Compositionally complex materials (CCMs) present a potential paradigm shift in the design of magnetic materials. These alloys exhibit long-range structural order coupled with limited or no chemical order. As a result, extreme local environments exist with a large variations in the magnetic energy terms, which can manifest large changes in the magnetic behavior. In the current work, the magnetic properties of (Cr, Mn, Fe, Ni) alloys are presented. These materials were prepared by room-temperature combinatorial sputtering, resulting in a range of compositions with a single bcc structural phase and no chemical ordering. The combinatorial growth technique allows CCMs to be prepared outside of their thermodynamically stable phase, enabling the exploration of otherwise inaccessible order. The mixed ferromagnetic and antiferromagnetic interactions in these alloys causes frustrated magnetic behavior, which results in an extremely low coercivity $(<1\mathrm{mT})$, which increases rapidly at 50 K. At low temperatures, the coercivity achieves values of nearly 500 mT, which is comparable to some high-anisotropy magnetic materials. Commensurate with the divergent coercivity is an atypical drop in the temperature dependent magnetization. These effects are explained by a mixed magnetic phase model, consisting of ferro-, antiferro-, and frustrated magnetic regions, and are rationalized by simulations. A machine-learning algorithm is employed to visualize the parameter space and inform the development of subsequent compositions. Annealing the samples at 600 °C orders the sample, more-than doubling the Curie temperature and increasing the saturation magnetization by as much as 5×. Simultaneously, the large coercivities are suppressed, resulting in magnetic behavior that is largely temperature independent over a range of 350 K. The ability to transform from a hard magnet to a soft magnet over a narrow temperature range makes these materials promising for heat-assisted recording technologies.

Electromigration (EM) is the motion of lattice atoms under high-density electric current. It is an important mechanism for structural changes in nanoelectronic devices and a major contributor to the electroplastic effect in structural metals. Recently, a phase-field-crystal (PFC) model for studying EM in metals was developed and shown to successfully capture many important EM-driven structural evolutions at experimentally relevant timescales. In this work, connections between the previous PFC EM model and existing EM theories are established. It is shown that the PFC model can be linked to both the electron-density-based quantum EM theory and the drift-diffusion-based continuum EM theory, therefore filling an important gap in the theoretical and computational study of EM-related phenomena. The numerical method for implementing the PFC EM model is discussed in detail. The well-established Blech effect is quantitatively reproduced using the model.

We investigate energetically favorable structures of $AB{\mathrm{O}}_2\mathrm{N}$ oxynitrides as functions of pressure and strain via swarm-intelligence-based structure prediction methods, density functional theory (DFT) lattice dynamics and first-principles molecular dynamics. We predict several thermodynamically stable polar oxynitride perovskites under high pressures. In addition, we find that ferroelectric polar phases of perovskite-structured oxynitrides can be thermodynamically stable and synthesized at high pressure on appropriate substrates. The dynamical stability of the ferroelectric oxynitrides under epitaxial strain at ambient pressure also implies the possibility to synthesize them using pulsed laser deposition or other atomic layer deposition methods. Our results have broad implications for further exploration of other oxynitride materials as well. We performed first-principles molecular dynamics and find that the polar perovskite of ${\mathrm{YSiO}}_2\mathrm{N}$ ($I4cm$) is metastable up to at least 600 K under compressive epitaxial strain before converting to the stable wollastonitelike structures ($I4/mcm$). We predict that ${\mathrm{YGeO}}_2\mathrm{N}, {\mathrm{LaSiO}}_2\mathrm{N}$, and ${\mathrm{LaGeO}}_2\mathrm{N}$ are metastable as ferroelectric perovskites ($P4mm$) at zero pressure even without epitaxial strain.

Medium-range ordering (MRO) and structural heterogeneity in Zr-Cu-Co-Al metallic glasses (MGs) are characterized and their influence on ductility is investigated. Angular correlation analysis and digital reconstruction of dark-field images of nanodiffraction patterns acquired using four-dimensional scanning transmission electron microscopy reveal structural symmetries of the MRO regions localized at the nanometer scale, as well as their size distribution. The type and size distribution of the MRO regions change as a function of MG composition, with some MRO types clearly resembling the symmetry of known intermetallic phases. The MRO appears to become more structurally frustrated (e.g., lack of sixfold symmetry) when compositional heterogeneity increases, which may be inherently connected to the observed increase in ductility. Based on this hypothesis, mesoscale deformation simulations incorporating the experimentally acquired MRO information (types and sizes) are performed to gain insights on potential MRO-ductility relationship. Different types of MRO are assumed to have different properties and their influences on the stress-strain curve, the largest connected-free-volume, and the total number of extreme strain value sites are investigated parametrically. The simulation results reveal that the degree of heterogeneity in the MRO structures, in terms of both type and size, correlates directly with the ductility of the MGs.

