Bestowing CMOS-compatible binary oxides with additional functionalities is a powerful strategy toward the realization of oxide electronics. Ideal candidates are thin films which display a strong sensitivity to strain, chemical doping or nanoscale confinement. Among these, crystalline tungsten trioxide WO3 exhibits exceptional structural flexibility, enabling a wide range of functionalities. Here, we reveal the emergence of a previously unreported polar phase in epitaxial WO3 thin films. We accomplish this by imposing epitaxial shear strain, which stabilizes a low-symmetry triclinic structure that persists up to large film thicknesses and elevated temperatures. At the atomic scale, a change in the oxygen octahedral tilt pattern facilitates this symmetry lowering into a polar phase, which manifests as a periodic in-plane polarized stripe domain configuration with needle-like bifurcations at the microscale. The stripe domain walls further exhibit a strongly enhanced electrical conductivity in conjunction with a pronounced reduction of a distortive structural mode, providing the first experimental evidence for the formation of anti-distortive polarons recently predicted in WO3.

Molecular dynamics simulations of the well-known ferroelastic material, CaTiO3, reproduce the experimentally observed bulk and wall properties. We show that Landau–Ginzburg potentials are an excellent approach to estimate bulk properties like the phase diagram and the ferroelastic bulk deformation. Most importantly, the known ferroelastic wall properties are reproduced and predicted, when unknown. The main emerging property is the wall polarization Pwall, which is as large as some of the best ferroelectric bulk materials. The temperature dependence of Pwall shows a near-linear decay from room temperature to the transition point. We argue that the most promising experimental technique to test our results is the measurement of the second harmonic generation of domain walls over large temperature intervals.

Calcite, CaCO3, has been reported to exist in as many as seven different structural forms. The structure at room temperature and pressure (space group R 3 c, ‘Phase I’) was established by Bragg many years ago. A phase transition to a higher temperature phase (space group R 3 m, ‘Phase V’) was noted to occur at around 1240 K—this may proceed via an intermediate phase (space group again R 3 c, referred to as ‘Phase IV’). These phases differ primarily in the disposition of the CO3 groups. Additional phases are found at higher pressures. We report a para-phase (parent phase, virtual prototype, aristotype) which assists in understanding the different phases, the phase transitions, and especially the domain structures and twin wall boundaries associated with these transitions. Molecular dynamics methods were used to study the temperature evolution of an isothermal-isobaric (NPT) ensemble of some 384,000 atoms. These computations reproduced the features of the known structures in R 3 c and R 3 m and then, at higher temperature, revealed a structure of the sodium chloride type (space group Fm 3 m) in which the entities were the Ca2+ cation and the CO32− anion, this latter with effectively spherical symmetry. On this basis we have upon cooling a necessarily first order ferroelastic transition from cubic Fm 3 m to rhombohedral R 3 m, computed to occur at a simulated temperature of 1900 K, and a possibly continuous transition from the R 3 m to rhombohedral (on a doubled cell) R 3 c computed to occur at about 1525 K. The computations also allowed us to follow the domain structure and twin walls as a function of temperature, during both heating and cooling. The structure just below the R 3 m to R 3 c transition shows strong disorder in the orientation of the CO3 groups, and this may be what is sometimes referred to as Phase IV. The domain structure just below the cubic to rhombohedral transition shows twinning of typical ferroelastic character. The doubling of the cell below the R 3 m to rhombohedral (on a doubled cell) R 3 c leads to a more complicated twin pattern. Indeed, the different structures can be identified from patterns of twinning. Differences between domain structures obtained on heating and cooling indicate extensive thermal metastabilities.

