Strain-induced boundary states and phase transitions in photonic graphene flakes

Structures and transitions in bcc tungsten grain boundaries and their role in the absorption of point defects

Strain-induced partial phase transition in TiO2 nanoparticles manifesting frequency dispersive pseudo-inductive switching of capacitance

Grain boundary (GB) phase transitions can change drastically the properties of nanograined polycrystals, leading to enhanced plasticity or brittleness, increasing diffusion permeability. They influence also liquid-phase and activated sintering, soldering, processing of semi-solid materials. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. The GB wetting tie line appears in the L+S area. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The liquid phase has to substitute the GB and to separate both grains. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. In case if two solid phase are in equilibrium, the GB “solid state wetting” (or covering) can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such covering GB phase transition occurs if the energy of two interphase boundaries between phase 1 and 2 is lower than the GB energy in the phase 1.

Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The experimental data on GB wetting phase transitions in numerous systems are analysed. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity obtained in various systems both in polycrystals and bicrystals are analysed. In case if two solid phases are in equilibrium, the GB “solid state wetting” can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such GB phase transition occurs if the energy of two interphase boundaries is lower than the GB energy in the phase 1.

Strain engineering demonstrates remarkable precision in inducing phase transitions, as well as high orientability, enabling tunable phase transitions with low energy consumption and rapid response. NbOCl2, an emerging ferroelectric (FE) layered two-dimensional (2D) material, exhibits pronounced in-plane FE properties and demonstrates a significant anisotropic second harmonic generation response. Here, we demonstrate that the FE phase transition in NbOCl2 can be modulated by applying strain relative to its intrinsic lattice orientation. It has been discovered that the strain-induced FE phase transition in NbOCl2 crystals depends on the direction of the polar axis. Specifically, when strain is applied along the polar axis and reaches a minimal threshold of just 0.65%, it induces a transition from the FE phase to the antiferroelectric phase. By releasing the strain, NbOCl2 reverts to the FE phase, enabling a tunable phase transition along the polar axis. Furthermore, it was discovered that the challenge in inducing phase transition behavior by applying uniaxial strain along the nonpolar axis is due to the difficulty in effectively coupling the strain field to the key atomic interaction system that determines the FE properties. This work not only provides valuable strategies and insights for inducing reversible phase transitions in other 2D materials but also establishes a robust foundation for the development of FE memory devices with enhanced directional controllability.

Grain boundary phase transitions and phase diagrams

Since the recent discovery of superelastic behavior in ferroelectric BaTiO3, understanding the mechanical and electromechanical properties of ferroelectric ceramics under high loading conditions has become crucial for engineering applications such as advanced functional nanodevices. Here, we shed light on paraelectric SrTiO3 in contrast ot ferroelectric BaTiO3, and investigate the mechanical strength, deformation behavior, and electromechanical response of the paraelectric ceramics SrTiO3 under high stress conditions with ferroelectric phase transition. Under low-strain conditions, that is, in the paraelectric phase, SrTiO3 shows mechanical properties that maintain the characteristic high stiffness of ceramics. On the other hand, after the strain-induced ferroelectric phase transition, we find that SrTiO3 demonstrates superelastic-like nonlinear deformation behavior. Therefore, SrTiO3 exhibits both high stiffness and deformability simultaneously due to the strain-induced ferroelectric phase transition. Additionally, we clarify the emergence of hysteresis loops in both mechanical deformation and electromechanical responses without accompanying structural phase transitions. We also reveal that giant piezoelectric coefficients manifest at discontinuous points of polarization curves. We conclude that these unique properties in SrTiO3 originate from the displacement of oxygen atoms and the chemical bonding network due to the ferroelectric phase transition, based on analysis of the electronic structure and interatomic distances of each atom with respect to strain. Our results advance fundamental understanding of SrTiO3 behavior under extreme loading conditions and establish a foundation for advanced engineering applications.

