Ferroelastic Twin Walls Research Articles

Strain coupling, microstructure dynamics, and acoustic mode softening in germanium telluride

GeTe is a material of intense topical interest due to its potential in the context of phase-change and nanowire memory devices, as a base for thermoelectric materials and as a ferroelectric. The combination of a soft optic mode and a Peierls distortion contributes large strains at the cubic - rhombohedral phase transition near 625 K and the role of these has been investigated through their influence on elastic and anelastic properties by resonant ultrasound spectroscopy. The underlying physics is revealed by softening of the elastic constants by ~30-45%, due to strong coupling of shear and volume strains with the driving order parameter and consistent with an improper ferroelastic transition which is weakly first order. The magnitude of the softening is permissive of the transition mechanism involving a significant order/disorder component. A Debye loss peak in the vicinity of 180 K is attributed to freezing of the motion of ferroelastic twin walls and the activation energy of ~0.07 eV is attributed to control by switching of the configuration of long and short Ge-Te bonds in the first coordination sphere around Ge. Precursor softening as the transition is approached from above can be described with a Vogel-Fulcher expression with a similar activation energy, which is attributed to coupling of acoustic modes with an unseen central mode that arises from dynamical clusters with local ordering of the Peierls distortion. The strain relaxation and ferroelastic behaviour of GeTe depend on both displacive and order/disorder effects but the dynamics of switching will be determined by changes in the configuration of distorted GeTe6 octahedra, with a rather small activation energy barrier.

Magnetostructural coupling behavior at the ferromagnetic transition in double-perovskiteSr2FeMoO6

The ordered double-perovskite $\mathrm{S}{\mathrm{r}}_{2}\mathrm{FeMo}{\mathrm{O}}_{6}$ (SFMO) possesses remarkable room-temperature low-field colossal magnetoresistivity and transport properties which are related, at least in part, to combined structural and magnetic instabilities that are responsible for a cubic-tetragonal phase transition near 420 K. A formal strain analysis combined with measurements of elastic properties from resonant ultrasound spectroscopy reveal a system with weak biquadratic coupling between two order parameters belonging to ${\mathrm{\ensuremath{\Gamma}}}_{4}^{+}$ and $m{\mathrm{\ensuremath{\Gamma}}}_{4}^{+}$ of parent space group $Fm\overline{3}m$. The observed softening of the shear modulus by \ensuremath{\sim}50% is due to the classical effects of strain/order parameter coupling at an improper ferroelastic $({\mathrm{\ensuremath{\Gamma}}}_{4}^{+})$ transition which is second order in character, while the ferromagnetic order parameter $(m{\mathrm{\ensuremath{\Gamma}}}_{4}^{+})$ couples only with volume strain. The influence of a third order parameter, for ordering of Fe and Mo on crystallographic B sites, is to change the strength of coupling between the ${\mathrm{\ensuremath{\Gamma}}}_{4}^{+}$ order parameter and the tetragonal shear strain due to the influence of changes in local strain heterogeneity at a unit cell scale. High anelastic loss below the transition point reveals the presence of mobile ferroelastic twin walls which become pinned by oxygen vacancies in a temperature interval near 340 K. The twin walls must be both ferroelastic and ferromagnetic, but due to the weak coupling between the magnetic and structural order parameters it should be possible to pull them apart with a weak magnetic field. These insights into the role of strain coupling and relaxational effects in a system with only weak coupling between three order parameters allow rationalization and prediction of how static and dynamic properties of the material might be tuned in thin film form by choice of strain contrast with a substrate.

Ferroelastic twin structures in epitaxial WO3 thin films

Tungsten trioxide is a binary oxide that has potential applications in electrochromic windows, gas sensors, photo-catalysts, and superconductivity. Here, we analyze the crystal structure of atomically flat epitaxial layers on YAlO3 single crystal substrates and perform nanoscale investigations of the ferroelastic twins revealing a hierarchical structure at multiple length scales. We have found that the finest stripe ferroelastic twin walls along pseudocubic 〈100〉 axes are associated with cooperative mosaic rotations of the monoclinic films and the larger stripe domains along pseudocubic 〈110〉 axes are created to reduce the misfit strain through a commensurate matching of an effective in-plane lattice parameter between film and substrate. The typical widths of the two fine and larger stripe domains increase with film thickness following a power law with scaling exponents of ∼0.6 and ∼0.4, respectively. We have also found that the twin structure can be readily influenced by illumination with an electron beam or a tip-based mechanical compression.

