Chuanwei Huang

Coexistence of ferroelectric triclinic phases in highly strained BiFeO3 films

The structural evolution of the strain-driven morphotropic phase boundary (MPB) in BiFeO3 films has been investigated using synchrotron x-ray diffractometry in conjunction with scanning probe microscopy. Our results demonstrate the existence of mixed-phase regions that are mainly made up of two heavily tilted ferroelectric triclinic phases. Analysis of first-principles computations suggests that these two triclinic phases originate from a phase separation of a single monoclinic state accompanied by elastic matching between the phase-separated states. These first-principle calculations further reveal that the intrinsic piezoelectric response of these two low-symmetry triclinic phases is not significantly large, which thus implies that the ease of phase transition between these two energetically close triclinic phases is likely responsible for the large piezoelectric response found in the BiFeO3 films near its MPB. These findings not only enrich the understandings of the lattice and domain structure of epitaxial BiFeO3 films but may also shed some light on the origin of enhanced piezoelectric response near MPB.

Nanoscale domains in strained epitaxial BiFeO3 thin Films on LaSrAlO4 substrate

BiFeO3 thin films with various thicknesses were grown epitaxially on (001) LaSrAlO4 single crystal substrates using pulsed laser deposition. High resolution x-ray diffraction measurements revealed that a tetragonal-like phase with c-lattice constant ∼4.65 A is stabilized by a large misfit strain. Besides, a rhombohedral-like phase with c-lattice constant ∼3.99 A was also detected at film thickness of ∼50 nm and above to relieve large misfit strains. In-plane piezoelectric force microscopy studies showed clear signals and self-assembled nanoscale stripe domain structure for the tetragonal-like regions. These findings suggest a complex picture of nanoscale domain patterns in BiFeO3 thin films subjected to large compressive strains.

Low symmetry monoclinic MC phase in epitaxial BiFeO3 thin films on LaSrAlO4 substrates

We reported that the tetragonal-like phase identified in strained epitaxial BiFeO3 films on a (001) LaSrAlO4 single crystal substrates is monoclinic MC, based on high resolution synchrotron x-ray studies and piezoresponse force microscopy measurements. This MC phase has different symmetry with the rhombohedral-like monoclinic MA phase found in BiFeO3 films grown on low mismatch SrTiO3 substrates. Transmission electron microscopy revealed that the films on LaSrAlO4 substrates have a high crystalline quality and coherent interface.

Study of strain effect on in-plane polarization in epitaxial BiFeO3 thin films using planar electrodes

Epitaxial strain plays an important role in determining physical properties of perovskite ferroelectric oxide thin films. However, it is very challenging to directly measure properties such as polarization in ultrathin strained films using traditional sandwich capacitor devices, because of high leakage current. We employed a planar electrode device with different crystallographical orientations between electrodes along different electric field orientation to directly measure the in-plane polarization-electric field (P-E) hysteresis loops in fully strained thin films. At high misfit strains such as -4.4%, the pure Tetrogonal-like phase is obtained and its polarization vector is constrained to lie in the (010) plane with a significantly large in-plane component, ~44 {\mu}C/cm2. First-principle calculations are carried out in parallel, and provide a good agreement with the experimental results. Our results pave the way to design in-plane devices based on T-like BFO and the strategy proposed here can be expanded to study all other similar strained multiferroic ultrathin films.

Strain-driven phase transitions and associated dielectric/piezoelectric anomalies in BiFeO3 thin films

Strain-driven phase transitions and related intrinsic polarization, dielectric, and piezoelectric properties for single-domain films were studied for BiFeO3 using phenomenological Landau–Devonshire theory. A stable and mixed structure between tetragonal and rhombohedral-like (monoclinic) phases is predicted at a compressive misfit strain of um=−0.0382 without an energy barrier. For a tensile misfit strain of um=0.0272, another phase transition between the monoclinic and orthorhombic phases was predicted with sharply high dielectric and piezoelectric responses.

Large tensile-strain-induced monoclinic MB phase in BiFeO3 epitaxial thin films on a PrScO3 substrate

110]o direction. For films thicknesses less than ∼40 nm, the presence of well-ordered domains is proved by the detection of satellite peaks in synchrotron x-ray diffraction studies. For thicker films, only the Bragg reflections from tilted domains were detected. This is attributed to the broader domain size distribution in thicker films. Using planar electrodes, the in-plane polarization of the MB phase is determined to be ∼85 μC/cm 2 , which is much larger than that of compressive-strained BiFeO3 films. Our results further reveal that the substrate monoclinic distortion plays an important role in determining the stripe domain formation of the rhombohedral ferroic epitaxial thin films, which sheds light on the problem of understanding elastic domain structure evolution in many other functional oxide thin films as well.

