Showing papers on "Phase (matter) published in 2017"

Structural phase transition in monolayer MoTe 2 driven by electrostatic doping

Abstract: Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and physical properties. Exploring the physics of transitions between these different structural phases in two dimensions may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chemical means; purely electrostatic control over crystal phases through electrostatic doping was recently proposed as a theoretical possibility, but has not yet been realized. Here we report the experimental demonstration of an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe2). We find that the phase transition shows a hysteretic loop in Raman spectra, and can be reversed by increasing or decreasing the gate voltage. We also combine second-harmonic generation spectroscopy with polarization-resolved Raman spectroscopy to show that the induced monoclinic phase preserves the crystal orientation of the original hexagonal phase. Moreover, this structural phase transition occurs simultaneously across the whole sample. This electrostatic-doping control of structural phase transition opens up new possibilities for developing phase-change devices based on atomically thin membranes.

Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites.

Abstract: The distinct physical properties of hybrid organic–inorganic materials can lead to unexpected nonequilibrium phenomena that are difficult to characterize due to the broad range of length and time scales involved. For instance, mixed halide hybrid perovskites are promising materials for optoelectronics, yet bulk measurements suggest the halides reversibly phase separate upon photoexcitation. By combining nanoscale imaging and multiscale modeling, we find that the nature of halide demixing in these materials is distinct from macroscopic phase separation. We propose that the localized strain induced by a single photoexcited charge interacting with the soft, ionic lattice is sufficient to promote halide phase separation and nucleate a light-stabilized, low-bandgap, ∼8 nm iodide-rich cluster. The limited extent of this polaron is essential to promote demixing because by contrast bulk strain would simply be relaxed. Photoinduced phase separation is therefore a consequence of the unique electromechanical propert...

Phase engineering of a multiphasic 1T/2H MoS2 catalyst for highly efficient hydrogen evolution

Abstract: Molybdenum disulfide (MoS2) has attracted much attention as a promising electrocatalyst for the hydrogen evolution reaction (HER). Although tremendous efforts have been made to enhance the HER performance of MoS2, the functional design of its intrinsic structures still remains challenging. In this work, a highly active and stable multiphasic catalyst (1T/2H MoS2) is developed through a facile hydrothermal route, in which the 1T phase is induced by the intercalation of guest ions and molecules, and the concentration of the 1T phase can be controlled by adjusting the preparation temperature. The existence of the 1T phase provides more active sites and better conductivity for the HER, resulting in an excellent activity with a small Tafel slope of 46 mV dec−1. More importantly, the integration with the 2H phase is beneficial to the stabilization of the metastable 1T phase, ensuring the excellent durability of 1T/2H MoS2.

Optimized Separation of Acetylene from Carbon Dioxide and Ethylene in a Microporous Material

Abstract: Selective separation of acetylene (C2H2) from carbon dioxide (CO2) or ethylene (C2H4) needs specific porous materials whose pores can realize sieving effects while pore surfaces can differentiate their recognitions for these molecules of similar molecular sizes and physical properties We report a microporous material [Zn(dps)2(SiF6)] (UTSA-300, dps = 4,4′-dipyridylsulfide) with two-dimensional channels of about 33 A, well-matched for the molecular sizes of C2H2 After activation, the network was transformed to its closed-pore phase, UTSA-300a, with dispersed 0D cavities, accompanied by conformation change of the pyridyl ligand and rotation of SiF62– pillars Strong C–H···F and π–π stacking interactions are found in closed-pore UTSA-300a, resulting in shrinkage of the structure Interestingly, UTSA-300a takes up quite a large amounts of acetylene (764 cm3 g–1), while showing complete C2H4 and CO2 exclusion from C2H2 under ambient conditions Neutron powder diffraction and molecular modeling studies clea

Phase transitions in bismuth-modified silver niobate ceramics for high power energy storage

