Data-driven, or machine learning (ML), approaches have become viable alternatives to semiempirical methods to construct interatomic potentials, due to their capacity to accurately interpolate and extrapolate from first-principles simulations if the training database and descriptor representation of atomic structures are carefully chosen. Here, we present highly accurate interatomic potentials suitable for the study of dislocations, point defects, and their clusters in bcc iron and tungsten, constructed using a linear or quadratic input-output mapping from descriptor space. The proposed quadratic formulation, called quadratic noise ML, differs from previous approaches, being strongly preconditioned by the linear solution. The developed potentials are compared to a wide range of existing ML and semiempirical potentials, and are shown to have sufficient accuracy to distinguish changes in the exchange-correlation functional or pseudopotential in the underlying reference data, while retaining excellent transferability. The flexibility of the underlying approach is able to target properties almost unattainable by traditional methods, such as the negative divacancy binding energy in W or the shape and the magnitude of the Peierls barrier of the $\frac{1}{2}\langle 111\rangle$ screw dislocation in both metals. We also show how the developed potentials can be used to target important observables that require large time-and-space scales unattainable with first-principles methods, though we emphasize the importance of thoughtful database design and degrees of nonlinearity of the descriptor space to achieve the appropriate passage of information to large-scale calculations. As a demonstration, we perform direct atomistic calculations of the relative stability of $\frac{1}{2}\langle 111\rangle$ dislocations loops and three-dimensional C15 clusters in Fe and find the crossover between the formation energies of the two classes of interstitial defects occurs at around 40 self-interstitial atoms. We also compute the kink-pair formation energy of the $\frac{1}{2}\langle 111\rangle$ screw dislocation in Fe and W, finding good agreement with density functional theory informed line tension models that indirectly measure those quantities. Finally, we exploit the excellent finite-temperature properties to compute vacancy formation free energies with full anharmonicity in thermal vibrations. The presented potentials thus open up many avenues for systematic investigation of free-energy landscape of defects with ab initio accuracy.

Three-dimensional (3D) carbon networks provide a great platform to search and investigate various topological phases. A lot of nodal-line and nodal-surface topological phases have been found in the 3D carbon networks. However, nodal points are few in them because most carbon networks have high symmetries including inversion symmetry. Here, we propose that nodal points can be generated in low-symmetry carbon structures without inversion symmetry. Two $s{p}^2$-hybridized 3D carbon networks, consisting of distorted six-membered rings and a double helix, respectively, are used as examples to show nodal points. There are several pairs of nodal points locating on the high-symmetry paths of the first Brillouin zone. These nodal points are Dirac points. However, they can also be treated as Weyl points because of the negligibly small spin-orbit coupling in the carbon structures. These nodal points only shift in the momentum space but do not disappear when perturbations such as strains are applied on the carbon structures. We have also studied the topological surface states and found Fermi arcs linking the nodal points. In addition, the synthesis of two carbon networks is discussed.

We theoretically investigate the electronic band structures and magnetic properties of ilmenites with edge-sharing ${\mathrm{IrO}}_6$ honeycomb layers, $A{\mathrm{IrO}}_3$ with $A= \mathrm{Mg}, \mathrm{Zn}, \mathrm{and} \mathrm{Mn}$, in comparison with a collinear antiferromagnet ${\mathrm{MnTiO}}_3$. The compounds with $A= \mathrm{Mg} \mathrm{and} \mathrm{Zn}$ were recently reported in Y. Haraguchi et al., Phys. Rev. Mater. 2, 054411 (2018), while ${\mathrm{MnIrO}}_3$ has not been synthesized yet but the honeycomb stacking structure was elaborated in a superlattice with ${\mathrm{MnTiO}}_3$ in K. Miura et al., Commun. Mater. 1, 55 (2020). We find that, in contrast to ${\mathrm{MnTiO}}_3$, where an energy gap opens in the Ti $3d$ bands by antiferromagnetic ordering of the high-spin $S=5/2$ moments, ${\mathrm{MgIrO}}_3$ and ${\mathrm{ZnIrO}}_3$ have a gap in the Ir $5d$ bands under the influence of both spin-orbit coupling and electron correlation. Their electronic structures are similar to those in the spin-orbit coupled Mott insulators with the $j_{\mathrm{eff}}=1/2$ pseudospin degree of freedom, as found in monoclinic $A_2{\mathrm{IrO}}_3$ with $A=$ Na and Li, which have been studied as candidates for the Kitaev spin liquid. Indeed, we find that the effective exchange interactions between the $j_{\mathrm{eff}}=1/2$ pseudospins are dominated by the Kitaev-type bond-dependent interaction and the symmetric off-diagonal interactions. On the other hand, for ${\mathrm{MnIrO}}_3$, we show that the local lattice structure is largely deformed, and both Mn $3d$ and Ir $5d$ bands appear near the Fermi level in a complicated manner, which makes the electronic and magnetic properties qualitatively different from ${\mathrm{MgIrO}}_3$ and ${\mathrm{ZnIrO}}_3$. Our results indicate that the ${\mathrm{IrO}}_6$ honeycomb network in the ilmenites $A{\mathrm{IrO}}_3$ with $A=$ Mg and Zn would offer a good platform for exotic magnetism by the spin-orbital entangled moments like the Kitaev spin liquid.

