Spontaneous Strain Research Articles

Antiferroelectric Heterostructures Memristors with Unique Resistive Switching Mechanisms and Properties.

A novel antiferroelectric material, PbSnO3 (PSO), was introduced into a resistive random access memory (RRAM) to reveal its resistive switching (RS) properties. It exhibits outstanding electrical performance with a large memory window (>104), narrow switching voltage distribution (±2 V), and low power consumption. Using high-resolution transmission electron microscopy, we observed the antiferroelectric properties and remanent polarization of the PSO thin films. The in-plane shear strains in the monoclinic PSO layer are attributed to oxygen octahedral tilts, resulting in misfit dislocations and grain boundaries at the PSO/SRO interface. Furthermore, the incoherent grain boundaries between the orthorhombic and monoclinic phases are assumed to be the primary paths of Ag+ filaments. Therefore, the RS behavior is primarily dominated by antiferroelectric polarization and defect mechanisms for the PSO structures. The RS behavior of antiferroelectric heterostructures controlled by switching spontaneous polarization and strain, defects, and surface chemistry reactions can facilitate the development of new antiferroelectric device systems.

Active shape programming drives Drosophila wing disc eversion.

How complex 3D tissue shape emerges during animal development remains an important open question in biology and biophysics. Here, we discover a mechanism for 3D epithelial shape change based on active, in-plane cellular events that is analogous to inanimate "shape programmable" materials, which undergo blueprinted 3D shape transformations from in-plane gradients of spontaneous strains. We study eversion of the Drosophila wing disc pouch, when the epithelium transforms from a dome into a curved fold, quantifying 3D tissue shape changes and mapping spatial patterns of cellular behaviors on the evolving geometry using cellular topology. Using a physical model inspired by shape programming, we find that active cell rearrangements are the major contributor to pouch eversion and validate this conclusion using a knockdown of MyoVI, which reduces rearrangements and disrupts morphogenesis. This work shows that shape programming is a mechanism for animal tissue morphogenesis and suggests that patterns in nature could present design strategies for shape-programmable materials.

Exploring Phase Transition and Structural Complexity in the Mixed Cation Uranium Oxide CaUNb2O8.

The structures and high-temperature phase transition of CaUNb2O8 were studied in situ using synchrotron X-ray and neutron powder diffraction. Rietveld refinements provided an accurate description of the crystal structures of both the monoclinic fergusonite-type I2/b structure observed at room temperature and the tetragonal scheelite-type I41/a structure found at high temperatures. Bond valence sum analysis showed Nb5+ to be octahedrally coordinated in the monoclinic fergusonite-type structure, akin to other ANbO4 materials. Rietveld analysis of the variable temperature data allowed for the determination of accurate unit cell parameters and atomic coordinates, as well as revealing a reversible phase transition around ∼750 °C. The Nb-O bond distances display anomalous behavior, with a discontinuity in the longer Nb-O(1') distance coinciding with the phase transition suggestive of a reconstructive phase transition. Mode analysis identified the Γ2+ mode as the primary mode that drives the phase transition; this is linearly coupled to the induced spontaneous strain within the monoclinic fergusonite-type structure. Analysis of the temperature dependence of the Nb(z) positional parameter, as well as of the ϵ1-ϵ2 and ϵ6 strain parameters, showed that the phase transition is not strictly second order, with the critical exponent β ≠ 1/2. This study demonstrates the complex structural features of mixed cation metal oxides at elevated temperatures.

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Molecular ferroelastic materials have the natural properties of spontaneous strain switching and potential applications in intelligent actuator. Under the guidance of the single/double protonation strategy, we successfully obtained the molecular ferroelasticity [Me2NH(CH2)2NH3](ReO4)2 (Me2NH(CH2)2NH3 = N,N‐dimethylethylenediamine). This work further enriches the family of ferroelastic molecules and demonstrates that single/double protonation is one of the effective molecular modification strategies for designing ferroelasticity. More details are discussed in the article by Fu et al. on page 1706—1712.image

Structural phase transitions in multicomponent La0.2Nd0.2Sm0.2Gd0.2RE50.2NbO4 (RE5 = Ho, Y, Tb, Eu, Pr) oxides