Abstract: Ag(Nb0.8Ta0.2)O3 is used here as a model system to shed light on the nature of the low temperature phase behavior of the unsubstituted parent compound AgNbO3, which is an important material for high-power energy storage applications. The three dielectric anomalies previously identified as M1 ↔ M2, Tf and M2 ↔ M3 transitions in AgNbO3 ceramics are found to be intimately related to the polarization the behavior of the B-site cations. In particular, the M1 ↔ M2 transition is found to involve the disappearance of original ferroelectric polar structure in the M1 phase. Analysis of weak-field and strong field hysteresis loops in the M2 region below Tf suggests the presence of a weakly-polar structure exhibiting antipolar behavior (i.e., a non-compensated antiferroelectric), which can be considered as ferrielectric (FIE). Modeling of the permittivity data using the Curie–Weiss law indicates that the Curie temperature is close to the freezing temperature, Tf, which can be regarded as the Curie point of the FIE phase. Substitution by Ta5+ in this system enhances the stability of the weakly polar/antiferroelectric state, giving rise to an increased energy storage density of 3.7 J cm−3 under an applied field of 27 MV m−1, one of the highest values ever reported for a dielectric ceramic. Furthermore, the energy storage capability remains approximately constant at around 3 J cm−3 up to 100 °C, at an applied field of 22 MV m−1.

Argon Plasma Induced Phase Transition in Monolayer MoS2

Abstract: In this work, we report a facile, clean, controllable and scalable phase engineering technique for monolayer MoS2. We found that weak Ar-plasma bombardment can locally induce 2H→1T phase transition in monolayer MoS2 to form mosaic structures. These 2H→1T phase transitions are stabilized by point defects (single S-vacancies) and the sizes of induced 1T domains are typically a few nanometers, as revealed by scanning tunneling microscopy measurements. On the basis of a selected-area phase patterning process, we fabricated MoS2 FETs inducing 1T phase transition within the metal contact areas, which exhibit substantially improved device performances. Our results open up a new route for phase engineering in monolayer MoS2 and other transition metal dichalcogenide (TMD) materials.

Amorphous nickel-cobalt complexes hybridized with 1T-phase molybdenum disulfide via hydrazine-induced phase transformation for water splitting.

Abstract: Highly active and robust eletcrocatalysts based on earth-abundant elements are desirable to generate hydrogen and oxygen as fuels from water sustainably to replace noble metal materials. Here we report an approach to synthesize porous hybrid nanostructures combining amorphous nickel-cobalt complexes with 1T phase molybdenum disulfide (MoS2) via hydrazine-induced phase transformation for water splitting. The hybrid nanostructures exhibit overpotentials of 70 mV for hydrogen evolution and 235 mV for oxygen evolution at 10 mA cm−2 with long-term stability, which have superior kinetics for hydrogen- and oxygen-evolution with Tafel slope values of 38.1 and 45.7 mV dec−1. Moreover, we achieve 10 mA cm−2 at a low voltage of 1.44 V for 48 h in basic media for overall water splitting. We propose that such performance is likely due to the complete transformation of MoS2 to metallic 1T phase, high porosity and stabilization effect of nickel-cobalt complexes on 1T phase MoS2. Electrocatalysts based on earth-abundant elements have emerged as promising candidates to replace noble metal materials. Here, the authors develop porous hybrid nanostructures combining amorphous Ni-Co complexes with 1T phase MoS2for enhanced electrocatalytic activity for overall water splitting.