Granular materials are ubiquitous in nature and commonly used in engineering. In geoscience, the movement of debris flow, the instability of slopes, and the slip of fault zones are all related to the movement of granular materials. The central issue in granular materials movement is its self-organizing intermittent behavior, which is called the avalanche process or crackling noise in physics and materials science. In this study, the influence of granular shape and hardness on avalanche characteristics has been investigated by continuous toroidal shear testing of granular materials and synchronous acoustic emission signal monitoring. Our results reveal that the energy distribution follows the form of P(E) E−εe−(E/Λ), with the exponent ε = 1.53 and Λ = 106 aJ. Omori aftershocks follow a power law with an exponent near unity for about four decades. The Omori sequence is identical for pre-shocks and aftershocks within experimental resolution, and this identity agrees with the previous epidemic-type aftershock sequence (ETAS) model prediction. The waiting times are power law distributed with 1 − υ = 0.9 and 2 + ξ = 2.1. The relative magnitudes between mainshock and the largest aftershock remain constant at 0.75. Changes of disk shapes (circles, octagons, and pentagons) show little influence on the avalanche characteristics. Variations in bead materials show the same exponent ε = 1.53 for polymethyl methacrylate and aluminum with excellent statistical relevance, and ε = 1.71 with a lower confidence level for soft nitrile rubber. Our experiments extract a fuller set of avalanche parameters with discs of different shapes and hardness. These results are particularly pertinent for research in geoscience, materials sciences, and physics where avalanches are commonly observed.

Timely detection of deformation mechanisms in metallic structural materials is essential for early-warning alerts on potential damages and fractures. Acoustic emission (AE) technologies are commonly used for this purpose due to their non-destructive nature. However, traditional methods often struggle with distinguishing AE signals associated with multiple co-existing deformation mechanisms. To address this challenge, we propose a knowledge-driven unsupervised learning approach. The novel method leverages a family of gradient-driven supervised base learners and integrates them with a knowledge-infused aggregate loss function, effectively transforming the approach into an unsupervised learning framework. Compared to existing methods, our approach excels in identifying co-existing deformation mechanisms associated with AE signals. Experiments on porous 316L stainless steel during tensile process show that the avalanche statistics of the identified dislocation and crack AE signals align closely with classical statistical methods and fracture theory. By integrating with the avalanche theory, our proposed approach can continuously monitor material deformation mechanisms in real-time and provide dynamic early failure warnings. Additionally, the framework demonstrates strong transferability in recognizing multiple co-existing deformation mechanisms in new materials, leveraging its unsupervised learning capability.

Ferroelastic materials (materials with switchable spontaneous strain) often are centrosymmetric, but their domain walls are always polar, as their internal strain gradients cause polarization via flexoelectricity. This polarization is generally not switchable by an external electric field, because reversing the domain wall polarity would require reversing the strain gradient, which in turn would require switching the spontaneous strain of the adjacent domains, destroying the domain wall in the process. However, domain wall polarization can also arise from biquadratic coupling between polar and non-polar order parameters (e.g. octahedral tilts in perovskites). Such coupling is independent of the sign of the polarization and thus allows switching between +P and -P. In this work, we seek to answer the question of whether the polarization of domain walls in ferroelastic perovskites is switchable, as per the symmetric biquadratic term, or non-switchable due to the unipolar flexoelectric bias. Using perovskite calcium titanate (CaTiO3) as a paradigm, molecular dynamics calculations indicate that high electric fields broaden the ferroelastic domain walls, thereby reducing flexoelectricity (as the domain wall strain gradient is inversely proportional to the wall width), eventually enabling switching. The polarization switching, however, is not ferroelectric-like with a simple hysteresis loop, but antiferroelectric-like with a double hysteresis loop. Ferroelastic domain walls thus behave as functional antiferroelectric elements, and also as nucleation points for a bulk phase transition to a polar state.

We study the dynamic elastic behavior of potassium thiocyanate (KSCN) in the temperature region of the order–disorder improper ferroelastic phase transition using dynamic mechanical analysis (0.05–40 Hz) and resonance ultrasound spectroscopy (100–600 kHz). The low frequency data show—in addition to the intrinsic phase transition anomalies—softening in a- and b-directions below Tc, which results from movements of ferroelastic domain walls under dynamic stress. In contrast to many other ferroelastic materials (LaAlO3, PbZrO3, SrTiO3, etc.), the domain wall motion in KSCN freezes already at a temperature below Tc–20 K. The corresponding increase of the domain wall relaxation time τDW with decreasing temperature can be well fitted by a Vogel–Fulcher law τDW=τ0exp[Ea/kB(T−TVF)] with τ0≈10−7 s, Ea≈0.035 eV, and TVF≈368 K, indicating domain glass behavior. The high frequency elastic moduli (∼f2) (100–600 kHz) do not show any precursor softening with decreasing temperature due to the large order parameter relaxation time τη. In contrast to the high frequency elastic moduli, the corresponding losses Q−1 exhibit peaks at Tc=415 K, which may be explained by order parameter fluctuations.

