The relaxation volume of point defects is known to be an important parameter for diffusion processes and interactions between defects in crystals. In this study, we investigate the effect of high pressure on the formation enthalpy and relaxation volume of point defects in magnesium oxide using first-principles calculations. Although energy corrections in charged systems are by now well established, the reliable evaluation of stress and pressure in charged periodic systems necessitates specific corrections to address the ill-defined electrostatic potential in first-principles codes. In this study, we extend a methodology based on the determination of the so-called hydrostatic absolute deformation potential applicable at high pressure. We show that, at ambient pressure, oxygen vacancies are associated with a positive relaxation volume, whereas the relaxation volume of magnesium vacancies is negative. With increasing pressure, all vacancy relaxation volumes decrease and become negative above 150 GPa. In addition, we demonstrate that the stability field of charged vacancies exhibits a pressure-dependent evolution, which correlates with the reduction of ionicity in MgO under pressure.

Although stress-induced amorphization has recently been identified as a new and distinctive plasticity mechanism, a formal framework for its systematic application in materials engineering is still lacking. In this study, we establish that stress-induced amorphization in Mg2SiO4 forsterite exhibits all the key features of phase transformation plasticity, a well-known phenomenon that has enabled the development of high-performance materials. By analogy with the Transformation-Induced Plasticity (TRIP) effect, we introduce the concept of amorphous-TRIP (a-TRIP). We demonstrate that the a-TRIP features, in particular the orientation effect of stress-induced amorphization (particularly strong in forsterite), can be exploited to design materials with enhanced toughness, a notable advance for a class of materials traditionally limited by brittle behavior. Stress-induced amorphization has been observed in some systems as a plasticity mechanism. Here, simulations and microscopy reveal that stress-induced amorphization in forsterite exhibits the features of phase transformation plasticity.

Abstract Understanding the rheology of the Earth’s lower mantle is essential for modelling mantle convection and its consequences for the geodynamical, thermal and chemical evolution of the planet. However, the extreme pressure–temperature conditions, the long natural timescales, and the complexity of deep-mantle mineralogy make it difficult to constrain mantle viscosity from experiments alone. In this review, we examine an alternative approach based on mineral physics and multiscale numerical modelling, tracing its theoretical foundations and its development over the last decades. We first outline the key physical mechanisms governing plastic deformation in lower-mantle minerals—point defects, dislocations, diffusion, and grain-boundary processes—and the conditions under which they operate. We then summarize the major experimental advances that provide essential constraints for these models. Building on this foundation, we review the evolution of multiscale modelling strategies, from first-principles calculations of defect energetics and dislocation cores to mesoscale dislocation dynamics and polycrystal-scale formulations. Using ferropericlase and bridgmanite as archetypal examples, we show how successive developments have progressively linked atomic-scale mechanisms with macroscopic rheology under deep-mantle conditions. Finally, we discuss current limitations and outstanding challenges, including the scarcity of diffusion data at high pressures, uncertainties in defect chemistry, and the need to integrate multiphase and multimechanism deformation. Together, these elements provide a coherent view of the present capabilities of mineral-physics-based rheology and outline a roadmap for future efforts to improve our understanding of deep-mantle dynamics.

Stress-induced amorphization has recently attracted attention as a potential deformation mechanism in ceramics, semi-conductors or minerals. Its activation is promoted when conventional plasticity, e.g. driven by dislocations, is inhibited. However, the mechanisms underlying this phenomenon are still unclear. Based on quantitative in situ TEM tensile testing of small-sized olivine bi-crystals, we demonstrate that stress-induced amorphization and grain boundary sliding can be activated under high stresses at room temperature in specimens with high angle grain boundaries. Low angle grain boundaries are less prone to this phenomenon. Varying the iron content in olivine demonstrates that iron inhibits amorphization and, consequently, promotes brittle failure. This contrast with the accepted view that iron promotes ductility but at high temperatures. These findings coming from natural minerals provide a novel approach regarding the control of the mechanical properties of hard materials at low temperatures.