Observation of extreme phase transition temperatures of water confined inside isolated carbon nanotubes

Abstract: Fluid phase transitions inside single, isolated carbon nanotubes are predicted to deviate substantially from classical thermodynamics. This behaviour enables the study of ice nanotubes and the exploration of their potential applications. Here we report measurements of the phase boundaries of water confined within six isolated carbon nanotubes of different diameters (1.05, 1.06, 1.15, 1.24, 1.44 and 1.52 nm) using Raman spectroscopy. The results reveal an exquisite sensitivity to diameter and substantially larger temperature elevations of the freezing transition (by as much as 100 °C) than have been theoretically predicted. Dynamic water filling and reversible freezing transitions were marked by 2–5 cm−1 shifts in the radial breathing mode frequency, revealing reversible melting bracketed to 105–151 °C and 87–117 °C for 1.05 and 1.06 nm single-walled carbon nanotubes, respectively. Near-ambient phase changes were observed for 1.44 and 1.52 nm nanotubes, bracketed between 15–49 °C and 3–30 °C, respectively, whereas the depression of the freezing point was observed for the 1.15 nm nanotube between −35 and 10 °C. We also find that the interior aqueous phase reversibly decreases the axial thermal conductivity of the nanotube by as much as 500%, allowing digital control of the heat flux. A vibrational spectroscopy technique is used to study vapour, liquid and solid water within isolated carbon nanotubes and reveals phase transitions that show an extreme sensitivity to nanotube diameter, with melting temperatures higher than 100 °C for 1.05 and 1.06 nm diameter nanotubes and below 0 °C for 1.24 and 1.44 nm diameter nanotubes.

Copper-substituted Na0.67Ni0.3−xCuxMn0.7O2 cathode materials for sodium-ion batteries with suppressed P2–O2 phase transition

Abstract: P2-type sodium layered oxides NaxMO2 (M = transition metal) are considered as one kind of promising cathode material for sodium-ion batteries because of their known structures, superior electrochemical properties, and their ease of synthesis. The Ni2+/Ni3+ and Ni3+/Ni4+ redox reactions endow the P2–Na2/3Ni1/3Mn2/3O2 electrode with a relatively high operating voltage and high specific capacity. However, the phase transition from P2 to O2 and Na+/vacancy ordering make P2–Na2/3Ni1/3Mn2/3O2 susceptible to severe voltage and capacity decay. Herein, we propose to employ the electrochemically active copper(II) as a unique substituent to stabilize the P2 phase, forming Na0.67Ni0.3−xCuxMn0.7O2 (x = 0, 0.1, 0.2 and 0.3). Our work highlights the importance of Cu(II) in the structural engineering of high performance cathode materials, whose existence can not only stabilize the P2 phase against the notorious phase transition, but also contribute to the rechargeable capacity due to the high potential Cu2+/Cu3+ redox. We identified that the cathode formulated as P2-type Na0.67Ni0.1Cu0.2Mn0.7O2 shows favorable battery performance with much-alleviated structural degradation.

Fe3O4-functionalized graphene nanosheet embedded phase change material composites: efficient magnetic- and sunlight-driven energy conversion and storage

Abstract: As an important energy utilization mode, thermal energy is closely related to human life and social production. Phase change materials have been widely adopted to store thermal energy to improve its utilization efficiency. However, the inherent low energy conversion ability of these materials is one of the key problems to be resolved urgently. In this paper, we report novel magnetic- and sunlight-driven energy conversion and storage nanocomposites based on Fe3O4-functionalized graphene nanosheet (Fe3O4–GNS) embedded form-stable polymer phase change materials. Owing to the excellent magnetocaloric performance of Fe3O4 and the universal photoabsorption and photothermal conversion of graphene, the nanocomposites can effectively convert magnetic or light energy into thermal energy under an alternating magnetic field or solar illumination. The energy is stored by phase change materials during the phase transition process. The obtained hybrid nanocomposites exhibit excellent thermal stability with high melting–freezing enthalpy and excellent reversibility. Furthermore, the novel nanocomposites show the characteristics of form-stable phase transformation. The Fe3O4–GNS embedded phase change material composites for energy conversion and storage are expected to open up a rich field of energy materials.