Magnetoelastic relaxations in EuTiO3

The multiferroic properties of EuTiO3 are greatly enhanced when a sample is strained, signifying that coupling between strain and structural, magnetic or ferroelectric order parameters is extremely important. Here resonant ultrasound spectroscopy has been used to investigate strain coupling effects, as well as possible additional phase transitions, through their influence on elastic and anelastic relaxations that occur as a function of temperature between 2 and 300 K and with applied magnetic field up to 14 T. Antiferromagnetic ordering is accompanied by acoustic loss and softening, and a weak magnetoelastic effect is also associated with the change in magnetization direction below . Changes in loss due to the influence of magnetic field suggest the existence of magnetic defects which couple with strain and may play a role in pinning of ferroelastic twin walls.

Functional twin boundaries and tweed microstructures: a comparison between minerals and device materials

AbstractIn ferroelastic materials, the existence of degenerate strain states leads to the formation of nanoscale microstructures, such as domain boundaries (twin walls) and tweed. As the symmetry properties of microstructures differ from those of the bulk, they may dramatically change the macroscopic properties of a crystal. In addition, they are likely to have functional properties (ferroelecricity, piezoelectricity, magnetism, conductivity and rapid chemical transport) that are absent in the bulk. The existence of functional properties of twin walls, along with the advances in nano-scale characterization, has opened the door to domain boundary engineering, which aims to use domain boundaries as active elements in device materials. Hence, this relatively new field puts ferroelastic twin walls and possibly tweed at the heart of future electronic devices. Ferroelasticity is very common among minerals. Similar to manmade materials, the same crystallographic principles apply, which means that there are many minerals that await discovery for their functional properties. Thus, this review aims to raise attention to the discovery of minerals with functional microstructures. The current development of functional twin boundaries and tweed structures in physics and materials sciences is compared with the traditional observation of such structures in minerals. With an emphasis on chemical transport and piezoelectric/ ferroelectric behaviour, examples of functional microstructures are given from both man-made materials and minerals in addition to a discussion of the origin of polar twin walls and the introduction of a recent experimental technique, resonant piezoelectric spectroscopy (RPS), for their discovery.

Elastic and anelastic relaxations associated with phase transitions inEuTiO3

Elastic and anelastic properties of single crystal samples of EuTiO3 have been measured between 10 and 300 K by resonant ultrasound spectroscopy at frequencies in the vicinity of 1 MHz. Softening of the shear elastic constants C44 and 1 (C11 − C12 )b y∼20–30% occurs with falling temperature in a narrow interval through the transition point, Tc = 284 K, for the cubic-tetragonal transition. This is accounted for by classical coupling of macroscopic spontaneous strains with the tilt order parameter in the same manner as occurs in SrTiO3. A peak in the acoustic loss occurs a few degrees below Tc and is interpreted in terms of initially mobile ferroelastic twin walls, which rapidly become pinned with further lowering of temperature. This contrasts with the properties of twin walls in SrTiO3, which remain mobile down to at least 15 K. No further anomalies were observed that might be indicative of strain coupling to any additional phase transitions above 10 K. A slight anomaly in the shear elastic constants, independent of frequency and without any associated acoustic loss, was found at ∼140 K. It marks a change from elastic stiffening to softening with falling temperature and perhaps provides evidence for coupling between strain and local fluctuations of dipoles related to the incipient ferroelectric transition. An increase in acoustic loss below ∼80 K is attributed to the development of dynamical magnetic clustering ahead of the known antiferromagnetic ordering transition at ∼5.5 K. Detection of these elastic anomalies serves to emphasize that coupling of strain with tilting, ferroelectric, and magnetic order parameters is likely to be a permeating influence in determining the structure, stability, properties, and behavior of EuTiO3.

Nano‐Domain Pinning in Ferroelastic‐Ferroelectrics by Extended Structural Defects

Most ferroelectrics are also ferroelastics (hysteretic stress‐strain relationship and response to mechanical stresses). The interactions between ferroelastic twin walls and ferroelectric domain walls are complex and only partly understood, hindering the technological potential of these materials. Here we study via atomic force microscopy the pinning of 180‐degree ferroelectric domain walls in lead zirconate titanate (PZT). Our observations satisfy all three categories of ferroelectric‐ferroelastic domain interaction proposed by Bornarel, Lajzerowicz, and Legrand.