Olivine is one of the most abundant silicate minerals in cosmic environments, occurring largely in amorphous form in the interstellar medium before crystallizing within protoplanetary disks. The kinetics and mechanisms governing this transition are central to understanding the thermal and chemical evolution of silicate dust. To investigate these processes at high spatial and structural resolution, we conducted an in situ transmission electron microscopy study on amorphous olivine thin films produced by pulsed laser deposition. Structural and chemical evolution during heating was monitored using selected-area electron diffraction, pair distribution function analysis, automated crystal orientation mapping, electron energy-loss spectroscopy, and aberration-corrected high-resolution transmission electron microscopy. The amorphous material begins to chemically reorganize at similar to 400 degrees C, where Fe2+ is oxidized to Fe3+ and nanoscale crystallites-presumably hematite-form within the glassy matrix. These crystallites appear to act as nucleation sites that promote subsequent olivine crystallization. Full crystallization occurs near 800 degrees C, yielding grains 50-150 nm in size. These observations provide direct nanoscale evidence for iron-assisted nucleation in amorphous olivine and offer constraints on the temperatures and pathways governing the crystallization of interstellar and protoplanetary silicate dust.

The diffusion of point defects in Mg2SiO4 forsterite was investigated using the kinetic activation-relaxation technique (k-ART) with the Pedone potential. Although point defects mobility has been previously studied in forsterite and other olivine structures, this approach captures mechanisms inaccessible to molecular dynamics, enabling detailed mapping of migration pathways and energies not seen before. It is found that vacancies generally diffuse faster than interstitials, with rates differing by three orders of magnitude. Novel SiO4 tetrahedral rotation mechanisms are predicted for O diffusion. These mechanisms are consistent with isotropic O vacancy transport and explain anisotropic O interstitial diffusion. Si vacancies do not diffuse but induce Mg migration, this is corroborated by Density Functional Theory (DFT) calculations using the Si -> Mg substitution plus Mg vacancy, which predict that this process is energetically favorable. In contrast, Si interstitials relax to tetrahedral sites, stabilize the lattice by altering stoichiometry, and suppress diffusion. Mg defects show strong site preferences with diffusion faster along [001] direction. Both interstitials and vacancies favor Mg migration via octahedral sites. These results highlight vacancies as the dominant drivers of diffusion and reveal complex defect-dependent mechanisms in Mg2SiO4. K-ART's ability to capture these processes highlights its value in probing long-time diffusion dynamics and provides new insights into defect behavior in forsterite.

Understanding crack tip - dislocation interaction is critical for improving the fracture resistance of semi-brittle materials like room-temperature plastically deformable ceramics. Here, we use a modified double cleavage drilled compression (DCDC) specimen geometry, which facilitates stable crack propagation, to achieve in situ observation of crack tip - dislocation interaction inside a scanning electron microscope. Single-crystal magnesium oxide specimens, furnished with dislocation-rich barriers across the intended crack path, were employed to study how localized dislocation structures influence crack dynamics. While individual dislocation behavior could not be resolved due to mechanical drift limitations, crack progression was clearly observed to decelerate within dislocation-rich regions, slowing to 15 % of its velocity as compared to the pristine crystal. Upon exiting these regions, cracks reaccelerated until reaching the next dislocation-rich barrier. Complementary phase field modeling coupled with crystal plasticity has been employed to replicate the experimental observations and provide mechanistic insight into crack tip - dislocation interactions, confirming that dislocation-rich zones serve as effective barriers to crack propagation. The aligned experiment and simulation results underscore the robustness of the technique and its potential to inform the design of more fracture-resistant ceramic materials.

The ductile deformation of strontium titanate SrTiO3 at low temperature (T less than or similar to 1000 K) is commonly associated with the activity of dislocations gliding in {110} slip planes. While dislocations with pure screw character keep essentially the same dissociated core structure in the whole temperature range, dislocations with edge character may adopt different atomic configurations, associated with different charge states and different mobilities. In this work we use atomic-scale simulations to investigate the core structure of charged and neutral edge dislocations. We report anew possible dislocation core that is charge-neutral, dissociated, and with the lowest Peierls stress reported so far, thus making it an efficient component of plastic deformation. In comparison, dislocations carrying a positive charge are slightly less mobile, while those with negative charge have a very low mobility. Our results indicate that glide of charge-neutral dislocations is favoured, and that they may locally acquire a charge by interacting with vacancies all while remaining glissile. Finally, we investigate edge dislocations that are dissociated in their climb plane, and confirm that they are energetically more favourable than their glissile counterparts. Computing the activation energy for the core transformation provides insight into the ductile-brittle transition.