The control and manipulation of many of light's fundamental properties, such as reflectivity, has become a topic of increasing interest since the advent of engineered electromagnetic structures—now known as metamaterials. Many of these metamaterial structures are based on the properties of dielectric materials. Magnetic materials, on the other hand, have long been known to interact with electromagnetic waves in unusual ways; in particular, their nonreciprocal properties have enabled rapid advances in millimeter wave technology. Here, we show how a structured magnetic grating can be employed to engineer electromagnetic response at frequencies upwards of hundreds of gigahertz. In particular, we investigate how nonreciprocal reflection can be induced and controlled in this spectral region through the composition of the magnetic grating. Moreover, we find that both surface and guided polaritons contribute to high-frequency nonreciprocity; the nature of these is also investigated. Control of electromagnetic radiation at high frequencies is a current challenge of communications technology where our magnetic gradient might be employed in devices including signal processing filters and unidirectional isolators.

Moiré excitons in twisted heterobilayer (HBL) transition metal dichalcogenides (TMDs) exhibit an interesting platform for engineering optical properties. The screening from the dielectric environment surrounding the HBL and the twist angle, arising from vertically stacking two layered TMD materials, allow the possibility to control and tune the interlayer exciton states. In this paper, we show in the first part that interlayer excitons in $\mathrm{Mo}{\mathrm{Se}}_2/\mathrm{W}{\mathrm{Se}}_2$ HBL localize due to the moiré potential arising within the $\text{TMD} \text{HBL}$ and discuss the conditions for which each localization site can be viewed as a local quantum emitter. In the second part, we show that the trapped exciton energy can be transferred, via the Förster resonance energy transfer (FRET) mechanism, to an adjacent two-dimensional sheet of doped graphene (Gr). The analysis of the localized exciton quenching rate is provided within the random-phase approximation, which allows one to access the collective electronic behavior within the Gr layer. Our model demonstrates the high sensitivity of FRET efficiency to external parameters, particularly the dielectric environment and the distance between the donor—here the HBL—and the acceptor (Gr). These properties show that FRET can be quite a useful tool for applications in biological systems and nanoscale sensors.

Interfaces often govern the thermal performance of nanoscale devices and nanostructured materials. As a result, accurate knowledge of thermal interface conductance is necessary to model the temperature response of nanoscale devices or nanostructured materials to heating. Here, we report the thermal boundary conductance between metals and insulators that are commonly used in spin-caloritronic experiments. We use time-domain thermoreflectance to measure the interface conductance between metals such as Au, Pt, Ta, Cu, and Al with garnet and oxide substrates, e.g., NiO, yttrium iron garnet (YIG), thulium iron garnet (TmIG), ${\mathrm{Cr}}_2{\mathrm{O}}_3$, and sapphire. We find that, at room temperature, the interface conductance in these types of material systems range from 50 to $300\mathrm{MW}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$. We also measure the interface conductance between Pt and YIG at temperatures between 80 and 350 K. At room temperature, the interface conductance of Pt/YIG is $170\mathrm{MW}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$ and the Kapitza length is ∼40 nm. A Kapitza length of 40 nm means that, in the presence of a steady-state heat current, the temperature drop at the Pt/YIG interface is equal to the temperature drop across a 40-nm-thick layer of YIG. At 80 K, the interface conductance of Pt/YIG is $60\mathrm{MW}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$, corresponding to a Kapitza length of ∼300 nm.