Domains within Domains and Walls within Walls: Evidence for Polar Domains in CryogenicSrTiO3

Resonant piezoelectric spectroscopy shows polar resonances in paraelectric SrTiO3 at temperatures below 80 K. These resonances become strong at T<40 K. The resonances are induced by weak electric fields and lead to standing mechanical waves in the sample. This piezoelectric response does not exist in paraelastic SrTiO3 nor at temperatures just below the ferroelastic phase transition. The interpretation of the resonances is related to ferroelastic twin walls which become polar at low temperatures in close analogy with the known behavior of CaTiO3. SrTiO3 is different from CaTiO3, however, because the wall polarity is thermally induced; i.e., there exists a small temperature range well below the ferroelastic transition point at 105 K where polarity appears on cooling. As the walls are atomistically thin, this transition has the hallmarks of a two-dimensional phase transition restrained to the twin boundaries rather than a classic bulk phase transition.

Ferroelastic aspects of relaxor ferroelectric behaviour in Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 perovskite

Elastic and anelastic properties of poled and depoled single crystals of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 with compositions close to the morphotropic boundary have been investigated over the temperature range 5–700 K by resonant ultrasound spectroscopy (RUS) at frequencies of 0.1–1.2 MHz. Steep elastic softening occurs in a temperature interval of at least 250 K as the Vogel-Fulcher freezing interval and cubic → tetragonal transition point, Tc, are approached from above. This is understood in terms of coupling between acoustic modes and central peak mode(s) associated with dynamic polar nano regions (PNR's) below the Burns temperature. Acoustic losses occur in a temperature interval of ∼50 K above Tc, associated with slowing down of the PNR dynamics. The cubic ↔ tetragonal and tetragonal ↔ rhombohedral transitions are accompanied by steep minima in elastic properties, closely analogous to the pattern of softening and stiffening observed in sequences of improper ferroelastic transitions in other perovskites. Variations in the magnitudes of acoustic losses at T &amp;lt; Tc correlate with the density of ferroelastic twin walls, from lowest for [001]c-poled and [111]c-poled crystals in the stability fields of the tetragonal and rhombohedral phases, respectively, to highest for unpoled crystals. A simple model of Debye-like peaks in acoustic loss near 100 K has yielded activation energies and attempt frequencies in the same range as those observed from dielectric data in the Vogel-Fulcher freezing interval. These highlight the fact that, in addition to conventional ferroelectric/ferroelastic twin walls, relaxor ferroelectrics contain local structural heterogeneities coupled to strain, which are probably related to the presence of static PNR's preserved even in poled crystals. RUS also provides a convenient and effective means of determining the mechanical quality factor of relaxor ferroelectrics, as functions of both poling history and temperature.

Elastic and anelastic relaxations accompanying magnetic ordering and spin-flop transitions in hematite, Fe2O3

Hematite, Fe2O3, provides in principle a model system for multiferroic (ferromagnetic/ferroelastic) behavior at low levels of strain coupling. The elastic and anelastic behavior associated with magnetic phase transitions in a natural polycrystalline sample have therefore been studied by resonant ultrasound spectroscopy (RUS) in the temperature range from 11 to 1072 K. Small changes in softening and attenuation are interpreted in terms of weak but significant coupling of symmetry-breaking and non-symmetry-breaking strains with magnetic order parameters in the structural sequence . The transition at TN = 946 ± 1 K is an example of a multiferroic transition which has both ferromagnetic (from canting of antiferromagnetically ordered spin moments) and ferroelastic (rhombohedral → monoclinic) character. By analogy with the improper ferroelastic transition in Pb3(PO4)2, W and W′ ferroelastic twin walls which are also 60° and 120° magnetic domain walls should develop. These have been tentatively identified from microstructures reported in the literature. The very low attenuation in the stability field of the C2/c structure in the polycrystalline sample used in the present study, in comparison with the strong acoustic dissipation reported for single crystal samples, implies, however, that the individual grains each consist of a single ferroelastic domain or that the twin walls are strongly pinned by grain boundaries. This absence of attenuation allows an intrinsic loss mechanism associated with the transition point to be seen and interpreted in terms of local coupling of shear strains with fluctuations which have relaxation times in the vicinity of ∼10−8 s. The first order (Morin) transition occurs through a temperature interval of coexisting phases but the absence of an acoustic loss peak suggests that the relaxation time for interface motion is short in comparison with the time scale of the applied stress (at ∼0.1–1 MHz). Below the Morin transition a pattern of attenuation which resembles that seen below ferroelastic transitions has been found, even though the ideal low temperature structure cannot contain ferroelastic twins. This loss behavior is tentatively ascribed to the presence of local ferromagnetically ordered defect regions which are coupled locally to shear strains.

