The diffusion mechanism of carbon in iron plays a vital role in carburization processes, steel fabrication, and metal dusting corrosion. In this paper, using the kinetic activation-relaxation technique (k-ART), an off-lattice kinetic Monte Carlo algorithm with on-the-fly catalog building that allows to obtain diffusion properties over large time scales taking full account of chemical and elastic effects coupled with an EAM potential, we investigate the effect of pressure on the diffusion properties of carbon in $85.{91}^{\circ }\langle 100\rangle$ symmetric tilt grain boundaries (GB) of $\alpha$-iron up to a pressure of 12 kbar at a single temperature of 600 K. We find that, while the effect of pressure can strongly modify the C stability and diffusivity in the GB in ways that depend closely on the local environment and the nature of the deformation, isotropic and uniaxial pressure can lead to opposite and nonmonotonous effects regarding segregation energy and activation barriers. These observations are relevant to understanding of the evolution of heterogeneous materials, where variations of local pressure can alter the carbon diffusion across the material.

Spin-driven ferroelectricity phenomena have drawn great interest in the scientific community due to potential application in spintronics and their complex physical mechanisms. A noticeable example of this is multiferroic $\mathrm{BaYFe}{\mathrm{O}}_4$ that exhibits an unconventional magnetoelectric (ME) coupling due to the uncorrelated behavior of the ferroelectric and cycloidal states under an applied magnetic field. To shed more light on this spin-driven ME effect, a high-quality sample of $\mathrm{BaYFe}{\mathrm{O}}_4$ was synthesized by a standard solid-state reaction method, and its high-field (up to 9 T) magnetic properties have been systematically investigated by means of magnetometry, magnetocaloric effect, and Mössbauer measurements over a wide temperature range (5–400 K). In addition, its crystal and magnetic structures have been studied using x-ray and neutron powder diffraction. Results obtained indicate that Fe spins form a long-range spin density wave (SDW) antiferromagnetic (AFM) order at $T_{\mathrm{N}1}\sim 50\mathrm{K}$, which transforms into the cycloidal AFM order at $T_{\mathrm{N}2}\sim 35\mathrm{K}$. A spin-glass-like state emerges below $T^{*}\sim 17\mathrm{K}$, and coexists with the long-range cycloidal AFM one in this temperature range. Magnetocaloric and Mössbauer measurements consistently confirm the robustness of both the long-range SDW and cycloidal AFM orders under applied magnetic fields up to 6 T, whereas the spin-glass state is converted into the ferromagnetic (FM) state when the applied magnetic field exceeds 1 T. These findings pinpoint the fact that the magnetic field evolution of spin correlations from the AFM to FM character in the spin-glass state is responsible for the magnetic field dependence of ferroelectricity in $\mathrm{BaYFe}{\mathrm{O}}_4$.

The anomalous thermal expansion in a layered $3d\text{−}5d$ based triple perovskite iridate ${\mathrm{Ba}}_3{\mathrm{CoIr}}_2{\mathrm{O}}_9$ is investigated using high resolution synchrotron diffraction. Below the magnetostructural transition at 107 K, the onset of antiferromagnetic order is associated with a monoclinic distortion of the hexagonal structure. Deeper within the magnetically ordered state, a part of the monoclinic phase distorts even further, and both these structural phases co-exist down to the lowest measured temperatures. We observe negative thermal expansion in this phase co-existence regime, which appears to be intimately connected to the temperature driven relative fractions of these monoclinic phases. The significant NTE observed in this system could be driven by magnetic exchange striction and is of relevance to a number of systems with pronounced spin orbit interactions.

Removing artificial bands from the back side of surface slabs with pseudohydrogen atoms has become the method of choice to boost the convergence of density-functional theory (DFT) surface calculation with respect to slab thickness. In this paper we apply this approach to semipolar compound semiconductor surfaces, which have recently become attractive for device applications. We show that approaches employing saturation of dangling bonds by pseudohydrogen atoms alone are inadequate to properly passivate the surfaces, remove spurious surface states from the fundamental band gap, and achieve flat band conditions in the slab. We propose and successfully apply to technologically interesting semipolar wurtzite surfaces of III-N, III-V, and II-VI semiconductors a reconstruction-inspired passivation scheme that utilizes native anions to passivate cation dangling bonds and pseudohydrogen atoms to obey the electron counting rule and compensate for polarization-induced surface-bound charges. This scheme is generic and robust and can be straightforwardly implemented in DFT investigations of low-symmetry surfaces as well as in high-throughput and machine learning studies.

