L. Q. Chen

A Strain-Driven Morphotropic Phase Boundary in BiFeO3

Biffed into Shape BiFeO3 is known to have a very large ferroelectric polarization. Although the bulk ground state is rhombohedral (with the electrical polarization along the [111] direction), in thin films and under epitaxial strain, the material can be tetragonally distorted (polarization along [001]). Zeches et al. (p. 977) show that under compressive strain, these films are monoclinic, a phase that is highly stable because it comprises the same symmetry as the monoclinic phase which has been reported at the so-called morphotropic phase boundaries in technologically important ferroelectrics. This work offers the possibility of obtaining large piezoelectric responses in lead-free systems. Growth of epitaxial films of BiFeO3 on various substrates may provide a route toward making lead-free ferroelectric devices. Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO3) films. Electric field–dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.

Lattice, elastic, polarization, and electrostrictive properties of BaTiO3 from first-principles

Predicting the domain structures and properties in both bulk single crystal and thin film ferroelectrics using the phase-field approach requires the knowledge of fundamental mechanical, electrical, and electromechanical coupling properties of a single-domain state. In this work, the elastic properties and structural parameters of cubic single crystals as well as tetragonal, orthorhombic, and rhombohedral BaTiO3 single domain states are obtained using first-principles calculations under the local density approximation. The calculated lattice constants, bulk modulus, and elastic constants are in good agreement with experiments for both the cubic paraelectric phase and the low-temperature ferroelectric phases. Spontaneous polarizations for all three ferroelectric phases and the electrostrictive coefficients of cubic BaTiO3 are also computed using the Berry’s phase approach, and the results agree well with existing experimentally measured values.

Computer simulation of stress-oriented nucleation and growth of θ′ precipitates inAl–Cu alloys

Many structural transformations result in several orientation variants whose volume fractions and distributions can be controlled by applied stresses during nucleation, growth or coarsening. Depending on the type of stress and the coupling between the applied stress and the lattice misfit strain, the precipitate variants may be aligned parallel or perpendicular to the stress axis. This paper reports our studies on the effect of applied stresses on nucleation and growth of coherent θ′ precipitates in Al–Cu alloys using computer simulations based on a diffuse-interface phase-field kinetic model. In this model, the orientational differences among precipitate variants are distinguished by non-conserved structural field variables, whereas the compositional difference between the precipitate and matrix is described by a conserved field variable. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg–Landau (TDGL) equations for the structural variables and the Cahn–Hilliard diffusion equation for composition. Random noises were introduced in both the composition and the structural order parameter fields to simulate the nucleation of θ′ precipitates. It is demonstrated that although an applied stress affects the microstructural development of a two-phase alloy during both the nucleation and growth stages, it is most effective to apply stresses during the initial nucleation stage for producing anisotropic precipitate alignment.

Morphological evolution of coherent multi-variant Ti11Ni14 precipitates in Ti-Ni alloys under an applied stress—a computer simulation study

Coherent precipitation of multi-variant Ti11Ni14 precipitates in TiNi alloys was investigated by employing a continuum field kinetic model. The structural difference between the precipitate phase and the matrix as well as the orientational differences between precipitate variants are distinguished by nonconserved structural field variables, whereas the compositional difference between the precipitate and matrix is described by a conserved field variable. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg–Landau (TDGL) equations for the structural variables and the Cahn–Hilliard diffusion equation for the composition. In particular, the interaction between precipitates, and the growth morphology of Ti11Ni14 precipitates under strain-constraints were studied, without a priori assumptions on the precipitate shape and distribution. The predicted morphology and distribution of Ti11Ni14 variants were compared with experimental observations. Excellent agreement between the simulation and experimental observations was found.

Computer simulation of morphological evolution and coarsening kinetics of δ′ (Al3Li) precipitates in Al–Li alloys

Abstract The morphological evolution and coarsening kinetics of L12 ordered (Al3Li) precipitates (δ′) in a f.c.c. disordered matrix (α) were investigated using computer simulations based on microscopic diffusion equations. The effective interatomic interactions were fitted to the phase diagram using a two-neighbor mean-field model whereas the kinetic parameter in the microscopic diffusion equation was fitted to the chemical diffusion coefficient in the equilibrium disordered phase. The coalescence or encounter among precipitates which belong to any one of the four different antiphase domains of the L12 ordered phase is automatically taken into account. Volume fractions ranging from 20 to 65% were studied. Structure, scaling and particle-size distribution (PSD) functions were calculated. It is shown that the PSDs become increasingly broad and their skewness changes sign from negative to positive with increasing precipitate volume fraction. It is found that the cube of the average particle radius varies approximately linearly with time in the scaling regime for all the volume fractions studied, with the rate constant increasing with volume fraction. During coarsening, the volume fraction is not constant, approaching the equilibrium value asymptotically with time. The results are compared with existing analytical theories and experimental measurements.