Ferrimagnetic/ferroelastic domain interactions in magnetite below the Verwey transition. Part I: electron holography and Lorentz microscopy

The crystallographic and magnetic microstructure of magnetite (Fe3O4) below the Verwey transition (∼120 K) is studied using transmission electron microscopy. The low temperature phase is found to have a monoclinic C-centered lattice with a c-glide plane perpendicular to the b-axis, which allows twin-related crystal orientations to be distinguished. Off-axis electron holography and Lorentz electron microscopy are used to show that magnetic domains present at room temperature become subdivided into sub-micron-sized magnetic domains below the Verwey transition, with the magnetization direction in each magnetic domain oriented along the monoclinic [001] axis. The nature of the interactions between the magnetic domain walls and the ferroelastic twin walls is investigated. Cooling and warming cycles through the transition temperature are used to show that a memory effect is likely to exist between the magnetic states that form above and below the transition. Our results suggest that ferroelastic twin walls have a strong influence on the low temperature magnetic properties of magnetite.

Ferrimagnetic/ferroelastic domain interactions in magnetite below the Verwey transition: Part II. Micromagnetic and image simulations

Micromagnetic simulations have been used to explore the interaction between ferrimagnetic domain walls (DWs) and ferroelastic twin walls (TWs) below the Verwey transition in magnetite (Fe3O4). Simulations were performed using a thin-foil geometry in order to replicate the domain patterns observed experimentally using transmission electron microscopy. The magnetic microstructure is shown to be highly sensitive to the physical dimensions and crystallographic orientation of the foil, the spatial distribution and crystallographic classification of the TWs and the temperature/field history of the sample. A method to calculate the phase shift of a beam of electrons passing through the micromagnetic simulations is applied. The resulting phase maps provide a robust interpretation of experimental images obtained using Fresnel-mode Lorentz microscopy and off-axis electron holography. The interaction between ferrimagnetic and ferroelastic DWs during field cycling provides an explanation for the low-temperature ‘field-memory effect’ in magnetite.

Domain Wall Damping and Elastic Softening inSrTiO3: Evidence for Polar Twin Walls

A marked change in anelastic properties, namely, elastic softening accompanied by increased damping, has been observed in a single crystal of SrTiO(3) below ~50 K by resonant ultrasound spectroscopy. This correlates with other subtle changes in structure and properties which have been explained in the past in terms of a novel quantum state and the formation of polar clusters in an incipient ferroelectric structure. Comparison of the new data, obtained at frequencies near 1 MHz, with mechanical spectroscopy data collected at a few Hz or a few kHz, reveals a distinct dispersion with frequency and is interpreted in terms of an acoustic loss mechanism which depends primarily on the mobility under stress of ferroelastic twin walls. In most ferroelastic materials, it is found that the twin walls become immobile below some low-temperature interval due to the pinning effects of defects. It is proposed instead for SrTiO(3) that associated with the local atomic displacements within the incipient ferroelectric clusters is a change in structure of the twin walls such that their mobility becomes enhanced. We propose that the structural change is not correlated with structural changes of the bulk material but relates to increasing polarity of the walls. This interpretation implies that ferroelastic domain walls in SrTiO(3) become ferroelectric at low temperatures.

Acoustic attenuation due to transformation twins in CaCl2: Analogue behaviour for stishovite

CaCl2 undergoes a tetragonal (P42/mnm) to orthorhombic (Pnnm) transition as a function of temperature which is essentially the same as occurs in stishovite at high pressures. It can therefore be used as a convenient analogue material for experimental studies. In order to investigate variations in elastic properties associated with the transition and possible anelastic loss behaviour related to the mobility of ferroelastic twin walls in the orthorhombic phase, the transition in polycrystalline CaCl2 has been examined using resonant ultrasound spectroscopy (RUS) at high frequencies (0.1–1.5MHz) in the temperature interval 7–626K, and dynamic mechanical analysis (DMA) at low frequencies (0.1–50Hz) in the temperature interval 378–771K. RUS data show steep softening of the shear modulus as the transition temperature is approached from above and substantial acoustic dissipation in the stability field of the orthorhombic structure. DMA data show softening of the storage modulus, which continues through to a minimum ∼20K below the transition point and is followed by stiffening with further lowering of temperature. There is no obvious acoustic dissipation associated with the transition, as measured by tanδ, however. The elastic softening and stiffening matches the pattern expected for a pseudoproper ferroelastic transition as predicted elsewhere. Acoustic loss behaviour at high frequencies fits with the pattern of behaviour expected for a twin wall loss mechanism but with relaxation times in the vicinity of ∼10−6s. With such short relaxation times, the shear modulus of CaCl2 at frequencies corresponding to seismic frequencies would include relaxations of the twin walls and is therefore likely to be significantly lower than the intrinsic shear modulus. If these characteristics apply also to twin wall mobility in stishovite, the seismic signature of the orthorhombic phase should be an unusually soft shear modulus but with no increase in attenuation.