AbstractIn this work, the influence of compositional complexity on the structural and thermal properties of multicomponent rare‐earth ortho‐niobates from the La0.2Nd0.2Sm0.2Gd0.2RE50.2NbO4 (RE5 = Ho, Y, Tb, Eu, Pr) series was investigated. Based on X‐ray powder diffraction studies using synchrotron radiation, it was found that all tested materials were pure single‐phase compositions and showed stability in the monoclinic I2/c crystal structure at room temperature. High‐temperature X‐ray powder diffraction studies and dilatometry studies confirmed the presence of a structural phase transition between low‐temperature (I2/c) and high‐temperature (I41/a) polymorphs. The structural phase transition temperatures are between 676°C and 701°C. Interestingly, despite their compositional complexity, the structural phase transition temperature behaves similarly to conventional ortho‐niobates, that is, it depends on the radius of the A‐cation; that is, as the ionic radius increases, the phase transition temperature decreases. The transition has been categorized as a second‐order phase transition based on the observed relationship between the Landau order parameter and spontaneous strain. The coexistence of the tetragonal and monoclinic phases has been seen in all compositions around the temperature of the structural phase transition. The presence of two orientation states in the monoclinic structure leads to the so‐called spontaneous strain, which consists of longitudinal (u) and shear (v) strain components. The values of these strains at 300°C range between 2.42 and 2.58 × 10−2 for longitudinal, 2.98 and 3.04 × 10−2 for shear, and 5.46 and 5.57 × 10−2 for scalar spontaneous strain. It was found that the spontaneous strain in each of the materials under test was very little impacted by the variation in the complexity of the A sublattice's composition. In addition, thermal expansion coefficients of both polymorphs were determined, which range from 12.7 × 10−6 K−1 to 13.2 × 10−6 K−1 for the monoclinic structure and 9.7 × 10−6 K−1 to 9.9 × 10−6 K−1 for the tetragonal one.

A phase-field model for study of ferroelastic deformation behavior in yttria stabilized zirconia

A modified phase-field model coupled with linear elasticity is formulated to study the effect of ferroelastic domain switching on the deformation behavior of tetragonal-prime yttria-stabilized zirconia. Simulations are performed under uniaxial tension and compression using realistic domain microstructures. This work provides new insights into the mechanism of ferroelastic deformation by studying the evolution of domains under different loading directions and strain rates. The model predicts realistic stress-strain curves consisting of stress-plateaus with accurate domain switching behavior without setting a switching criterion based on a critical stress for nucleation of new domains. The results reveal that the origin of coercive stress lies in the balance between the energy absorbed by the domain switching process and the externally applied energy. Loading in different directions but parallel to the spontaneous strain results in different levels of coercive stress as domain switching absorbs different amounts of energy. For the same reason when the strain-rate increases, the coercive stress also increases. This work establishes the relation between the coercive stress and microstructural changes during ferroelastic domain switching.

Raman spectroscopy, phase transitions, microstructures, and mechano‐luminescence in Ca‐Sr feldspars (CaAl2Si2O8‐SrAl2Si2O8)

AbstractThe Raman spectra of Ca1‐xSrxAl2Si2O8 (0 ≤ x ≤ 1) feldspars have been measured, to investigate at a short‐range scale the effect of changing composition on the P ➔ I and I ➔ I2/c phase transitions and how phase transitions affect the mechano‐luminescence found in these feldspars. Transmission electron microscopy (TEM) and X‐ray diffraction (XRD) were done on the same samples, to pinpoint the transitions, by the analysis of selected area diffraction patterns (SAED) and of the spontaneous strain. Natural anorthite (CaAl2Si2O8) shows a large number of well‐resolved peaks, together with sharp h + k even, l odd diffractions (c‐type) in SAED patterns, indicative of a P symmetry. In synthetic anorthite, the Raman spectrum is similar to that of natural anorthite, but the peaks are broader, and c‐type reflections are elongated. As Sr increases, the c‐type reflections become more diffuse and elongated, as the Raman peaks do. At x > 0.6, new peaks appear in Raman spectra, and other peaks show a change in their wavenumber. Moreover, the c‐type reflections disappear completely, and the lattice becomes I‐centred. Between x ~ 0.6 and x = 0.86(3), the XRD patterns prompt for a triclinic structure, with S.G. I . The transition to the monoclinic I2/c structure is well constrained by the ferroelastic strain, whereas the Raman spectra are more similar between the monoclinic and triclinic I and I2/c than between I and P . The absence of mechano‐luminescence in triclinic I compositions suggests that this property is enhanced in local P configurations at conditions close to the P to I transition. A possible mechanism for mechano‐luminescence involving a compression induced P to I transition in local Eu2+ bearing P configurations is proposed.