Geophysical observations indicate that iron enrichment of various spatial scales may be present in the lowermost mantle. Various mechanisms have been proposed to explain the process of iron infiltration from the core to the mantle, each with its own inherent limitations. Grain boundary (GB) diffusion significantly outpaces bulk diffusion within crystal interiors, and may facilitate iron transport across the core‐mantle boundary (CMB). In this study, we investigate diffusion in two symmetric tilt GBs of ferropericlase and ferropericlase single crystals using large‐scale molecular dynamics simulations. The GB diffusivities in pure periclase and their temperature dependence agree well with previous studies. In addition, we study the GB diffusion of Fe in (Mg, Fe)O GBs for the first time. The results suggest that GB diffusion of Fe is likely to be sluggish near the CMB, and thus may not be an effective mechanism for transporting iron from the core to the mantle.

To accurately identify local structures in atomic-scale simulations of complex materials is crucial for the study of numerous physical phenomena including dynamic plasticity, crystal nucleation and glass formation. In this work, we propose a data-driven method to characterize local atomic environments, and assign them to crystal phases or lattice defects. After constructing a reference database, our approach uses descriptors based on Steinhardt's parameters and a Gaussian mixture model to identify the most probable environment. This approach is validated against several test cases : polymorph identification in alumina, and dislocation and grain boundary analysis in the olivine structure.

Potassium tantalate KTaO3 is a cubic, paraelectric perovskite ceramic that exhibits surprising ductility at room temperature as most recently reported. Much like strontium titanate (SrTiO3), plastic deformation is accommodated by dislocations gliding in 110 planes. In this work we propose a new interatomic potential for KTaO3, and apply it to model dislocations with <110> Burgers vector. We demonstrate that dislocations dissociate, and finely characterize their core structure and Peierls potential. Dislocations of edge character can carry a positive or negative electric charge, but we show that charge-neutral configurations are energetically more favorable. We also perform high-resolution electron microscopy to validate our simulation methodology. Comparing our results with other ductile perovskites, we confirm KTaO3 to be ductile, but stiffer than SrTiO3.

In this work, we investigate twist grain boundary (GB) energies and structures in Mg2SiO4 forsterite using atomistic simulations. We first present a new bond orientational order parameter allowing to highlight disordered regions in this low-symmetry crystal for which classical visualization tools are ineffective. Then we examine three GB planes, (010), (120) and (001), corresponding to the most favorable free surfaces of this crystal. We show that twist GB follow the same energy ordering as corresponding free surfaces. In addition, except for some misorientation angles and for the (120) GB plane, GB energies and structures are quite insensitive to microscopic translational degrees of freedom. The dislocation composition of low-angle twist GB can be related to γ-surfaces in the corresponding planes, and are in good agreement with first-principle calculations. It is also shown that the dislocation core structures in low angle twist GB can strongly differ from the ones of intracrystalline dislocations.

An understanding of the rheological behavior of the solid Earth is fundamental to provide a quantitative description of most geological and geophysical phenomena. The continuum mechanics approach to describing large-scale phenomena needs to be informed by a description of the mechanisms operating at the atomic scale. These involve crystal defects, mainly vacancies and dislocations. This often leads to a binary view of creep reduced to diffusion creep or dislocation creep. However, the interaction between these two types of defects leading to dislocation climb plays an important role, and may even be the main one, in the high-temperature, low strain rate creep mechanisms of interest to the Earth sciences. Here we review the fundamentals of dislocation climb, highlighting the specific problems of minerals. We discuss the importance of computer simulations, informed by experiments, for accurately modeling climb. We show how dislocation climb increasingly appears as a deformation mechanism in its own right. We review the contribution of this mechanism to mineral deformation, particularly in Earth's mantle. Finally, we discuss progress and challenges, and we outline future work directions. ▪Dislocations can be sources or sinks of vacancies, resulting in a displacement out of the glide plane: climb.▪Dislocation climb can be a recovery mechanism during dislocation creep but also a strain-producing mechanism.▪The slow natural strain rates promote the contribution of climb, which is controlled by diffusion.▪In planetary interiors where dislocation glide can be inhibited by pressure, dislocation climb may be the only active mechanism.