High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi

Abstract: High-entropy alloys, near-equiatomic solid solutions of five or more elements, represent a new strategy for the design of materials with properties superior to those of conventional alloys. However, their phase space remains constrained, with transition metal high-entropy alloys exhibiting only face- or body-centered cubic structures. Here, we report the high-pressure synthesis of a hexagonal close-packed phase of the prototypical high-entropy alloy CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed to suppression of the local magnetic moments, destabilizing the initial fcc structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the transformation is sluggish, occurring over a range of >40 GPa. However, the behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This demonstrates a means of tuning the structures and properties of high-entropy alloys in a manner not achievable by conventional processing techniques. High-entropy alloys represent a new strategy for the design of materials with properties superior to those of conventional alloys, but are largely limited to simple phases of cubic symmetry. By applying high pressures on CrMnFeCoNi, here authors demonstrate synthesis of a hexagonal close-packed phase.

A comprehensive study on the structural evolution of HfO2 thin films doped with various dopants

Abstract: The origin of the unexpected ferroelectricity in doped HfO2 thin films is now considered to be the formation of a non-centrosymmetric Pca21 orthorhombic phase. Due to the polycrystalline nature of the films as well as their extremely small thickness (∼10 nm) and mixed orientation and phase composition, structural analysis of doped HfO2 thin films remains a challenging task. As a further complication, the structural similarities of the orthorhombic and tetragonal phase are difficult to distinguish by typical structural analysis techniques such as X-ray diffraction. To resolve this issue, the changes in the grazing incidence X-ray diffraction (GIXRD) patterns of HfO2 films doped with Si, Al, and Gd are systematically examined. For all dopants, the shift of o111/t101 diffraction peak is observed with increasing atomic layer deposition (ALD) cycle ratio, and this shift is thought to originate from the orthorhombic to P42/nmc tetragonal phase transition with decreasing aspect ratio (2a/(b + c) for orthorhombic and c/a for the tetragonal phase). For quantitative phase analysis, Rietveld refinement is applied to the GIXRD patterns. A progressive phase transition from P21/c monoclinic to orthorhombic to tetragonal is confirmed for all dopants, and a strong relationship between orthorhombic phase fraction and remanent polarization value is uniquely demonstrated. The concentration range for the ferroelectric properties was the narrowest for the Si-doped HfO2 films. The dopant size is believed to strongly affect the concentration range for the ferroelectric phase stabilization, since small dopants can strongly decrease the free energy of the tetragonal phase due to their shorter metal–oxygen bonds.

Decoding the phase structure of QCD via particle production at high energy

Abstract: Recent studies based on non-perturbative lattice Monte-Carlo solutions of Quantum Chromodynamics, the theory of strong interactions, demonstrated that at high temperature there is a phase change from confined hadronic matter to a deconfined quark-gluon plasma where quarks and gluons can travel distances largely exceeding the size of hadrons. The phase structure of such strongly interacting matter can be decoded via analysis of particle abundances in high energy nuclear collisions within the framework of the statistical hadronization approach. The results imply quark-hadron duality at and experimental delineation of the location of the phase boundary of strongly interacting matter.

Probing slow relaxation and many-body localization in two-dimensional quasiperiodic systems

Abstract: While many-body localization is well understood in one-dimensional systems, its behavior in two or more dimensions is largely unknown. New experiments hint at a many-body localized phase in a two-dimensional system and provide insight into how a system transitions between this phase and a normal thermal phase.

Self-Optimization of the Active Site of Molybdenum Disulfide by an Irreversible Phase Transition during Photocatalytic Hydrogen Evolution

Abstract: The metallic 1T-MoS2 has attracted considerable attention as an effective catalyst for hydrogen evolution reactions (HERs). However, the fundamental mechanism about the catalytic activity of 1T-MoS2 and the associated phase evolution remain elusive and controversial. Herein, we prepared the most stable 1T-MoS2 by hydrothermal exfoliation of MoS2 nanosheets vertically rooted into rigid one-dimensional TiO2 nanofibers. The 1T-MoS2 can keep highly stable over one year, presenting an ideal model system for investigating the HER catalytic activities as a function of the phase evolution. Both experimental studies and theoretical calculations suggest that 1T phase can be irreversibly transformed into a more active 1T' phase as true active sites in photocatalytic HERs, resulting in a "catalytic site self-optimization". Hydrogen atom adsorption is the major driving force for this phase transition.