Pt-Ge eutectic alloy droplets are scrutinized by microscopic analysis of electrons excited by 4.9-eV photons. Using only the work function and contact angle as fitting parameters, we determine the exact droplet shape that reproduces the experimental electron emission profile. The local inclination of the droplets' surface plays a decisive role in the generated emission profile. Intensity variations at the illuminated side of the eutectic droplets are a consequence of standing waves in the electromagnetic field responsible for excitation. Intensity variations at the nonilluminated side are ascribed to a diffraction pattern of the photons after their interaction with the droplet.

We report the crystal structures, initial magnetic and charge transport characterization, and calculated electronic structures of the hexagonal oxyhalide perovskites, ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Cl}}_2$ and ${\mathrm{Ba}}_7{\mathrm{Ru}}_4{\mathrm{O}}_{15}{\mathrm{Br}}_2$. The experimental information is obtained through the study of single crystals. Face-sharing $\mathrm{Ru}{\mathrm{O}}_6$ octahedra form ${\mathrm{Ru}}_2{\mathrm{O}}_9$ dimers in a layered triangular geometry in these materials; minor amounts of off-magnetic-site structural disorder are present in our crystals. Both are magnetically isotropic, with 2.5 μB/mol-Ru, Curie-Weiss theta −185 K, and 2.9 μB/mol-Ru, Curie-Weiss theta −168 K, respectively. Broad features in the magnetic susceptibility and heat capacity associated with magnetic ordering (at 35 and 37 K, respectively) are observed. The charge transport band gaps are 0.03 eV for the oxychloride and 0.006 eV for the oxybromide. There is no gap at the Fermi level for either of these compounds in density-functional theory electronic structure calculations.

High-entropy alloys (HEAs) have stimulated great interest due to their remarkable mechanical and irradiation performance. Experiments suggest that delayed defect evolution in HEAs, compared to conventional metals and dilute alloys, is the main reason for their improved irradiation resistance. However, the mechanism responsible for the observation remains elusive. Here we show that the potential energy landscape of defects under the influence of random arrangement of different species is the reason for the delayed defect evolution. We arrive at the conclusion by investigating the diffusion of defects and defect clusters under three cases: the averaged-atom model, random model, and the model with local short-range ordering. Our results suggest that, compared to the average model, the chemical fluctuation inherent in HEAs can suppress interstitial motion more than vacancy motion. The effects are more pronounced when SRO develops. For defect clusters, the chemical disorder can reduce their jump frequencies significantly and enhance correlation effects, leading to suppressed defect motion. Notably, we find that with SRO, such defect motion can be entirely trapped in local regions. This work demonstrates that chemical fluctuations and SRO are the main reason responsible for the suppressed defect evolution in HEAs, which dictates a promising way to improve the irradiation performance of HEAs through manipulating its chemical disorder states, such as local ordering.