Here, we trace in situ the slip-line formation and morphological signature of dislocation avalanches in a high-entropy alloy with the aim of revealing their microstructural degree of localization. Correlating the intermittent microplastic events with their corresponding slip-line patterns allows defining two main event types, one of which is linked to the formation of new slip lines, whereas the other one involves reactivation of already existing slip lines. The formation of new slip lines reveals statistically larger and faster avalanches. The opposite tendency is seen for avalanches involving reactivation of already existing slip lines. The combination of both these types of events represents the highest degree of spatial avalanche delocalization that spans the entire sample, forming a group of events that determine the truncation length scale of the truncated power-law scaling. These observations link the statistics of dislocation avalanches to a microstructural observable.

The computational design of materials with ionic bonds poses a critical challenge to thermodynamic modeling since density functional theory yields inaccurate predictions of their formation enthalpies. Progress requires leveraging physically insightful correction methods. The recently introduced coordination corrected enthalpies (CCE) method delivers accurate formation enthalpies with mean absolute errors close to room temperature thermal energy, i.e., $\approx 25$ meV/atom. The CCE scheme, depending on the number of cation-anion bonds and oxidation state of the cation, requires an automated analysis of the system to determine and apply the correction. Here, we present AFLOW-CCE—our implementation of CCE into the AFLOW framework for computational materials design. It features a command line tool, a web interface, and a Python environment. The workflow includes a structural analysis, automatically determines oxidation numbers, and accounts for temperature effects by parametrizing vibrational contributions to the formation enthalpy per bond.

CoSn is a Pauli paramagnet with relatively flat $d$ bands centered about 100 meV below the Fermi energy, $E_F$. Single crystals of CoSn lightly doped with Fe, In, or Ni are investigated using x-ray and neutron scattering, magnetic susceptibility and magnetization, AC susceptibility, specific heat, and resistivity measurements. Within the rigid-band approximation, hole doping with a few percent of Fe or In should move the flat bands closer to $E_F$, whereas electron doping with Ni should move the flat bands further away from $E_F$. We provide evidence that this indeed occurs. Fe and In doping drive CoSn toward magnetism, while Ni doping suppresses CoSn's already weak magnetic response. The resulting ground state is different for Fe versus In doping. For Fe-doped crystals, ${\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x\mathrm{Sn}$, with $0.02<x<0.27$, the magnetic and specific-heat data are consistent with the formation of a spin glass, with a glass temperature, $T_g$, ranging from 1 K for $x=0.02$ to 10 K for $x=0.27$. Powder and single-crystal neutron diffraction found no evidence of long-range magnetic order below $T_g$ for samples with $x\approx 0.17$. For In-doped crystals, $\mathrm{Co}{\mathrm{Sn}}_{1-y}{\mathrm{In}}_y$, both the magnetic susceptibility and the Sommerfeld coefficient, γ, increase substantially relative to pure CoSn, but with no clear indication of a magnetic transition for $0.05<y<0.2$. CoSn crystals doped with Ni (${\mathrm{Co}}_{0.93}{\mathrm{Ni}}_{0.07}\mathrm{Sn}$) have a significantly smaller magnetic susceptibility and γ than pure CoSn, consistent with flat bands further from $E_F$.

$A{B}_2{\mathrm{O}}_4$ are a special class of spinel structured compounds exhibiting unique multiferroic properties. Recent experimental observation confirms switchable polarization in $A{\text{Cr}}_2{\text{O}}_4$ $(A=\text{Co}, \mathrm{Mn}, \mathrm{Fe})$, arising out of noncollinear magnetic spin order. In this paper we unfold the microscopic origin behind such magnetic spin order, hysteresis, polarization, and the so-called magnetic compensation effect in $A{\text{Cr}}_2{\text{O}}_4$ $(A=\text{Co}, \mathrm{Mn}, \mathrm{Fe}, \mathrm{Ni})$ using Monte Carlo simulation. We have carefully constructed a set of exchange interaction parameters, which help to explain various experimental findings such as magnetization vs temperature $\left(T\right)$, conical stability, magnetic ordering, and polarization in a representative compound ${\text{CoCr}}_2{\text{O}}_4$. When substituted with Fe, ${\text{CoCr}}_2{\text{O}}_4$ shows few exotic phenomena such as sign reversible exchange bias effect and magnetic compensation. ${\text{CoMn}}_2{\text{O}}_4$ and ${\text{CoFe}}_2{\text{O}}_4$ are two other compounds in the spinel family where we found no conical spin order and polarization, and thus facilitate a distinct contrast compared to others.