Coexistence of ferroelectric vortex domains and charged domain walls in epitaxial BiFeO3 film on (110)O GdScO3 substrate

The occurrence of ferroelectric charged domain walls (CDWs) which was thought to be energetically unstable is observed, together with a ferroelectric vortex structure composed of 109° and 180° domains near an epitaxial BiFeO3/GdScO3 interface. The CDW and vortex affect the domain arrangement, domain configuration, and hence tune the domain size distribution.

Abnormal Poisson's ratio and Linear Compressibility in Perovskite Materials

Materials with unusual properties, such as negative thermal expansion ( NTE ), [ 1 ] negative linear compressibility ( NLC ), [ 2 ] and a negative Poisson’s ratio ( NPR ) [ 3 ] have attracted considerable attention recently not only because of the underlying basic physics but also for the potential applications in tailoring thermal/mechanical related properties. The Poisson’s effect is a fundamental mechanical phenomenon that refers to a material’s deformation in an orthogonal direction to an uniaxial external force. Poisson’s Ratio ( PR ) ( νi j = −ε ( j )/ε (i ) ) , is defi ned as the negative ratio between the transverse strain ε ( j ) and longitudinal strain ε ( i ). Modern materials’ PR remains a stirring area of research since the original publication by Siméon Denis Poisson 200 years ago. [ 4 ] PR was predicted to vary from −1.0 to 0.5 in the isotropic elastic theory, [ 5 ] but practically, a vast majority of materials become thinner laterally when stretched longitudinally giving a positive Poisson’s ratio ( PPR ). Negative Poisson’s ratio ( NPR ) has been observed in a few materials such as SiO 2 , [ 6 ] polymers and composites. [ 7–9 ] Driven by a variety of potential applications of NPR and its mechanically coupled properties such as fracture toughness and acoustic performance, many theoretical studies were carried out. The theoretical limits of PR have been described mathematically for different crystal symmetries. [ 10–14 ] The studies based on the single crystal materials’ elastic stiffness data predicted that NPR could occur in many cubic metallic alloys, paratellurite, and the zeolite mineral natrolite, along two specifi ed orthogonal directions. For example, ( [110] , [ 110 ]) ), [ 3 , 15 , 16 ] where [110] is the load direction and [ 110 ] is the transverse direction exhibiting the Poisson’s effect. Some of these predictions have been observed experimentally in cubic iron-gallium and iron-aluminum alloys. [ 17 ] Furthermore, a recent statistical study showed that many materials, from organics to inorganics, possess not only NPRs but also unusual PPRs larger than the isotropic upper limit of 0.5 along specifi c directions. [ 18 ]

Periodic elastic nanodomains in ultrathin tetragonal-like BiFeO3 films

We present a synchrotron grazing incidence x-ray diffraction analysis of the domain structure and polar symmetry of highly strained BiFeO3 (BFO) thin films grown on LaAlO3 substrate. We reveal the existence of periodic elastic nanodomains in the pure tetragonal-like BFO ultrathin films down to a thickness of 6 nm. A unique shear strain-accommodation mechanism is disclosed. We further demonstrate that the periodicity of the nanodomains increases with film thickness but deviates from the classical square root law in the ultrathin thickness regime (6-30 nm). Temperature-dependent experiments further reveal the disappearance of periodic modulation above similar to 90 degrees C due to a M-C-M-A structural phase transition.

Nanoscale phase separation in quasi-uniaxial and biaxial strained multiferroic thin films

Nanoscale phase separation was investigated in epitaxial strained BiFeO3 thin films on LaAlO3 single crystal substrate. In biaxial strained thin films, nanoscale mixtures of the tetragonal-like and rhombohedral-like phases occur with a film thickness above 35 nm. For 10-30 nm ultrathin ones, tetragonal-like single phase is confirmed using synchrotron x-ray and the atomic force microscopy studies. However, nanoscale phase separations are still observed in quasi-uniaxial transmission electron microscopy foil specimens for those ultrathin films, indicating the phase separation emerges in a much smaller thickness in uniaxial constraint films than that in biaxial ones.