Ti sublattice (the continuum of states) in disordered lead-free Ba(Zr,Ti)O3. The microscopic origins of the discrete state and continuum of states are discussed in the context of relaxor properties. Furthermore, our simulations suggest that the T* characteristic temperature of relaxor is related to a hardening of the vibrational frequencies associated with fluctuation of the Ti sublattice. Finally, a terahertz relaxation mode reflecting reorientations of Ti dipoles and showing a thermally activated behaviour is predicted, in agreement with previous experiments.