Colloidal crystals formed by size-asymmetric binary particles coassemble into a wide variety of colloidal compounds with lattices akin to ionic crystals. Recently, a transition from a compound phase with a sublattice of small particles to a metal-like phase in which the small particles are delocalized has been predicted computationally and observed experimentally. In this colloidal metallic phase, the small particles roam the crystal maintaining the integrity of the lattice of large particles, as electrons do in metals. A similar transition also occurs in superionic crystals, termed sublattice melting. Here, we use energetic principles and a generalized molecular dynamics model of a binary system of functionalized nanoparticles to analyze the transition to sublattice delocalization in different coassembled crystal phases as a function of temperature ($T$), number of grafted chains on the small particles, and number ratio between the small and large particles $n_s:n_l$. We find that $n_s:n_l$ is the primary determinant of crystal type due to energetic interactions and interstitial site filling, while the number of grafted chains per small particle determines the stability of these crystals. We observe first-order sublattice delocalization transitions as $T$ increases, in which the host lattice transforms from low- to high-symmetry crystal structures, including A20 $\to$ bct $\to$ bcc, ${\mathrm{A}}_{\mathrm{d}}$ $\to$ bct $\to$ bcc, and bcc $\to$ bcc/fcc $\to$ fcc transitions and lattices. Analogous sublattice transitions driven primarily by lattice vibrations have been seen in some atomic materials exhibiting an insulator-metal transition also referred to as metallization. We also find minima in the lattice vibrations and diffusion coefficient of small particles as a function of $n_s:n_l$, indicating enhanced stability of certain crystal structures for $n_s:n_l$ values that form compounds.

We analyze the effect of temperature on the yielding transition of amorphous solids using different coarse-grained model approaches. On one hand, we use an elastoplastic model, with temperature introduced in the form of an Arrhenius activation law over energy barriers. On the other hand, we implement a Hamiltonian model with a relaxational dynamics, where temperature is introduced in the form of a Langevin stochastic force. In both cases, temperature transforms the sharp transition of the athermal case in a smooth crossover. We show that this thermally smoothed transition follows a simple scaling form that can be fully explained using a one-particle system driven in a potential under the combined action of a mechanical and a thermal noise, namely, the stochastically driven Prandtl-Tomlinson model. Our work harmonizes the results of simple models for amorphous solids with the phenomenological $\sim T^{2/3}$ law proposed by Johnson and Samwer [Phys. Rev. Lett. 95, 195501 (2005)] in the framework of experimental metallic glasses yield observations, and extend it to a generic case. Conclusively, our results strengthen the interpretation of the yielding transition as an effective mean-field phenomenon.

Amorphous films of tantalum oxide $\left({\mathrm{Ta}}_2{\mathrm{O}}_5\right)$ are widely applied to build highly reflective mirrors used in interferometric gravitational wave detectors, such as the Laser Interferometer Gravitational Wave Observatory. Despite a large number of studies, the structural properties of amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ on the local scale still deserve several further investigations. Such information is essential to any attempt to understand the properties of this important material, and no property modeling efforts can be expected to yield reliable information until the local structure in the amorphous phase is better understood. In this paper we report the results obtained by analyzing the synchrotron x-ray pair distribution functions of pure and Ti-doped amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ films, deposited by ion-beam sputtering as prepared and after a thermal treatment. As a result, it is found that: (1) the arrangement of Ta atoms in amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ strongly resembles that characterizing the high-pressure $Z\text{−}{\mathrm{Ta}}_2{\mathrm{O}}_5$ polymorph, whereas the topological properties of O atoms resemble that observed in $\delta \text{−}{\mathrm{Ta}}_2{\mathrm{O}}_5$; (2) structural correlations in amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ start to vanish above ∼5 Å and are completely suppressed for $r$ > 10 Å on account of disorder; (3) Ti substitution retains the short-range topological ordering characterizing the as-prepared ${\mathrm{Ta}}_2{\mathrm{O}}_5$ amorphous film even after thermal treatment; conversely, pure ${\mathrm{Ta}}_2{\mathrm{O}}_5$ films undergo a significant rearrangement of the local structure after thermal treatment.

Poly- and single-crystalline samples of ${\mathrm{Ba}}_5{\left(\mathrm{Os}{\mathrm{O}}_5\right)}_3\mathrm{Cl}$ have been successfully synthesized. ${\mathrm{Ba}}_5{\left(\mathrm{Os}{\mathrm{O}}_5\right)}_3\mathrm{Cl}$ crystallizes in the noncentrosymmetric space group $P{6}_{3}cm$. ${\mathrm{Ba}}_5{\left(\mathrm{Os}{\mathrm{O}}_5\right)}_3\mathrm{Cl}$ shows a ferromagnetic order below 5 K and a possible ferroelectric order below 150 K. A multiferroic state with both ferromagnetic and ferroelectric ordering is established, although the ferromagnetic and ferroelectric orders are not coupled. The experimental effective moment and the saturated magnetization for ${\mathrm{Ba}}_5{\left(\mathrm{Os}{\mathrm{O}}_5\right)}_3\mathrm{Cl}$ are close to the spin-only value, indicating that the spin-orbit coupling does not play an important role, which is due to the large crystal field and spin splitting in ${\mathrm{Ba}}_5{\left(\mathrm{Os}{\mathrm{O}}_5\right)}_3\mathrm{Cl}$.