Principles behind antiferromagnetic spintronics often claim that an antiferromagnet (AF) cannot be manipulated by external magnetic fields and that domain walls (DWs) are highly mobile and do not suffer strong pinning while maintaining robustness during thermal fluctuations. Using dysprosium (Dy) as an example, we take the opposite view. The functionality of a helical AF in Dy uses two intrinsic properties of chiral DWs: their intrinsic magnetization and their freezing just below the N & eacute;el temperature. Experiments with Dy single crystal show that field cooling through the fluctuation regime converts a helical AF into a domain wall-dominated system with striking properties related with robustness of frozen chiral DWs. On the other hand, chiral DWs are easily controlled by external fields throughout the fluctuation regime.

Acoustic emission, AE, spectroscopy identifies dislocation avalanches and crack propagation in high-purity Cu with self-stabilized dislocation networks. These samples were produced by additive manufacturing where thermal gradients form cellular structures. These structures confine avalanches of dislocation movements (∼confined dislocations) with an energy exponent ε = 1.82 ± 0.04. Free dislocation movement follows force-integrated mean-field behavior with ε = 1.6 ± 0.02. Additional crack propagation under tension exhibits a critical failure exponent of ε = 1.45 ± 0.01. These three mechanisms combine to generate sample failure under tension. We use this example to demonstrate how different avalanche mechanisms can be disentangled in AE spectroscopy of additive manufactured metal and how the specific self-stabilized dislocation networks influence these avalanche dynamics.

For decades, SrTiO3 has been in the focus of research with seemingly never-ending new insights regarding its ground state properties, application potentials, its surface and interface properties, the superconducting state, the twin boundaries, domain functionalities, etc. Here, we focus on the already well-investigated lattice dynamics of STO and show that four different temperature regimes can be identified which dominate the elastic properties, the thermal conductivity, and the birefringence. These regimes are a low-temperature quantum fluctuation-dominated one, followed by an intermediate regime, a region of structural phase transition at ~105 K and its vicinity, and at high temperatures, a regime characterized by precursor and saturation effects. They can all be elucidated by lattice dynamical aspects. The relevant temperature dependences of the soft modes are discussed and their relationship to lattice polarizability is emphasized.

Kidney stones have a prevalence rate of > 10% in some countries. There has been a significant increase in surgery to treat kidney stones over the last 10 years, and it is crucial that such techniques are as effective as possible, while limiting complications. A selection of kidney stones with different chemical and structural properties were subjected to compression. Under compression, they emit acoustic signals called crackling noise. The variability of the crackling noise was surprisingly great comparing weddellite, cystine and uric acid stones. Two types of signals were found in all stones. At high energies of the emitted sound waves, we found avalanche behaviour, while all stones also showed signals of local, uncorrelated collapse. These two types of events are called 'wild' for avalanches and 'mild' for uncorrelated events. The key observation is that the crossover from mild to wild collapse events differs greatly between different stones. Weddellite showed brittle collapse, extremely low crossover energies (< 5 aJ) and wild avalanches over 6 orders of magnitude. In cystine and uric acid stones, the collapse was more complicated with a dominance of local "mild" breakings, although they all contained some stress-induced collective avalanches. Cystine stones had high crossover energies, typically [Formula: see text] 750 aJ, and a narrow window over which they showed wild avalanches. Uric acid stones gave moderate values of crossover energies, [Formula: see text] 200 aJ, and wild avalanche behaviour for [Formula: see text] 3 orders of magnitude. Further research extended to all stone types, and measurement of stone responses to different lithotripsy strategies, will assist in optimisation of settings of the laser and other lithotripsy devices to insight fragmentation by targeting the 'wild' avalanche regime.