Toughening mechanism in superstructure Hf6Ta2O17 thin film

Fracture toughness is a key indicator to estimate the anti-peeling performance of thermal barrier coatings (TBCs). A unique superstructure endows Hf6Ta2O17 with an outstanding phase stability and mechanical property, which also poses great challenges for studying its intrinsic toughening mechanism. Here, the superstructure Hf6Ta2O17 single crystal film was epitaxially grown on YSZ(011) substrate by pulsed laser deposition. An uncommon ripple pattern near the Vickers indentation crack tips was first reported. The pop-in signals in the load-displacement curve indicated it may be relevant to ferroelastic domain transformation, which was further confirmed by spontaneous strain distribution in HRTEM. The transformation path, predicted by DFT, was conducted in b→c ferroelastic domain through in-plane rotation of Hf–O/Ta–O polyhedrons. The corresponding critical stress for domain transformation was up to 5.83 GPa. In addition, an intrinsic ductility was confirmed with a high Pugh ratio B/G of 2.49, much larger than the critical value (1.75) for brittle-ductile transition. These two mechanisms make contribution to the superior fracture toughness of Hf6Ta2O17. This work provides a deep insight into the toughening mechanism of ceramics and paves the way to design of next-generation TBCs.

Three-Step Ferroelastic Transitions from Hexagonal to Triclinic Phases in a Hybrid Perovskite: (1-Fluoromethyl-1-methylpyrrolidine)[CdCl3

Hybrid ferroelastic crystals have emerged as a hot research topic in recent years owing to their prospective applications in piezoelectric sensors, mechanical switches, and optoelectronic devices. Nevertheless, most of the documented materials exhibit one-step or two-step ferroelastic phase transition(s), and those with multistep ferroelastic transitions are extremely scarce. We present a new hexagonal molecular perovskite based on a fluoro-substituted flexible cyclic ammonium cation, (1-fluoromethyl-1-methylpyrrolidine)[CdCl3] (1), undergoing unusual three-step ferroelastic phase transitions from hexagonal paraelastic phase to orthorhombic, monoclinic, and triclinic ferroelastic phases at 388, 376, and 311 K, respectively, with Aizu notation of 6/mmmFmmm, mmmF2/m, and 2/mF-1, featuring spontaneous strain of 0.002, 0.023, and 0.110, respectively. Furthermore, variable-temperature single-crystal diffraction reveals that the phase-transition mechanism in 1 principally originates from intriguing dynamic change of organic cations and synchronous displacement of inorganic chains. This scarce instance of multistep hybrid ferroelastic provides important clues for finding advanced ferroelastic materials.

Molecular Ferroelastic Induced by Mono‐/Double‐Protonation Strategy

Comprehensive SummaryMolecular ferroelastics with the natural features of mechanical flexibility and switchable spontaneous strain have attracted widespread attention in the scientific community due to their potential applications in tunable gratings, flexible memorizers, strain sensors, and intelligent actuators. However, most designs of molecular ferroelastics remain in the stage of blind exploration, posing a challenge to achieve a functional ferroelastic more effectively. Herein, we have successfully obtained a molecular ferroelastic, [Me2NH(CH2)2NH3](ReO4)2 (Me2NH(CH2)2NH3 = N,N‐dimethylethylenediammonium), under the guidance of the mono‐/double‐protonation strategy. The double‐protonated [Me2NH(CH2)2NH3](ReO4)2 undergoes a paraelastic‐ferroelastic phase transition with the Aizu notation of 2/mF at 322 K. Meanwhile, the theoretical calculation and experimental measurement simultaneously show that [Me2NH(CH2)2NH3](ReO4)2 possesses good mechanical flexibility, because its elastic modulus (E) of 8.26 GPa and hardness (H) of 0.45 GPa are smaller than the average values of organic crystals (E of 12.05 GPa and H of 0.5 GPa), which makes it promising to apply in wearable pressure sensors, implantable medical sensors, high‐precision tuners, etc. This work further enriches the molecular ferroelastic family and demonstrates that mono‐/double‐protonation is one of the effective molecular modification strategies for designing ferroelastics.