We apply machine learning algorithms to optimize thermodynamic and elastic properties of multicomponent Fe-Cr alloys with additions of Ni, Mo, Al, W, V, and Nb. The target properties are mixing enthalpy, Young's elastic modulus, and the ratio between shear and bulk moduli, which is often used as a phenomenological criterion for a material's ductility. We thoroughly analyze the descriptors that provide the robust performance of the machine learning models. Next, the iterative active learning method is used for the optimization of the chemical composition to simultaneously improve both thermodynamic stability and the elastic properties of Fe-Cr-based alloys. As a result, we predict compositions of thermodynamically stable alloys with improved mechanical properties, demonstrating the high potential of data-driven computational design in the field of materials for nuclear energy applications.

Although often conceived as random solid solutions, alloys may exhibit a local short-range order (SRO) where the distribution of atoms deviates from a perfect randomness, owing to complex interactions among alloying elements. SRO can affect various properties of alloys, but understanding their exact forms, roles, and origins remains challenging from experiment alone. Here we show, through combining statistical sampling and ab initio calculations, that a strong and special SRO dominates the structure of SiSn alloy, which is a key subset of group IV alloys for mid-infrared technology. Remarkably, the SRO in SiSn is found to be reflected primarily by a strong depletion in the second Sn-Sn coordination shell. This is distinguished from the main character of the SRO in the closely related GeSn alloy, which is reflected by the depletion within the first Sn-Sn coordination shell. The unique nature of SRO in SiSn alloy is further attributed to the competition between the two unfavorable local configurations in SiSn: A Sn-Sn first-nearest neighbor through a direct bond and a Sn-Sn second-nearest neighbor through a Sn-Si-Sn motif. The large energy penalty induced by Sn-Si-Sn local structural motif is found to be responsible for the difference between SiSn and GeSn in their forms of SRO. The SRO in SiSn alloy is further demonstrated to substantially raise the direct band gap. Our finding thus constitutes a new knowledge of the origin and form of SRO and lays the foundation for understanding the complex structure of SiGeSn ternary alloy.

At ambient conditions, potassium dihydrogen phosphate (KDP) usually exists in tetragonal structure, and only transforms to monoclinic structure at a temperature around 503 K. In this study, stable monoclinic KDP is developed at room temperature using a mixture of graphene oxides and unsaturated KDP solution. Furthermore, a reversible phase transition between monoclinic and tetragonal structure is observed by tuning the distance between graphene oxide layers. The first-principles calculation suggests that the tetragonal structure is more energetically favorable when KDP is sandwiched by two graphitic surfaces with a large interlayer distance, whereas the monoclinic structure is more stable with a small interlayer distance. This is attributed to the interaction between ${\mathrm{K}}^{+}$ cations and π electrons in the graphitic surface, which distorts the local structural configurations. The increase of interaction strength with decreasing interlayer distance not only reduces the crystal-liquid interfacial free energy, which promotes the formation of the monoclinic structure in unsaturated solution, but also stimulates transformation of existing tetragonal KDP to monoclinic structure. The observations shed light on the nucleation mechanism for crystal structure that is usually infeasible and unstable at ambient conditions, and provide an avenue to tune crystal structures for a wide range of metal salts.

A micromechanics model was developed to evaluate the elastic binding energy between carbide precipitates and hydrogen interstitials using Eshelby's equivalent inclusion method. Density functional theory (DFT) simulations were performed to obtain the material-specific quantities, e.g., lattice constants and the elastic constants, for the continuum model. Using this model, we find that for coherent carbide precipitates, hydrogen atoms are more likely to bind on the broad surfaces of the disk-like precipitates, which is consistent with experimental observations. For semicoherent and incoherent precipitates, our model suggests that it is possible for semicoherent precipitates to have significant hydrogen binding capability while there is no hydrogen-binding capability of incoherent precipitates, which also agrees with experimental findings. In addition, several factors that influence the binding energies between hydrogen atoms and carbide precipitates were quantitatively analyzed, including the precipitate size, morphology, orientation, and interface. These collective results include both the position and the value of the strongest hydrogen-binding interaction for a wide range of carbide stoichiometries, which contributes to our understanding of hydrogen trapping in steel-based materials.