Neuromorphic computation is based on memristors, which function equivalently to neurons in brain structures. These memristors can be made more efficient and tailored to neuromorphic devices by using ferroelastic domain boundaries as fast diffusion paths for ionic conduction, such as of oxygen, sodium, or lithium. In this paper, we show that the local memristor generates a second, unexpected feature, namely, weak magnetic fields that emerge from moving ferroelastic needle domains and vortices. The vortices appear near ferroelastic “junctions” that are common when the external stimulus is a combination of electric fields and structural phase transitions. Many ferroelastic materials show such phase transitions near room temperatures so that device applications display a “multiferroic” scenario where the memristor is driven electrically and read magnetically. Our computer simulation study of an elastic spring model suggests magnetic fields in the order of 10−7 T, which opens the way for a fundamentally new way of running neuromorphic devices. The magnetism in such devices emerges entirely from intrinsic displacement currents and not from any intrinsic magnetism of the material.

Calcite, a CaCO3 polymorph, is one of the most significant materials in nature. In addition to its fundamental role in the global carbon cycle, it fills an unparalleled range of applications, both as a natural biomineral and more directly engineered materials. One of the most notable characteristics of calcite is its tendency to form twins, the boundaries of which are expected to determine key attributes of the host mineral at mesoscopic scales. Using a classical molecular dynamics simulation performed with a thermodynamically consistent rigid-ion force field, we show that the walls between the two twin domains aligned along the hexagonal (101¯4) plane of calcite have some unexpected properties: (1) rather than monoatomic planes, these twin walls are composed of two layers with fundamentally different distortions, (2) atomic shifts within the twin walls create strong polarity while the bulk remains centrosymmetric, and (3) the temperature evolution of the structural order parameter is mostly, but not completely, related to the tilt and rotation of CO32− molecules. Molecular dynamics simulations using thermodynamically accurate interatomic potentials demonstrate that the temperature evolution of twin walls is different from that of the bulk material. We show that kinks in twin walls have a low energy and generate high structural disorder near the kink position. Our methods are applicable to other molecular systems and emphasize the dominance of twin walls in materials. Published by the American Physical Society 2024

The elastic interaction between kinks (and antikinks) within domain walls plays a pivotal role in shaping the domain structure, and their dynamics. In bulk materials, kinks interact as elastic monopoles, dependent on the distance between walls, and typically characterised by a rigid and straight domain configuration. In this work we investigate the evolution of the domain structure as the sample size decreases, by means of in-situ heating techniques on free-standing samples. A significant transformation is observed: domain walls exhibit pronounced curvature, accompanied by an increase in both domain wall and junction density. This transformation is attributed to the pronounced influence of kinks, inducing sample warping, where 'dipole dipole' interactions are dominant. Moreover, we experimentally identify a critical thickness range that delineates a cross-over between the monopolar and dipolar regimens and corroborated this by detailed atomic simulations. These findings are relevant for in-situ TEM studies and for the development of novel devices based on free-standing ferroic thin films and nanomaterials.

Calcite, a CaCO3 polymorph, is one of the most significant materials in nature. In addition to its fundamental role in the global carbon cycle, it fills an unparalleled range of applications, both as a natural biomineral and more directly engineered materials. One of the most notable characteristics of calcite is its tendency to form twins, the boundaries of which are expected to determine key attributes of the host mineral at mesoscopic scales. Using a classical molecular dynamics simulation performed with a thermodynamically consistent rigid-ion force field, we show that the walls between the two twin domains aligned along the hexagonal (1014) plane of calcite have some unexpected properties: (1) rather than monoatomic planes, these twin walls are composed of two layers with fundamentally different distortions, (2) atomic shifts within the twin walls create strong polarity while the bulk remains centrosymmetric, and (3) the temperature evolution of the structural order parameter is mostly, but not completely, related to the tilt and rotation of CO32-molecules. Molecular dynamics simulations using thermodynamically accurate interatomic potentials demonstrate that the temperature evolution of twin walls is different from that of the bulk material. We show that kinks in twin walls have a low energy and generate high structural disorder near the kink position. Our methods are applicable to other molecular systems and emphasize the dominance of twin walls in materials.