Robust Ferroelasticity and Carrier Dynamics Across the Domain Wall in Perovskite‐Like van der Waals WO2I2

AbstractAs a new group of van der Waals (vdWs) ferroic materials, transition metal dioxydihalides MO2X2 (M: Mo, W; X: halogen) with a perovskite‐like structure are theoretically predicted to exhibit intriguing physics and versatile ferroic characteristics, which is not achieved experimentally as far as it is known. In this work, the robust ferroelasticity in vdWs WO2I2 with the switching strain as low as ≈0.3%, accompanied with the striped optical contrast between adjacent domains, spot splitting of selected area electron diffraction (SAED) patterns at domain wall, and 90° domain wall is demonstrated. With the aid of ab‐initio calculations, the origin of ferroelasticity in WO2I2 is unveiled, where the imaginary phonon mode in the high‐symmetry paraelastic phase leads to the spontaneous displacement of W atom away from the center of the [WO4I2] octahedron, resulting in the switchable spontaneous strain under an external strain field. Moreover, transient absorption microscopy (TAM) measurements demonstrate that the diffusion of photogenerated carriers is significantly hindered by the ferroelastic domain walls. This study provides deep insights into the ferroic order and domain wall in perovskite‐like vdWs MO2X2 for new physics and functionalities.

Experimental observation of two-dimensional phase in compressed FeF2

Iron difluoride (FeF2) has attracted considerable attention for its physical characteristics and practical applications, and its compression behaviors usually play a key role in the in-depth understanding of this compound. Since its high-pressure crystal structure evolution determining a more profound comprehension remains disputable, we carried out extensive experiments to focus on the pressure-induced structural phase transitions of FeF2. Through in situ high-pressure synchrotron x-ray diffraction measurements, we not only confirmed a reported high-pressure orthorhombic Pbca phase at 11 GPa but also identified an interesting two-dimensional structure with hexagonal close packed symmetry (P-3m1) that appears above 25 GPa at room temperature. Furthermore, the spontaneous strain fitting and electronic transport measurements suggest that its ambient rutile-type structure (P42/mnm) evolves into an orthorhombic structure (Pnnm) through a second-order phase transition at 5 GPa. These experimental results elaborate on the pressure-induced phase transitions of FeF2 on the order of P42/mnm → Pnnm → Pbca → P-3m1, shedding light on a rare three-dimensional to two-dimensional configuration transition in difluorides.

Exceptional stress-director coupling at the crack tip of a liquid crystal elastomer

A liquid crystal elastomer (LCE) is a special elastomer containing rod-like liquid crystals, which align in a certain direction, called the director. The director of a LCE can rotate under stress, resulting in large spontaneous strain and soft elastic behavior. This study unravels how the strong stress-director coupling in a monodomain LCE induces unique crack-tip fields and fracture behavior. Through stretching edge-cracked LCEs with various initial directors, we characterize the displacement and director fields theoretically and experimentally. The results reveal that the directors undergo significant and inhomogeneous rotation at the crack tips, leading to very different stress/strain distributions from traditional elastomers. Particularly, when the initial director is tilted to the loading direction, the stress/strain distributions are asymmetrical about the crack plane. Notably, we discover a domain wall forms along a certain polar angle at the crack tip, with opposite director rotation, and thereby shear strain, on the two sides of the domain wall. Moreover, LCEs with a tilted initial director to the loading exhibit much smaller crack openings and energy release rates than those of neo-Hookean materials, while LCEs with a parallel director exhibit higher values. We attribute these findings to a combined effect of bulk softening at the remote region and the formation of domains of opposite director rotation near the crack tip. This study provides an understanding of how the stress-director coupling of LCEs triggers their unique crack-tip fields, and insights into strategies to enhance the fracture properties of LCEs for future applications.

The effect of ferroelastic domains and ultrahigh‐density dislocations on fracture toughness of high entropy niobates

AbstractThis study investigates the fracture toughness and toughening mechanism of (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4, a high entropy material characterized by a high thermal expansion coefficient and low thermal conductivity. Our finding demonstrates a noteworthy 50% increase in fracture toughness for (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 (∼3.09 MPa m0.5) compared to YNbO4 (∼2.11 MPa m0.5). The superior fracture toughness for (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 can be attributed to the two key factors: relatively easy ferroelastic domain switching and the presence of ultrahigh‐density dislocations (∼7 × 108 mm−2). The enhanced ferroelastic toughening in (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 is associated with its distinct characteristics, large atomic radius (RA), significant tetragonality (c/a ratio), and low monoclinic β angle. These factors contribute to larger spontaneous strain, larger coercive strain, and lower transformation strain, respectively. These properties facilitate the formation and switching of the ferroelastic domains. In addition, the interactions between the large strain field surrounding the dislocations and the cracks facilitate crack deflection and bridging. The combined effect of ferroelastic toughening and ultrahigh‐density dislocations ultimately leads to the significant improvement in the fracture toughness of (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4.