Prediction of ferroelectricity in BaTiO3/SrTiO3 superlattices with domains

The phase transitions of superlattices into single- and multidomain states were studied using a mesoscale phase-field model incorporating structural inhomogeneity, micromechanics, and electrostatics. While the predictions of transition temperatures of BaTiO3∕SrTiO3 superlattices into multidomains show remarkably good, quantitative agreement with ultraviolet Raman spectroscopic and variable-temperature x-ray diffraction measurements, the single-domain assumption breaks down for superlattices in which the nonferroelectric layer thickness exceeds the characteristic domain size in the ferroelectric layers.

Stripe domain structure in epitaxial (001) BiFeO3 thin films on orthorhombic TbScO3 substrate

We have analyzed the ferroelastic and ferroelectric domain structure of high crystalline quality (001) BiFeO3 films on orthorhombic (110) TbScO3 substrates. Two domains were present in stripes separated by (010) vertical boundaries, with spontaneous polarizations in adjacent domains rotated by 109°. The striped morphology was caused by nucleation of only two ferroelastic domains on the low symmetry GdFeO3-type substrate. Domain engineering through substrate symmetry is an important finding for rhombohedral ferroelectric epitaxial thin films. The stripe pattern with vertical walls may be useful for extracting domain wall contributions to magnetism and electrical transport properties of BiFeO3 materials.

Computer simulation of the kinetics of order-disorder and phase separation during precipitation of δ′ (Al3Li) in AlLi alloys

Abstract A computer simulation study has been performed of the reaction paths for the precipitation of δ′ (Al 3 Li) ordered particles from a disordered matrix (α) in AlLi alloys, using microscopic Langevin diffusion equations. It is found that the precipitation of δ′ occurs either by a congruent ordering process followed by decomposition, or by a non-classical nucleation mechanism which requires critical fluctuations of both composition and order parameter, except in a narrow range of compositions near the equilibrium phase boundary of the disordered phase, where classical nucleation theory seems to be applicable. Composition and order parameter profiles across a critical nucleus were obtained for different compositions in the nucleation and growth regime, and compared with those obtained from the continuum non-classical nucleation theory. Possible origins for the discrepancies in the precipitation mechanism, obtained from different theoretical and experimental studies, are suggested.

Ferroelectric domain structures in SrBi2Nb2O9 epitaxial thin films: Electron microscopy and phase-field simulations

Ferroelectric domain structures of (001)SrBi2Nb2O9 epitaxial films, investigated using both transmission electron microscopy and phase-field simulations, are reported. Experiment and numerical simulation both reveal that the domain structures consist of irregularly shaped domains with curved domain walls. It is shown that the elastic contribution to domain structures can be neglected in SrBi2Nb2O9 due to its small ferroelastic distortion, less than 0.0018%. Two-beam dark-field imaging using reflections unique to domains of each of the two 90° polarization axes reveal the domain structure. Phase-field simulation is based on the elastic and electrostatic solutions obtained for thin films under different mechanical and electric boundary conditions. The effects of ferroelastic distortion and dielectric constant on ferroelectric domains are systematically analyzed. It is demonstrated that electrostatic interactions which favor straight domain walls are not sufficient to overcome the domain wall energy which favors curved domains in SrBi2Nb2O9.Ferroelectric domain structures of (001)SrBi2Nb2O9 epitaxial films, investigated using both transmission electron microscopy and phase-field simulations, are reported. Experiment and numerical simulation both reveal that the domain structures consist of irregularly shaped domains with curved domain walls. It is shown that the elastic contribution to domain structures can be neglected in SrBi2Nb2O9 due to its small ferroelastic distortion, less than 0.0018%. Two-beam dark-field imaging using reflections unique to domains of each of the two 90° polarization axes reveal the domain structure. Phase-field simulation is based on the elastic and electrostatic solutions obtained for thin films under different mechanical and electric boundary conditions. The effects of ferroelastic distortion and dielectric constant on ferroelectric domains are systematically analyzed. It is demonstrated that electrostatic interactions which favor straight domain walls are not sufficient to overcome the domain wall energy which fa...

Interfacial coherency and ferroelectricity of BaTiO3∕SrTiO3 superlattice films

We studied the phase transitions, domain morphologies, and polarizations in BaTiO3∕SrTiO3 superlattices grown on SrTiO3 substrates. Using the phase field approach, we discovered the remarkable influence of film/substrate interfacial coherency on the ferroelectricity of the SrTiO3 layers within a superlattice: it is an orthorhombic ferroelectric for an incoherent interface while it exhibits only induced polarization by the adjacent BaTiO3 layers for a coherent interface. We presented the domain morphologies within individual BaTiO3 and SrTiO3 layers which have different ferroelectric symmetries. The results are compared to ultraviolet Raman spectroscopy and variable temperature x-ray diffraction measurements.