Structure of a model TiO2 photocatalytic interface

Abstract: The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.

Stabilization of the Metastable Lead Iodide Perovskite Phase via Surface Functionalization

Abstract: Metastable structural polymorphs can have superior properties and applications to their thermodynamically stable phases, but the rational synthesis of metastable phases is a challenge. Here, a new strategy for stabilizing metastable phases using surface functionalization is demonstrated using the example of formamidinium lead iodide (FAPbI3) perovskite, which is metastable at room temperature (RT) but holds great promises in solar and light-emitting applications. We show that, through surface ligand functionalization during direct solution growth at RT, pure FAPbI3 in the cubic perovskite phase can be stabilized in nanostructures and thin films at RT without cation or anion alloying. Surface characterizations reveal that long-chain alkyl or aromatic ammonium (LA) cations bind to the surface of perovskite structure. Calculations show that such functionalization reduces the surface energy and plays a dominant role in stabilizing the metastable perovskite phase. Excellent photophysics and optically pumped la...

Quantitative Phase-Change Thermodynamics and Metastability of Perovskite-Phase Cesium Lead Iodide

Abstract: The perovskite phase of cesium lead iodide (α-CsPbI3 or “black” phase) possesses favorable optoelectronic properties for photovoltaic applications. However, the stable phase at room temperature is a nonfunctional “yellow” phase (δ-CsPbI3). Black-phase polycrystalline thin films are synthesized above 330 °C and rapidly quenched to room temperature, retaining their phase in a metastable state. Using differential scanning calorimetry, it is shown herein that the metastable state is maintained in the absence of moisture, up to a temperature of 100 °C, and a reversible phase-change enthalpy of 14.2 (±0.5) kJ/mol is observed. The presence of atmospheric moisture hastens the black-to-yellow conversion kinetics without significantly changing the enthalpy of the transition, indicating a catalytic effect, rather than a change in equilibrium due to water adduct formation. These results delineate the conditions for trapping the desired phase and highlight the significant magnitude of the entropic stabilization of thi...

Computation of Dendrites Using a Phase Field Model

Abstract: A phase field model is used to numerically simulate the solidification of a pure material. We employ it to compute growth into an undercooled liquid for a one-dimensional spherically symmetric geometry and a planar two-dimensional rectangular region. The phase field model equation are solved using finite difference techniques on a uniform mesh. For the growth of a sphere, the solutions from the phase field equations for sufficiently small interface widths are in good agreement with a numerical solution to the classical sharp interface model obtained using a Green's function approach. In two dimensions, we simulate dendritic growth of nickel with four-fold anisotropy and investigate the effect of the level of anisotropy on the growth of a dendrite. The quantitative behavior of the phase field model is evaluated for varying interface thickness and spatial and temporal resolution. We find quantitatively that the results depend on the interface thickness and with the simple numerical scheme employed it is not practical to do computations with an interface that is sufficiently thin for the numerical solution to accurately represent a sharp interface model. However, even with a relatively thick interface the results from the phase field model show many of the features of dendritic growth and they are in surprisingly good quantitative agreement with the Ivantsov solution and microscopic solvability theory.

Directly Probing Charge Separation at Interface of TiO2 Phase Junction.