In this study, we fabricate high-quality epitaxial ${\mathrm{Li}}_{1\text{–}x}\mathrm{Co}{\mathrm{O}}_2$ (LCO) thin films on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates with different orientations and systematically investigate the change in the crystal orientation of thin-film grains by tuning the oxygen deposition pressure. Further tensile strain study reveals the depth dependence in these epitaxial LCO thin films. Further, synaptic transistors are fabricated based on these high-quality epitaxial LCO films, and long-duration nonvolatile potentiation and depression states are established. By reducing the thickness of the LCO channel, the signal-to-noise ratio of the nonvolatile switching is substantially improved. Changes in the potentiation and depression states are found to significantly depend on the lattice orientation of the LCO channel, suggesting that an anisotropic Li-ion diffusion rate in highly crystallized LCO films plays an important role in the device performance.

Oxide interfaces between insulating perovskites can provide a two-dimensional electron gas (2DEG), which was observed initially beneath polar $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ (LAO) grown on $\mathrm{Ti}{\mathrm{O}}_2$-terminated $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$(001). Here, we suggest to use a solid electrolyte $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ (LiNO) instead of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ to create in LAO/LiNO a robustly switchable and magnetic quasi 2DEG (q2DEG). This prospective phenomenon is achieved by charging and discharging lithium niobate while its $n\to p$ doping induces the alteration of magnetic order in LAO/LiNO and, thus, the spin degrees of freedom in q2DEG. On the basis of ab initio calculations and starting from the defectless and insulating LAO/LiNO superlattice, we demonstrate that either a ferromagnetic q2DEG or weakly magnetic quasi-2D hole gas emerge there due to the presence of excessive electrons or holes, respectively. These defects were simulated by one extra Li and a lithium vacancy in LiNO. By varying the $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ thicknesses up to 18 formula units, we found that (i) the spatial extent of q2DEG is 1.5 nm along the stacking direction and that (ii) the dopant position at the Al-terminated interface is energetically preferable by more than 0.5 eV compared to the deep layers. In the context of the advanced design of q2DEG, the out-of-plane electric polarization, which intrinsically appears in multiferroic LAO/LiNO, allows to switch externally the electric-field dependence of the Rashba effect. Moreover, the in-plane charge current via q2DEG may generate there a transverse spin density due to the Rashba-Edelstein effect. We discuss this scenario using the relativistic electronic structures and locally projected density of states which differ remarkably in $\mathrm{L}\mathrm{A}\mathrm{O}/{\mathrm{Li}}_{1+x}\mathrm{N}\mathrm{O}$ and $\mathrm{L}\mathrm{A}\mathrm{O}/{\mathrm{Li}}_{1-x}\mathrm{N}\mathrm{O}$.

Data assimilation techniques are attracting increasing attention because they enable researchers to estimate material parameters by integrating an advanced computational model with experimental data of microstructure evolution. In this study, we develop an adjoint model to integrate a phase-field model for spinodal decomposition with time-series measurement data of compositional field maps to estimate the unknown parameters in the phase-field model. As a case study, simultaneous estimation of six parameters (Gibbs energy parameters, gradient energy coefficient, etc.) in the phase-field model is considered. To confirm the effectiveness of the developed adjoint model, numerical tests called “twin experiments” are conducted using synthetic measurement data prepared in advance through phase-field simulation. In the twin experiments, the optimum estimates of six model parameters of interest are shown to coincide with true values, indicating that several model parameters can be successfully estimated. Furthermore, the effects of the standard deviation of measurement noise $\left(\sigma \right)$ and the time interval of measurements $\left(\mathrm{\Delta }{t}_{\mathrm{meas}.}^{\prime }\right)$ on uncertainties of optimum estimates of parameters $\left({\sigma }^{\mathrm{est}}\right)$ are examined by conducting twin experiments. It is shown that there is a positive correlation between $\sigma$ and ${\sigma }^{\mathrm{est}}$ or between $\mathrm{\Delta }{t}_{\mathrm{meas}.}^{\prime }$ and ${\sigma }^{\mathrm{est}}$, which is essential information for designing experiments to estimate model parameters. The developed adjoint model is assumed to be useful for estimating unknown parameters (e.g., Gibbs energy parameters of a nonequilibrium phase) using time-series measurement data of microstructure evolution during spinodal decomposition.