Asymmetry and evolution of craton-influenced rifted margins

Abstract Rifting can result in asymmetric conjugate margins. In numerical models with laterally homogeneous lithosphere, the polarity of the asymmetry is random and results from spontaneous strain localization on a dominant fault/shear zone. However, along the central South Atlantic, margin width is well correlated to the proximity of the rift to a craton during rifting. To understand the evolution of rifting close to a craton, we used numerical experiments that included a thick craton lithosphere adjacent to a thinner, mobile-belt lithosphere. When rifting starts close to the craton, i.e., ≤ 100 km, deformation focuses along a fault/shear zone dipping toward the craton, leading to cratonward asymmetric asthenospheric uplift and rift migration. This results in a narrow margin on the craton side and a wide margin on the mobile-belt side. Craton-related asymmetry results in the wide margin subsiding more than the narrow one, inducing the formation of large synrift sag basins in the wide conjugate, as in the South Atlantic. This differential subsidence is preserved for ~100 m.y. after breakup and may partly explain the present-day higher dynamic topography close to cratons in the central South Atlantic. Mantle flow during asymmetric rifting leads to weakening of the craton lithosphere, partial intermixing, and recycling into the convective mantle, as interpreted for this sector of the South Atlantic ridge. Thus, rifting adjacent to a craton may have wide-ranging tectonic, stratigraphic, and compositional consequences that affect not only margin architecture, but also the development of subsequent dynamic topography and the spreading system.

Revisiting the structures and phase transitions of Ba2NaNb5O15.

The room-temperature and low-temperature structure(s) of Ba2NaNb5O15 (BNN) have been debated since the structure was proposed in the 1960s. This work revisits the structures and phase transitions of BNN, combining high-resolution X-ray and neutron powder diffraction with density functional theory calculations. Temperature-dependent high-resolution X-ray powder diffraction patterns are collected from 4 to 918 K, and sequential batch Rietveld refinement using a symmetry mode approach to describe the structure is used to extract the main structural changes as a function of temperature. The data show that the average structure of BNN is best described by the Ama2 space group, and no other structural phase transitions were observed below the ferroelastic transition. The symmetry mode analysis, combining results from diffraction and density functional theory, shows significant octahedral tilting and corrugations of both the A1 and A2 sites along the c direction. A strong correlation between the spontaneous strain and the octahedral tilting was observed, and a potential connection with emerging microstructure at low temperatures is proposed, all enabled by the symmetry mode approach used in this work.

The coenzyme B12 precursor 5,6-dimethylbenzimidazole is a flavin antagonist in Salmonella.

Salmonella enterica subsp. enterica sv. Typhimurium str. LT2 (hereafter S. Typhimurium) synthesizes adenosylcobalamin (AdoCbl, CoB12) de novo only under anoxic conditions, but it can assemble the lower ligand loop (a.k.a. the nucleotide loop) and can form the unique C-Co bond present in CoB12 in the presence or absence of molecular oxygen. During studies of nucleotide loop assembly in S. Typhimurium, we noticed that the growth of this bacterium could be arrested by the lower ligand nucleobase, namely 5,6-dimethylbenzimidazole (DMB). Here we report in vitro and in vivo evidence that shows that the structural similarity of DMB to the isoalloxazine moiety of flavin cofactors causes its deleterious effect on cell growth. We studied DMB inhibition of the housekeeping flavin dehydrogenase (Fre) and three flavoenzymes that initiate the catabolism of tricarballylate, succinate or D-alanine in S. Typhimurium. Notably, while growth with tricarballylate was inhibited by 5-methyl-benzimidazole (5-Me-Bza) and DMB, growth with succinate or glycerol was arrested by DMB but not by 5-Me-Bza. Neither unsubstituted benzimidazole nor adenine inhibited growth of S. Typhimurium at DMB inhibitory concentrations. Whole genome sequencing analysis of spontaneous mutant strains that grew in the presence of inhibitory concentrations of DMB identified mutations effecting the cycA (encodes D-Ala/D-Ser transporter) and dctA (encodes dicarboxylate transporter) genes and in the coding sequence of the tricarballylate transporter (TcuC), suggesting that increased uptake of substrates relieved DMB inhibition. We discuss two possible mechanisms of inhibition by DMB.