Magnetoelastic coupling and multiferroic ferroelastic/magnetic phase transitions in the perovskite KMnF3

The magnetic, elastic, and anelastic behavior of single-crystal KMnF${}_{3}$ have been investigated by superconducting quantum interference device (SQUID) magnetometry and resonant ultrasound spectroscopy (RUS) through the sequence of phase transitions: phase I, $Pm$$\overline{3}$$m$ $\ensuremath{\rightarrow}\phantom{\rule{0.16em}{0ex}}({T}_{\mathrm{c}1}$ $=$ 185 K) \ensuremath{\rightarrow} phase II, $I$4/mcm $\ensuremath{\rightarrow}({T}_{\mathrm{c}2}$ $=\phantom{\rule{0.16em}{0ex}}{T}_{\mathrm{N}}$ $=$ 87 K) \ensuremath{\rightarrow} phase III, antiferromagnetic, Cmcm \ensuremath{\rightarrow} (${T}_{\mathrm{c}3}$ $=$ 82 K) \ensuremath{\rightarrow} phase IV, canted ferromagnet, Pnma. It is concluded that observed changes in the elastic properties can be explained simply in terms of strain/order parameter coupling for the octahedral tilting transitions. There appears to be no evidence in the present data or in data from the literature for coupling between the magnetic order parameter and shear strains. Any coupling between the magnetic and structural transitions is therefore weak, probably occurring only biquadratically through a small common volume strain. The combined data show unambiguously that, for the crystal used, the N\'eel point and the structural transition at 87 K are coincident. In other crystals, with slightly different stoichiometries and defect contents, this need not be the case, however, and the overlap of transition temperatures in KMnF${}_{3}$ is essentially accidental. Strong acoustic dissipation at \ensuremath{\sim}0.1--1 MHz in the stability field of phase II is attributed to the local mobility of transformation twin walls under externally applied stress. A Debye-like loss peak near 130 K is attributed to pinning of at least some twin walls by defects, but relatively high levels of acoustic dissipation below this freezing temperature imply that some of the twin walls remain mobile due to weak pinning or the absence of any pinning. Acoustic losses continue in the stability field of phase III (Cmcm) but diminish substantially in the stability field of phase IV (Pnma), implying quite different twin mobilities in the different structure types. Overlap of the structural and magnetic instabilities in KMnF${}_{3}$ opens up possibilities for manipulation of ferroelastic twinning by application of a magnetic field and for creation of materials in which the ferroelastic twin walls have quite different magnetic properties from the matrix in which they lie.

Interfacial polarization and pyroelectricity in antiferrodistortive structures induced by a flexoelectric effect and rotostriction

Theoretical analysis based on the Landau-Ginzburg-Devonshire (LGD) theory is used to show that the product effect of flexoelectricity and rotostriction can lead to a spontaneous polarization in the vicinity of antiphase boundaries, ferroelastic twin walls, surfaces and interfaces in the octahedrally tilted phase of otherwise non-ferroelectric perovskites such as CaTiO3, SrTiO3, and EuTiO3. As an example, we numerically demonstrate a large spontaneous polarization on the order of 1-5muC/cm2 at the SrTiO3 antidistortive phase boundaries at temperatures lower than the antiferrodistortive structural phase transition temperature of ~105 K

Anelasticity maps for acoustic dissipation associated with phase transitions in minerals