Quantum anomalous Hall effect (QAHE) has been experimentally realized in the intrinsic antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. However, at ambient condition the magnetization of the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ sample decreases significantly after it is exposed in air for a couple of days, which hinders its potential application. Here, we theoretically propose to cover 2D van der Waals materials such as hexagonal boron nitride (h-BN) monolayer onto the surface of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. This not only protects the stability of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$, but also leads to interlayer ferromagnetic coupling and further realizes the QAHE at higher temperature. We find that interlayer ferromagnetic transition occurs generally when monolayer h-BN, $\mathrm{Mo}{\mathrm{S}}_2$, or $\mathrm{W}{\mathrm{Se}}_2$ is covered onto two or three septuple-layer $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ with interlayer antiferromagnetic coupling. Band-structure and topological properties calculations show that $\mathrm{h}-\mathrm{BN}/\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ heterostructure exhibits a topologically nontrivial band gap around 64–75 meV, hosting QAHE with a Chern number of $C=1$. Our proposed materials system should be considered as an ideal platform to explore high-temperature QAHE due to their relatively simple and stable structures.

Recent work analyzing the impact of nonsymmorphic symmetries on electronic states has given rise to the discovery of multiple types of topological matter. Here we report the single-crystal synthesis and magnetic properties of $\mathrm{Eu}{\mathrm{Ga}}_2{\mathrm{Sb}}_2$, a Eu-based antiferromagnet structurally consisting of pseudo-1D chains of Eu ions related by a nonsymmorphic glide plane. We find the onset of antiferromagnetic order at $T_N=8\mathrm{K}$. Above $T_N$ the magnetic susceptibility is isotropic. Curie-Weiss analysis suggests competing ferromagnetic and antiferromagnetic interactions, with $p_{\mathrm{eff}}=8.1{\mu }_B$ as expected for $4f^7 J=S=7/2 {\mathrm{Eu}}^{2+}$ ions. Below $T_N$ and at low applied magnetic fields, an anisotropy develops linearly, reaching ${\chi }_{\perp }/{\chi }_{\parallel }=6$ at $T=2\mathrm{K}$. There is concomitant metamagnetic behavior along ${\chi }_{\parallel }$, with a magnetic field of ${\mu }_{0}H\approx 0.5$ T sufficient to suppress the anisotropy. Independent of crystal orientation, there is a continuous evolution to a field-polarized paramagnetic state with $M=7{\mu }_B/{\mathrm{Eu}}^{2+}$ at ${\mu }_{0}H=2$ T as $T\to 0\mathrm{K}$. Specific-heat measurements show a recovered magnetic entropy of $\mathrm{\Delta }{S}_{\mathrm{mag}}\approx 16.4\mathrm{J}\mathrm{mo}{\mathrm{l}}^{–1}{\mathrm{K}}^{–1}$ from $T\sim 0$ K to $T=T_N$, close to the expected value of $R\ln \left(8\right)$ for an $S=7/2$ ion, indicating negligible low-dimensional spin fluctuations above $T_N$. We find no evidence of unusual behaviors arising either from the dimensionality or the presence of the nonsymmorphic symmetries.

Characterization of the primary damage is the starting point in describing and predicting the irradiation-induced damage in materials. So far, primary damage has been described by traditional interatomic potentials in molecular dynamics simulations. Here, we employ a Gaussian approximation machine-learning potential (GAP) to study the primary damage in silicon with close to ab initio precision level. We report detailed analysis of cascade simulations derived from our modified Si GAP, which has already shown its reliability for simulating radiation damage in silicon. Major differences in the picture of primary damage predicted by machine-learning potential compared to classical potentials are atomic mixing, defect state at the heat spike phase, defect clustering, and recrystallization rate. Atomic mixing is higher in the GAP description by a factor of two. GAP shows considerably higher number of coordination defects at the heat spike phase and the number of displaced atoms is noticeably greater in GAP. Surviving defects are dominantly isolated defects and small clusters, rather than large clusters, in GAP's prediction. The pattern by which the cascades are evolving is also different in GAP, having more expanded form compared to the locally compact form with classical potentials. Moreover, recovery of the generated defects at the heat spike phase take places with higher efficiency in GAP. We also provide the attributes of the new defect cluster that we had introduced in our previous study. A cluster of four defects, in which a central vacancy is surrounded by three split interstitials, where the surrounding atoms are all 4-folded bonded. The cluster shows higher occurrence in simulations with the GAP potential. The formation energy of the defect is 5.57 eV and it remains stable up to 700 K, at least for 30 ps. The Arrhenius equation predicts the lifetime of the cluster to be $0.0725\mu \mathrm{s}$ at room temperature.