The atomic structure of crystal defects such as dislocations, grain or phase boundaries, control these defects' properties: their mobility, ability to cross-slip, or solute segregation. These crystal defects can be conveniently studied by atomistic simulations and one then needs to transfer relevant information at the upper scale to model microstructures containing a large number of defects, e.g., a polycrystal. Here, we propose an atomistic to continuum mechanics crossover method that (i) represents the atomic structure of dislocations cores by an appropriate Nye dislocation density tensor field and (ii), captures quantitatively the short and long range mechanical fields of defects. For (i), we propose a modified and improved interpolation method based on the original work by Hartley and Mishin. For (ii), we use a field dislocation mechanics framework that rigorously calculates/evaluates the mechanical fields associated with any Nye dislocation density distribution. The transfer method relies on molecular static calculations using two energetic models - ab-initio for screw dislocation core simulations in tungsten, and EAM potential for low and large angle grain boundaries in copper. Our findings demonstrate the effectiveness of the proposed approach in reconstructing the Burgers vector, and continuous strain and rotation fields. The framework is further applied to analyze the elastic interactions between extrinsic edge dislocations and a low angle grain boundary in copper.

The ductile deformation of strontium titanate SrTiO3 at low temperature (T⪅1000 K) is commonly associated with the activity of dislocations gliding in {110} slip planes. While dislocations with pure screw character keep essentially the same dissociated core structure in the whole temperature range, dislocations with edge character may adopt different atomic configurations, associated with different charge states and different mobilities. In this work we use atomic-scale simulations to investigate the core structure of charged and neutral edge dislocations. We report a new possible dislocation core that is charge-neutral, dissociated, and with the lowest Peierls stress reported so far, thus making it an efficient component of plastic deformation. In comparison, dislocations carrying a positive charge are slightly less mobile, while those with negative charge have a very low mobility. Our results indicate that glide of charge-neutral dislocations is favoured, and that they may locally acquire a charge by interacting with vacancies all while remaining glissile. Finally, we investigate edge dislocations that are dissociated in their climb plane, and confirm that they are energetically more favourable than their glissile counterparts. Computing the activation energy for the core transformation provides insight into the ductile–brittle transition.

We report an initial investigation of the rheology of Mg2SiO4 glass through classical molecular dynamics simulations. We performed simple shear tests at different temperatures (1-300 K), shear rates (108-1010 s-1) and pressures (0-10 GPa), from which we investigate the atomic rearrangements induced by loading. At low strain, atomic rearrangements, as detected by a non-affine displacement criteria, nucleate randomly in the glass. They then give rise to the formation of shear bands once the steady state of plastic flow is reached. We show that the flow stress follows a Herschel-Buckley law modified to account for the thermal activation of plastic events.

Transport of heat from the interior of the Earth drives convection in the mantle, which involves the deformation of solid rocks over billions of years. The lower mantle of the Earth is mostly composed of iron-bearing bridgmanite MgSiO 3 and approximately 25% volume periclase MgO (also with some iron). It is commonly accepted that ferropericlase is weaker than bridgmanite 1 . Considerable progress has been made in recent years to study assemblages representative of the lower mantle under the relevant pressure and temperature conditions 2,3 . However, the natural strain rates are 8 to 10 orders of magnitude lower than in the laboratory, and are still inaccessible to us. Once the deformation mechanisms of rocks and their constituent minerals have been identified, it is possible to overcome this limitation thanks to multiscale numerical modelling, and to determine rheological properties for inaccessible strain rates. In this work we use 2.5-dimensional dislocation dynamics to model the low-stress creep of MgO periclase at lower mantle pressures and temperatures. We show that periclase deforms very slowly under these conditions, in particular, much more slowly than bridgmanite deforming by pure climb creep. This is due to slow diffusion of oxygen in periclase under pressure. In the assemblage, this secondary phase hardly participates in the deformation, so that the rheology of the lower mantle is very well described by that of bridgmanite. Our results show that drastic changes in deformation mechanisms can occur as a function of the strain rate.