Abstract: Phase junction is often recognized as an effective strategy to achieve efficient charge separation in photocatalysis and photochemistry. As a crucial factor to determine the photogenerated charges dynamics, there is an increasingly hot debate about the energy band alignment across the interface of phase junction. Herein, we reported the direct measurement of the surface potential profile over the interface of TiO2 phase junction. A built-in electric field up to 1 kV/cm from rutile to anatase nanoparticle was detected by Kelvin Probe Force Microscopy (KPFM). Home-built spatially resolved surface photovoltage spectroscopy (SRSPS) supplies a direct evidence for the vectorial charge transfer of photogenerated electrons from rutile to anatase. Moreover, the tunable anatase nanoparticle sizes in TiO2 phase junction leads to high surface photovoltage (SPV) by creating completely depleted space charge region (SCR) and enhancing the charge separation efficiency. The results provide a strong basis for understanding...

Understanding the Cubic Phase Stabilization and Crystallization Kinetics in Mixed Cations and Halides Perovskite Single Crystals

Abstract: The spontaneous α-to-δ phase transition of the formamidinium-based (FA) lead halide perovskite hinders its large scale application in solar cells. Though this phase transition can be inhibited by alloying with methylammonium-based (MA) perovskite, the underlying mechanism is largely unexplored. In this Communication, we grow high-quality mixed cations and halides perovskite single crystals (FAPbI3)1–x(MAPbBr3)x to understand the principles for maintaining pure perovskite phase, which is essential to device optimization. We demonstrate that the best composition for a perfect α-phase perovskite without segregation is x = 0.1–0.15, and such a mixed perovskite exhibits carrier lifetime as long as 11.0 μs, which is over 20 times of that of FAPbI3 single crystal. Powder XRD, single crystal XRD and FT-IR results reveal that the incorporation of MA+ is critical for tuning the effective Goldschmidt tolerance factor toward the ideal value of 1 and lowering the Gibbs free energy via unit cell contraction and cation ...

Chemical Vapor Deposition Growth of Few-Layer MoTe2 in the 2H, 1T′, and 1T Phases: Tunable Properties of MoTe2 Films

Abstract: Chemical vapor deposition allows the preparation of few-layer films of MoTe2 in three distinct structural phases depending on the growth quench temperature: 2H, 1T′, and 1T. We present experimental and computed Raman spectra for each of the phases and utilize transport measurements to explore the properties of the 1T MoTe2 phase. Density functional theory modeling predicts a (semi-)metallic character. Our experimental 1T films affirm the former, show facile μA-scale source-drain currents, and increase in conductivity with temperature, different from the 1T′ phase. Variation of the growth method allows the formation of hybrid films of mixed phases that exhibit susceptibility to gating and significantly increased conductivity.

A classical view on nonclassical nucleation

Abstract: Understanding and controlling nucleation is important for many crystallization applications. Calcium carbonate (CaCO3) is often used as a model system to investigate nucleation mechanisms. Despite its great importance in geology, biology, and many industrial applications, CaCO3 nucleation is still a topic of intense discussion, with new pathways for its growth from ions in solution proposed in recent years. These new pathways include the so-called nonclassical nucleation mechanism via the assembly of thermodynamically stable prenucleation clusters, as well as the formation of a dense liquid precursor phase via liquid–liquid phase separation. Here, we present results from a combined experimental and computational investigation on the precipitation of CaCO3 in dilute aqueous solutions. We propose that a dense liquid phase (containing 4–7 H2O per CaCO3 unit) forms in supersaturated solutions through the association of ions and ion pairs without significant participation of larger ion clusters. This liquid acts as the precursor for the formation of solid CaCO3 in the form of vaterite, which grows via a net transfer of ions from solution according to z Ca2+ + z CO32− → z CaCO3. The results show that all steps in this process can be explained according to classical concepts of crystal nucleation and growth, and that long-standing physical concepts of nucleation can describe multistep, multiphase growth mechanisms.

Phase coexistence and electric-field control of toroidal order in oxide superlattices.

Abstract: Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO3/SrTiO3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a1/a2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Our findings suggest new cross-coupled functionalities.