The wide band gap semiconducting perovskite $\mathrm{BaSn}{\mathrm{O}}_3$ is of high current interest due to outstanding room temperature mobility at high electron density, fueled by potential applications in oxide, transparent, and power electronics. Due in part to a lack of lattice-matched substrates, $\mathrm{BaSn}{\mathrm{O}}_3$ thin films suffer from high defect densities, however, limiting electron mobility. Additionally, the vast majority of $\mathrm{BaSn}{\mathrm{O}}_3$ thin film research has focused on pulsed laser deposition or molecular beam epitaxy. Here, we present an exhaustive optimization of the mobility of ${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_3$ films grown by a scalable, high-throughput method: high-pressure-oxygen sputter deposition. Considering target synthesis conditions, substrate selection, buffer layer structure, deposition temperature, deposition rate, thickness, and postdeposition annealing conditions, and by combining high-resolution x-ray diffraction, reciprocal space mapping, rocking curve analysis, scanning transmission electron microscopy, atomic force microscopy, and temperature-dependent electronic transport measurements, detailed understanding of synthesis-structure-property relationships is attained. Optimized room temperature mobility of $96\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{–}1}{\mathrm{s}}^{\text{–}1}$ is achieved in vacuum-annealed $\mathrm{GdSc}{\mathrm{O}}_3$(110)/$\mathrm{BaSn}{\mathrm{O}}_3$(120 nm)/${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_3$(200 nm) heterostructures, as well as $92\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{–}1}{\mathrm{s}}^{\text{–}1}$ on unbuffered substrates and $87\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{–}1}{\mathrm{s}}^{\text{–}1}$ without postdeposition annealing. These results, including important trends in defect densities and a surprising dependence of mobility on lattice mismatch, substantially expand the understanding of the interplay between deposition conditions, microstructure, and transport in doped $\mathrm{BaSn}{\mathrm{O}}_3$ films, establishing competitive mobilities in films fabricated via a scalable, high-throughput, industry-standard technique.

Very recently, a type of two-dimensional $\left(2D\right)$ crystals, $M{\mathrm{Si}}_2{\mathrm{N}}_4(M=\mathrm{Mo},\mathrm{W})$, has been successfully synthesized in experiment [Y.-L. Hong et al., Science 369, 670 (2020)]. Here, using first-principles calculations, we systematically investigate the potential of two-dimensional $\left(2D\right)$ $\mathrm{M}{\mathrm{Si}}_2{\mathrm{N}}_4$ as hydrogen evolution reaction (HER) catalysts, and propose two effective ways to trigger their HER activity. Our computations reveal that while the metal vacancies cannot activate the inertia planes for HER, $2D$ $\mathrm{M}{\mathrm{Si}}_2{\mathrm{N}}_4$ with $N$ vacancy exhibits excellent HER performance. Especially for $2D$ $\mathrm{W}{\mathrm{Si}}_2{\mathrm{N}}_4$, the introduction of $N$ vacancy can yield an ideal value of hydrogen adsorption Gibbs free energy $(\mathrm{\Delta }{G}_{{\mathrm{H}}^{*}}=-0.02\mathrm{eV})$, even superior to that of Pt $(\mathrm{\Delta }{G}_{{\mathrm{H}}^{*}}=-0.09\mathrm{eV})$. Moreover, by high-throughput screening of $3d$, $4d$, and $5d$ transition metals, we find that introducing V/Fe/Nb/Tc/Ta atom into $2D$ $\mathrm{M}{\mathrm{Si}}_2{\mathrm{N}}_4$ can significantly trigger their HER activity. The underlying physics are discussed in detail. Our work not only highlights a family of potential HER electrocatalysts, but also provides feasible strategies for triggering their HER activity.

We study free, capped, and encapsulated bilayer jacutingaite $\left({\mathrm{Pt}}_2{\mathrm{HgSe}}_3\right)$ from first principles. While the freestanding bilayer is a large-gap trivial insulator, we find that the encapsulated structure has a small trivial gap due to the competition between sublattice symmetry breaking and sublattice-dependent next-nearest-neighbor hopping. Upon the application of a small perpendicular electric field, the encapsulated bilayer undergoes a topological transition towards a quantum spin Hall insulator. We find that this topological transition can be qualitatively understood by modeling the two layers as uncoupled and can be described by an imbalanced Kane-Mele model that takes into account the sublattice imbalance and the corresponding inversion-symmetry breaking in each layer. Within this picture, bilayer jacutingaite undergoes a transition from a $0+0$ state, where each layer is trivial, to a $0+1$ state, where an unusual topological state relying on Rashba-like spin orbit coupling emerges in only one of the layers.