Acoustic dissipation due to structural phase transitions in minerals could give rise to large seismic attenuation effects superimposed on the high temperature background contribution from dislocations and grain boundaries in the Earth. In addition to the possibility of a sharp peak actually at a transition point for both compressional and shear waves, significant attenuation might arise over wider temperature intervals due to the mobility of transformation twins or other defects associated with the transition. Attenuation due to structural phase transitions in quartz, pyroxenes, perovskites, stishovite and hollandite, or to spin state transitions of Fe2+ in magnesiowustite and perovskite and the hcp/bcc transition in iron–nickel (Fe–Ni) alloy, are reviewed from this perspective. To these can be added possible loss behaviour associated with reconstructive transitions which might occur by a ledge mechanism on topotactic interfaces (orthopyroxene/clinopyroxene, olivine/spinel and perovskite/postperovskite), with impurities (Snoek effect) or with mobility of protons. There are experimental difficulties associated with measuring dissipation effects in situ at simultaneous high pressures and temperatures, so reliance is currently placed on investigation of analogue phases such as LaCoO3 for spin-state behaviour and LaAlO3 for the dynamics of ferroelastic twin walls. Similarly, it is not possible to measure loss dynamics simultaneously at the low stresses and low frequencies that pertain in seismic waves, so reliance must be placed on combining different techniques, such as dynamic mechanical analysis (low frequency, relatively high stress) and resonant ultrasound spectroscopy (high frequency, low stress), to extrapolate acoustic loss behaviour over wide frequency, temperature and stress intervals. In this context ‘anelasticity maps’ provide a convenient means of representing different loss mechanisms. Contouring of the inverse mechanical quality factor, Q−1, can be achieved if the appropriate constitutive laws are known. The overall approach is illustrated using the examples of spin-state transitions of Co3+ in LaCoO3 and twin mobility in single crystals of the rhombohedral phase of LaAlO3. Anelasticity maps of this type should give seismologists a clearer view of the characteristic patterns of seismic velocity and attenuation that could be used to detect (or rule out) the presence of particular phase transitions or loss behaviour in the core and mantle.

Microstructure dynamics in orthorhombic perovskites

Anelastic loss mechanisms associated with phase transitions in ${\text{BaCeO}}_{3}$ have been investigated at relatively high frequency $\ensuremath{\sim}1\text{ }\text{MHz}$ and low stress by resonant ultrasound spectroscopy (RUS), and at relatively low frequency $\ensuremath{\sim}1\text{ }\text{Hz}$ and high stress by dynamic mechanical analysis (DMA). Changes in the elastic moduli and dissipation behavior clearly indicate phase transitions due to octahedral tilting: $Pnma\ensuremath{\leftrightarrow}Imma\ensuremath{\leftrightarrow}R\overline{3}c\ensuremath{\leftrightarrow}Pm\overline{3}m$ structures at 551 K, 670 K, and 1168 K, and strain analysis shows that they are tricritical, first-order, and second-order phase transitions, respectively. Structures with intermediate tilt states ($R\overline{3}c$ and $Imma$ structures) show substantial anelastic softening and dissipation associated with the mobility of twin walls under applied stress. The $Pnma$ structure shows elastic stiffening which may be due to the simultaneous operation of two discrete order parameters with different symmetries. In contrast with studies of other perovskites, ${\text{BaCeO}}_{3}$ shows strong dissipation at both DMA and RUS frequencies in the stability field of the $Pnma$ structure. This is evidence that ferroelastic twin walls might become mobile in $Pnma$ perovskites and suggests that shearing of the octahedra may be a significant factor.

Trapping of oxygen vacancies in the twin walls of perovskite

The binding and pairwise interaction of oxygen vacancies in the ferroelastic (100) twin walls of the orthorhombic phase of ${\text{CaTiO}}_{3}$ perovskite $(Pbnm)$ have been investigated by numerical simulations using empirical force fields. An oxygen vacancy finds it energetically favorable to reside inside a twin wall, particularly when bridging two titanium ions located in the twin-wall plane. In such case, the binding energy of the vacancy to the wall is $0.7\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{eV}$, the error bar reflecting variability within two different force fields. A different disposition of the vacancy in the wall sees its binding energy reduced by a factor of 2. This implies that depending on the relative time scales for twin-wall motion and for oxygen-vacancy diffusion, anelastic motion of twin walls can display two different energy dissipation mechanisms associated to these point defects. The strongest interactions among oxygen vacancies in a twin wall are due to short-range repulsion so that no defect clusters were found. The vacancy-wall binding energy is found to increase substantially with pressure.

Effect of ferroelastic twin walls on local polarization switching: Phase-field modeling

Local polarization switching in epitaxial ferroelectric thin films in the presence of ferroelastic domain walls was studied using phase-field approach. The nucleation bias profile across a twin wall was analyzed, and the localization of preferential nucleation sites was established. This analysis was further extended to a realistic domain structure with multiple twin boundaries. It was observed that the local nucleation voltage required for a 180° domain switching is closely related to the number of such local defects.