Turing structuring with multiple nanotwins to engineer efficient and stable catalysts for hydrogen evolution reaction

Low-dimensional nanocrystals with controllable defects or strain modifications are newly emerging active electrocatalysts for hydrogen-energy conversion and utilization; however, a crucial challenge remains in insufficient stability due to spontaneous structural degradation and strain relaxation. Here we report a Turing structuring strategy to activate and stabilize superthin metal nanosheets by incorporating high-density nanotwins. Turing configuration, realized by constrained orientation attachment of nanograins, yields intrinsically stable nanotwin network and straining effects, which synergistically reduce the energy barrier of water dissociation and optimize the hydrogen adsorption free energy for hydrogen evolution reaction. Turing PtNiNb nanocatalyst achieves 23.5 and 3.1 times increase in mass activity and stability index, respectively, compared against commercial 20% Pt/C. The Turing PtNiNb-based anion-exchange-membrane water electrolyser with a low Pt mass loading of 0.05 mg cm−2 demonstrates at least 500 h stability at 1000 mA cm−2, disclosing the stable catalysis. Besides, this new paradigm can be extended to Ir/Pd/Ag-based nanocatalysts, illustrating the universality of Turing-type catalysts.

The GGDEF protein Dgc2 suppresses both motility and biofilm formation in the filamentous cyanobacterium Leptolyngbya boryana.

Colony pattern formations of bacteria with motility manifest complicated morphological self-organization phenomena. Leptolyngbya boryana is a filamentous cyanobacterium, which has been used as a genetic model organism for studying metabolism including photosynthesis and nitrogen fixation. A widely used type strain [wild type (WT) in this article] of this species has not been reported to show any motile activity. However, we isolated a spontaneous mutant strain that shows active motility (gliding activity) to give rise to complicated colony patterns, including comet-like wandering clusters and disk-like rotating vortices on solid media. Whole-genome resequencing identified multiple mutations in the genome of the mutant strain. We confirmed that inactivation of the candidate gene dgc2 (LBDG_02920) in the WT background was sufficient to give rise to motility and morphologically complex colony patterns. This gene encodes a protein containing the GGDEF motif which is conserved at the catalytic domain of diguanylate cyclase (DGC). Although DGC has been reported to be involved in biofilm formation, the dgc2 mutant significantly facilitated biofilm formation, suggesting a role for the dgc2 gene in suppressing both gliding motility and biofilm formation. Thus, Leptolyngbya is expected to be an excellent genetic model for studying dynamic colony pattern formation and to provide novel insights into the role of DGC family genes in biofilm formation. IMPORTANCE Self-propelled bacteria often exhibit complex collective behaviors, such as formation of dense-moving clusters, which are exemplified by wandering comet-like and rotating disk-like colonies; however, the molecular details of how these structures are formed are scant. We found that a strain of the filamentous cyanobacterium Leptolyngbya deficient in the GGDEF protein gene dgc2 elicits motility and complex and dynamic colony pattern formation, including comet-like and disk-like clusters. Although c-di-GMP has been reported to activate biofilm formation in some bacterial species, disruption of dgc2 unexpectedly enhanced it, suggesting a novel role for this GGDEF protein for inhibiting both colony pattern formation and biofilm formation.

Ferroelastic twin domain patterns and polar domain walls of BiVO4 thin films via phase-field simulations

Polar domain walls in centrosymmetric ferroelastics (e.g., BiVO4 films) give rise to inhomogeneity and offer promises for solar energy conversion applications via expediting charge-carrier separation. However, the origin of polar domain walls and the relationship between them and spontaneous strain from structural phase transition remain elusive at the mesoscale in BiVO4 films. Here, from the perspective of phenomenological thermodynamics, the relationship between four variants of monoclinic (M) phase and the corresponding lattice parameters during the tetragonal-monoclinic (T-M) phase transition is revealed. Using phase-field simulations, it suggests that the formation of four variants of ferroelastic twin domains arises from spontaneous strain from T-M phase transition. Moreover, it is demonstrated that (i) polar domain walls in BiVO4 films originate from the flexoelectric effect induced by strain gradient across the domain walls during phase transition process; (ii) shear strain gradient dominates the contribution to the polarization vectors at the domain walls; (iii) 180° head-to-head or tail-to-tail in-plane polarization configurations emerge at the adjacent domain walls. The stability of several types of ferroelastic domain walls is also analyzed from the point of view of total free energy. These results not only provide a fundamental understanding of the origin of polar domain walls and the formation mechanisms of ferroelastic twin patterns, but also stimulate potential applications of polar domain walls of non-polar BiVO4 films in energy conversion.