The oxygenic motifs (e.g., ${\mathrm{O}}^{2–}$, ${{\mathrm{O}}_2}^{2\text{–}}$ ${{\mathrm{O}}_2}^{2\text{–}}$, and ${{\mathrm{O}}_2}^{\text{–}}$) that are present in compounds have a substantial effect on their electronic structure and behavior. Herein, first-principles swarm-intelligence crystal structural searches reveal that the reaction between Y and O under high pressure leads to the formation of novel compounds with unique properties. Several O-rich Y-O compounds (e.g., ${\mathrm{YO}}_2$, ${\mathrm{Y}}_2{\mathrm{O}}_5$, and ${\mathrm{YO}}_3$) emerge as being stable. It is shown that the oxygenic motifs found within the stable species depend upon the oxygen content and pressure (e.g., ${\mathrm{O}}^{2\text{–}}$ in YO and ${\mathrm{Y}}_2{\mathrm{O}}_3$, the coexistence of ${\mathrm{O}}^{2\text{–}}$ and ${{\mathrm{O}}_2}^{2\text{–}}$ in ${\mathrm{YO}}_2$ and ${\mathrm{Y}}_2{\mathrm{O}}_5$, ${{\mathrm{O}}_2}^{2\text{–}}$ in $Pm\text{−}3$ ${\mathrm{YO}}_3$, and ${\mathrm{O}}^{2\text{–}}$ in Cmcm ${\mathrm{YO}}_3$), and are accompanied by a transition in the electronic structure from superconducting to metallic to semiconducting. Notably, the Cmcm symmetry ${\mathrm{YO}}_3$ phase, consisting of a 13-fold coordinated face-sharing polyhedron with 15 faces, can be classified as a transition metal (TM) superoxide. The long sought-after bulk yttrium monoxide (YO) is shown to become stable at high pressure. NaCl-type YO is superconducting with a critical temperature $\left(T_c\right)$ of 13.0 K at 25 GPa, becoming the TM monoxide with the highest known $T_c$. Our work will inspire future studies exploring the chemistry and properties of O-rich TM oxides at high pressure.

Atomistic simulations provide insights into structure-property relations on an atomic size and length scale, that are complementary to the macroscopic observables that can be obtained from experiments. Quantitative predictions, however, are usually hindered by the need to strike a balance between the accuracy of the calculation of the interatomic potential and the modeling of realistic thermodynamic conditions. Machine-learning techniques make it possible to efficiently approximate the outcome of accurate electronic-structure calculations, that can therefore be combined with extensive thermodynamic sampling. We take elemental nickel as a prototypical material, whose alloys have applications from cryogenic temperatures up to close to their melting point, and use it to demonstrate how a combination of machine-learning models of electronic properties and statistical sampling methods makes it possible to compute accurate finite-temperature properties at an affordable cost. We demonstrate the calculation of a broad array of bulk, interfacial, and defect properties over a temperature range from 100 to 2500 K, modeling also, when needed, the impact of nuclear quantum fluctuations and electronic excitations. The framework we demonstrate here can be easily generalized to more complex alloys and different classes of materials.

We investigated temperature dependent current driven spin-orbit torques in magnetron sputtered ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ (4 and 10 nm)/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ (5 nm) layered structures with in-plane magnetic anisotropy. The room temperature dampinglike and fieldlike spin torque efficiencies of the amorphous ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ films were measured to be $0.14\pm 0.008\left(0.07\pm 0.012\right)$ and $–0.03\pm 0.006\left(–0.20\pm 0.009\right)$, for the four (10 nm) films, respectively, by utilizing the second-harmonic Hall technique. The large fieldlike torque in the relatively thicker ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ (10 nm) thin film is unique compared to the traditional spin Hall materials interfaced with thick magnetic layers with in-plane magnetic anisotropy which typically have dominant dampinglike and negligible fieldlike torques. Additionally, the observed room temperature fieldlike torque efficiency in ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$ (10 nm)/CoFeB (5 nm) is up to three times larger than the dampinglike torque ($–0.20\pm 0.009$ and $0.07\pm 0.012$, respectively) and 30 times larger at 50 K ($–0.29\pm 0.014$ and $0.009\pm 0.017$, respectively). The temperature dependence of the fieldlike torques shows dominant contributions from the intrinsic spin Hall effect while the dampinglike torques show dominate contributions from the extrinsic spin Hall effects, skew scattering, and side jump. Through macrospin calculations, we found that including fieldlike torques on the order of or larger than the dampinglike torque can reduce the switching critical current and decrease magnetization procession for a perpendicular ferromagnetic layer.