We have determined the post‐perovskite phase transition boundary in MgSiO3 by in‐situ X‐ray diffraction measurements, using Au and MgO as internal pressure standards. Results demonstrate that phase transition occurs at 113 GPa and 2400 K with a positive Clapeyron slope of +4.7 MPa/K, based on the Au pressure scale. On the other hand, the simultaneous measurements of MgO standard show that the phase boundary is located at 119 GPa and 2400 K with a Clapeyron slope of +11.5 MPa/K, consistently with the previous experimental results based on the Pt scale and the theoretical calculations. Our experiments also indicate that the stabilities of perovskite and post‐perovskite in all of (Mg0.89Fe0.11)2SiO4, natural pyrolitic mantle, and MORB compositions are consistent with the phase transition boundary in pure MgSiO3, when all the data are compared using the same Au scale. The compositional variations in natural systems have little effect on the post‐perovskite phase transition.

In this paper, we report on the structural phase transition, surface morphologies, ferromagnetic and optical properties that have been observed in Bi1−xTbxFeO3 (BTFO) thin films with (x = 0, 0.03, 0.06 and 0.09). X-ray diffraction (XRD) indicated that all the films have a single phase with rhombohedral structure and exhibit gradual transition behavior to pseudo-tetragonal structure with the increase of x, the strain values were estimated to be 0.1–0.8, depending on composition. A possible mechanism, strain-induced structural phase transition, is discussed. The micro-Raman spectroscopy analysis was in good agreement with structural phase transition verified by the XRD patterns. Moreover, with an increase in Tb3+ ions from x = 0–0.06, the saturation magnetization of the BTFO thin films shows a linear increase, which is mainly due to the Tb3+ ions. However, a sharp increase emerges with x from 0.06 to 0.09, the phase transition mainly dominates the higher ferromagnetism in BTFO films. Moreover, BTFO films with x > 0 exhibit a lower band-gap than that with x = 0, originating from local rhombohedral to pseudo-tetragonal structure transition. These results are helpful for a deeper understanding of structural transition, magnetic, and optical properties in perovskite oxides and show the potential roles, which such materials can play in multiferroic applications and solar energy devices.

Monolayer transition metal dichalcogenides (TMDs) can host exotic phenomena such as correlated insulating and charge-density-wave (CDW) phases. Such properties are strongly dependent on the precise atomic arrangements. Strain, as an effective tuning parameter in atomic arrangements, has been widely used for tailoring material's structures and related properties, yet to date, a convincing demonstration of strain-induced dedicate phase transition at nanometer scale in monolayer TMDs has been lacking. Here, a strain engineering technique is developed to controllably introduce out-of-plane atomic deformations in monolayer CDW material 1T-NbSe2 . The scanning tunneling microscopy and spectroscopy (STM and STS) measurements, accompanied by first-principles calculations, demonstrate that the CDW phase of 1T-NbSe2 can survive under both tensile and compressive strains even up to 5%. Moreover, significant strain-induced phase transitions are observed, i.e., tensile (compressive) strains can drive 1T-NbSe2 from an intrinsic-correlated insulator into a band insulator (metal). Furthermore, experimental evidence of the multiple electronic phase coexistence at the nanoscale is provided. The results shed new lights on the strain engineering of correlated insulator and useful for design and development of strain-related nanodevices.

The combination of high pressure and severe plastic deformation in the process of high-pressure torsion in a rotational diamond anvil cell (RDAC) produces a variety of important mechanical and chemical effects. In this Review, mechanochemical phenomena that appeared with compression and plastic shear of samples in RDAC have been systematized, which are associated with strain-induced structural changes (SCs) under high pressure and shear, containing phase transitions (PTs) and chemical reactions (CRs). This Review aims to summarize the current phenomena based on the results of multiscale atomic and continuum theory and macroscale modeling. By analyzing and using the advanced phase field theory and simulation, the nano-scale mechanism of phase nucleation induced by plastic strain was studied. The results show that the concentration of the stress tensor near the edge dislocation stacking tip may reduce the nucleation pressure by ten times or more. These results promote the development of a microscopic analysis dynamic equation of strain-induced PTs. On the micro-scale, a simple strain control dynamic kinetics of strain-induced SCs is derived by thermodynamics. On the macro-scale, a macro-model of pressure and strain-induced PTs in RDAC is established based on the finite element method, and it could be used to explain various experimental phenomena. The application of RDAC provides important insights for the coupling of PT and material plastic flow, which is of great significance for optimizing the experimental design and extracting PT parameters of materials, as well as optimizing and controlling PT.