The possibility to use ferroelastic materials as components of neuromorphic devices is discussed. They can be used as local memristors with the advantage that ionic transport is constraint to twin boundaries where ionic diffusion is much faster than in the bulk and does not leak into adjacent domains. It is shown that nano-scale ferroelastic memristors can contain a multitude of domain walls. These domain walls interact by strain fields where the interactions near surfaces are fundamentally different from bulk materials. We show that surface relaxations (∼image forces) are curtailed to short range dipolar interactions which decay as 1/d2 where d is the distance between domain walls. In bigger samples such interactions are long ranging with 1/d. The cross-over regime is typically in the range of some 200–1500 nm using a simple spring interaction model.

We investigate perovskite oxides from different perspectives, namely their pseudo-harmonic dynamical properties, their dynamical properties when strong anharmonicity exists, and the intriguing functionalities arising from domain walls. Taking these viewpoints together yields a rather complex picture of this material class, which has not been found in previous approaches. It opens pathways to novel applications and reveals the rich ground states beyond the fictitious belief in the ‘simplicity of perovskites and such structures’.

Precursor elastic effects are investigated in a displacive anharmonic spring model and shown to extend greatly into the paraelastic phase. Weak precursor effects can be detected near 2T(tr), where T-tr is the ferroelastic transition temperature. The precursor effects become strong at T<1.7T(tr.) Two effects were identified in our two-dimensional model: the symmetry-breaking strain e(3) (epsilon(xy)) leads to softening of the elastic modulus C-33, while the nonsymmetry-breaking strain e(1)+e(2) (epsilon(xx)+epsilon(yy)) leads to hardening of C11. The strain e(3 )is proportional to the order parameter and scales as |e(1)+e(2)| similar to e(23). The temperature evolutions of the elastic moduli are surprisingly well described by power laws and Vogel-Fulcher equations. The power-law exponents are similar to-0.5 for Delta C-33 and similar to-1 for Delta C-11, Delta(C-11+C-12) and Delta(C-11-C-12). The Vogel-Fulcher temperatures are very similar, while the Vogel-Fulcher energies differ between the excess elastic moduli. The origin of the precursor effect is the evolution of short-range order in the paraelastic phase which gives rise to a characteristic local nanostructure. In the case of the symmetry-breaking strain, this microstructure resembles dynamical twinning patterns corresponding to the ferroelastic nanostructure, which weakens the material. In the case of the nonsymmetry-breaking strain, we find density fluctuations which make the material harder.

Domain walls have distinct properties from the bulk, and tailoring them to suit the needs for device applications is critical. Tungsten trioxide, WO3, is of great interest for device applications that make use of domain wall properties; it exhibits a phenomenologically rich sequence of phase transitions, virtually all of which are ferroelastic in character, resulting in many sets of domain walls at low temperatures, each with their own unique properties. Domain wall motion and its contribution to the piezoelectric response have been investigated in WO3 from 300 to 180 K using resonant ultrasound spectroscopy (RUS) and resonant piezoelectric spectroscopy (RPS), which showed that the 2(1)/,1, and 2(1)/ phases give a piezoelectric response despite the bulk being nominally centrosymmetric. Second harmonic generation (SHG) confirmed that polarity was strongest within the domain walls, and additional weak signals were found in the domains. Domain wall mobility was investigated in the , 2(1)/, and 2(1)/ phases from 685 to 5 K. Domain walls in the 2(1)/ and 1 were more mobile than those in the and 2(1)/ structures, and soon after the 1 -> 2(1)/ transition the walls become pinned at similar to 140K.

The filiform corrosion of Mg in NaCl solution was characterized by acoustic emission spectroscopy, measurements of the free-corrosion potential, and optical observations under in situ conditions. Three stages of corrosion were identified: an incubation period, localized attack, and filament propagation. The corrosion mechanisms include the Mg dissolution, the breakdown of hydroxide/oxide surface films, and the burst of gas bubbles. Localized attack and filament propagation are distinguished by a gradual transition from correlated avalanches (wild events) to predominantly stochastic processes (mild events), which still contain some weak avalanche signals. Our approach provides a way to monitor corrosion under in situ conditions.