Solid-state systems whose properties at high pressure (exceeding 1 GPa) mimic those of $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ are of large importance in the study of the interior of planets. By means of density functional theory (DFT) calculations we studied the high-pressure properties of a $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ analog, $\mathrm{Na}\mathrm{Zn}{\mathrm{F}}_3$. We reproduce the phase-transition sequence previously reported for this compound ($\mathrm{Gd}\mathrm{Fe}{\mathrm{O}}_3\to \mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3\to \mathrm{L}{\mathrm{a}}_2{\mathrm{S}}_3$), and predict that it should undergo a two-step dissociation: decomposition into an equimolar mixture of ${\mathrm{Na}}_2\mathrm{Zn}{\mathrm{F}}_4$ and $\mathrm{Na}{\mathrm{Zn}}_2{\mathrm{F}}_5$ at 25.4 GPa, followed by a breakdown into $\mathrm{Zn}{\mathrm{F}}_2$ and NaF at 66.8 GPa. These processes are analogous to those predicted for compressed $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$. Moreover, both ${\mathrm{Na}}_2\mathrm{Zn}{\mathrm{F}}_4$ and $\mathrm{Na}{\mathrm{Zn}}_2{\mathrm{F}}_5$ are isostructural with analogous phases from the Mg-Si-O system. We also find that both these compounds are thermodynamically stable at ambient conditions (${\mathrm{Na}}_2\mathrm{Zn}{\mathrm{F}}_4$) or at a low pressure of 19 GPa ($\mathrm{Na}{\mathrm{Zn}}_2{\mathrm{F}}_5$). Our study indicates that $\mathrm{Na}\mathrm{Zn}{\mathrm{F}}_3$ could serve as a good low-pressure analog of $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ exhibiting the same sequence of phase transitions, and pressure induced decomposition, but at pressures an order of magnitude lower.

Transition metal borides, which are three-dimensional (3D) layered materials containing covalently bonded B networks, have shown a number of excellent properties, such as radiation resistance and the ability to act as a diffusion barrier in integrated circuits. However, defect behavior, which controls many of the materials’ properties, has remained unknown in these materials. Here, we investigate the effects of the B networks on the defect chemistry in both binary borides (CrB, ${\mathrm{Cr}}_3{\mathrm{B}}_4$, ${\mathrm{Cr}}_2{\mathrm{B}}_3$) and ternary MAB phases (${\mathrm{Cr}}_2\mathrm{Al}{\mathrm{B}}_2$, ${\mathrm{Cr}}_3\mathrm{Al}{\mathrm{B}}_4$, ${\mathrm{Cr}}_4\mathrm{Al}{\mathrm{B}}_6$) using first-principles calculations. We find that increasing the number of B rings in the structure leads to lower formation energies and higher concentrations of Frenkel pairs. The results can be explained by the fact that the strongest Cr-B bond is weakened when borides have more B rings, leading to a reduction in the formation energy of Cr and B vacancies. Also, the bonds associated with Cr atoms bonded within B rings are softer in structures containing more B rings, which allows Cr interstitials to form with a lower energy cost and contributes to an increase in the concentration of Cr interstitials.

Using atomistic simulations based on the creation-relaxation algorithm, we explore the evolution of microstructure in irradiated zirconium over a broad range of radiation exposure. In agreement with experimental observations, we find that at relatively low temperatures, microstructure evolves towards an asymptotic dynamic steady state forming at doses close to 1 dpa. Simulations show the spontaneous formation of $a$-type interstitial dislocation loops, gradually transforming into an $a$-type extended dislocation network and giving rise to macroscopic anisotropic dimensional changes in a textured material. A fully developed $a$-type interstitial/vacancy and $c$-type vacancy dislocation microstructure corresponds to the highest degree of irradiation growth anisotropy and vanishingly small volumetric swelling of the material.