Enhanced susceptibilities in ferroelectrics often arise near phase boundaries between competing ground states. While chemically-induced phase boundaries have enabled ultrahigh electrical and electromechanical responses in lead-based ferroelectrics, precise chemical tuning in lead-free alternatives, such as (K,Na)NbO3 thin films, remains challenging due to the high volatility of alkali metals. Here, we demonstrate strain-induced morphotropic phase boundary-like polymorphic nanodomain structures in chemically simple, lead-free, epitaxial NaNbO3 thin films. Combining ab initio simulations, thin-film epitaxy, scanning probe microscopy, synchrotron X-ray diffraction, and electron ptychography, we reveal a labyrinthine structure comprising coexisting monoclinic and bridging triclinic phases near a strain-induced phase boundary. The coexistence of energetically competing phases facilitates field-driven polarization rotation and phase transitions, giving rise to a multi-state polarization switching pathway and large enhancements in dielectric susceptibility and tunability across a broad frequency range. Our results open new possibilities for engineering lead-free thin films with enhanced functionalities for next-generation applications.

Diffusion of Bi and Ag in a series of polycrystalline Cu–Bi alloys is investigated using a radiotracer technique and applying the 207Bi and 110m Ag isotopes, respectively. Together with the previous measurements (Divinski S., Lohmann M., Herzig C., Straumal B., Baretzky B., Gust W. Grain-boundary Melting Phase Transition in the Cu−Bi System. Phys. Rev. B 2005, 71, 104104), a temperature–concentration interval of strong, by orders of magnitude, enhancements of Bi grain boundary diffusion rates is distinguished and the results are interpreted in terms of a grain boundary pre-wetting/wetting phase transition. Grain boundary diffusivity of Ag exhibits as well a step-wise increase with rising Bi content, mirroring the behaviour observed for the Bi tracer. However, contrary to the Bi tracer atoms for which grain boundary enhancement is observed at about 60 ppm of Bi in Cu–Bi alloys, this transition is revealed by the Ag tracer atoms at a significantly higher concentration, specifically between 90 and 100 ppm of Bi at 1080 K. The Ag diffusion rates in alloys with a moderate Bi content turn out to be not affected by the Bi-induced grain boundary phase transition and the measured grain boundary diffusion coefficients of Ag are nearly the same as those determined for pure polycrystalline Cu. This spectacular result suggests a strong heterogeneity of Bi segregation and Bi-induced phase transition for general high-angle grain boundaries in a given alloy. The behaviour is discussed in terms of the extrinsic grain boundary defects and their impact on mechano-chemical coupling which is accompanying the grain boundary phase transitions.

We investigated the electronic properties and phase transitions of Na3Bi in four structural phases (space groups P63/mmc, Pc1, Fmm and Cmcm) under constant-volume uniaxial strain using the first-principles method. For P63/mmc and Pc1-Na3Bi, an important phase transition from a topological Dirac semimetal (TDS) to a topological insulator appears under compression strain around 4.5%. The insulating gap increases with the increasing compressive strain and up to around 0.1 eV at a strain of 10%. However, both P63/mmc and Pc1-Na3Bi still keep the properties of a TDS within a tensile strain of 0–10%, although the Dirac points move away from the Γ point along Γ–A in reciprocal space as the tensile strain increases. The Na3Bi with space group Fmm is identified as a topological semimetal with the inverted bands between Na-3s and Bi-6p and a parabolic dispersion in the vicinity of Γ point. Interestingly, for Fmm-Na3Bi, both compression and tensile strain lead to a TDS which is identified by calculating surface Fermi arcs and topological invariants at time-reversal planes (kz = 0 and kz = π/c) in reciprocal space. Additionally, we confirmed the high pressure phase Cmcm-Na3Bi is an ordinary insulator with a gap of about 0.62 eV. It is noteworthy that its gap almost keeps constant around 0.60 eV within a compression strain of 0–10%. In contrast, a remarkable phase transition from an insulator to a metal phase appears under tensile strain. Moreover, this phase transition is highly sensitive to tensile strain and takes place only at a strain 1.0%. These strain-induced electronic structures and phase transitions of the Na3Bi system in various phases are important due to their possible applications under high pressure in future electronic devices.

Strain-induced structural phase transition and the rotation of polarization in BaTiO3 films