Gallium nitride (GaN) has emerged as a quintessential wide band-gap semiconductor for an array of high-power and high-frequency electronic devices. The phonon thermal resistances that arise in GaN thin films can result in detrimental performances in these applications. In this work, we report on the thermal conductivity of submicrometer and micrometer thick homoepitaxial GaN films grown via two different techniques (metal-organic chemical vapor deposition and molecular beam epitaxy) and measured via two different techniques (time domain thermoreflectance and steady-state thermoreflectance). When unintentionally doped, these homoepitaxial GaN films possess higher thermal conductivities than other heteroepitaxially grown GaN films of equivalent thicknesses reported in the literature. When doped, the thermal conductivities of the GaN films decrease substantially due to phonon-dopant scattering, which reveals that the major source of phonon thermal resistance in homoepitaxially grown GaN films can arise from doping. Our temperature-dependent thermal conductivity measurements reveal that below 200 K, scattering with the defects and GaN/GaN interface limits the thermal transport of the unintentionally doped homoepitaxial GaN films. Further, we demonstrate the ability to achieve the highest reported thermal boundary conductance at metal/GaN interfaces through in situ deposition of aluminum in ultrahigh vacuum during molecular beam epitaxy growth of the GaN films. Our results inform the development of low thermal resistance GaN films and interfaces by furthering the understanding of phonon scattering processes that impact the thermal transport in homoepitaxially grown GaN.

The effect of Cu off-stoichiometry and Zn alloying on the fundamental absorption region of ${\mathrm{Cu}}_2\mathrm{CdSn}{\mathrm{S}}_4$ (CCTS) absorbers in complete solar cells has been investigated using electroreflectance (ER) spectroscopy at room temperature. It is found that ER spectra consist of contributions from two different sources, one of which corresponds to band gap transition in the absorber layer and the other to the interference effect in the window layer. ER measurements on CCTS samples reveal a near-constant band gap energy of 1.37–1.38 eV and a relatively small broadening of 60–90 meV in the probed $0.8<\mathrm{Cu}/\left(\mathrm{Cd}+\mathrm{Sn}\right)<0.89$ compositional range, in contrast to related kesterites ${\mathrm{Cu}}_2\mathrm{ZnSn}{\left(\mathrm{S},\mathrm{Se}\right)}_4$. The analysis of the band gap in ${\mathrm{Cu}}_2\left({\mathrm{Cd}}_{1-x},{\mathrm{Zn}}_x\right)\mathrm{Sn}{\mathrm{S}}_4$ alloys yields a quadratic dependence on Zn content with a bowing parameter of 0.4 eV. Finally, the broadening parameters of the band gap transitions as well as their compositional dependence are evaluated and discussed.

The topological transitions that occur to the grain boundary network during grain growth in a material with uniform grain boundary energies are believed to be known. The same is not true for more realistic materials, since more general grain boundary energies in principle allow many more viable grain boundary configurations. A grain growth simulation for such a material therefore requires a procedure to enumerate all possible topological transitions and select the most energetically favorable one. Such a procedure is developed and implemented here for a microstructure represented by a volumetric finite element mesh. As a specific example, all possible transitions for a typical configuration with five grains around a junction point are enumerated, and some exceptional transitions are found to be energetically similar to the conventional ones even for a uniform boundary energy. A general discrete formulation to calculate grain boundary velocities is used to simulate grain growth for an example microstructure. The method is implemented as a $\mathrm{C}++$ library based on SCOREC, an open source massively parallelizable library for finite element simulations with adaptive meshing.

Henmilite {${\mathrm{Ca}}_2\mathrm{Cu}{\left(\mathrm{OH}\right)}_4{\left[\mathrm{B}{\left(\mathrm{OH}\right)}_4\right]}_2$} is a blue calcium copper borate mineral found only in the Fuka mine, Okayama Prefecture, Japan. Crystal structure refinement, magnetic and specific heat measurements, and density functional theory (DFT) calculations are performed to clarify its magnetic properties. Synchrotron x-ray diffraction experiments reveal that the hydrogen-bonded chains are arranged in an antiferroelectric manner, doubling the unit cell along the $a$ axis. An antiferromagnetic transition is observed at 0.2 K at zero magnetic field. Furthermore, a dome-shaped antiferromagnetic ordering region exists in the temperature–magnetic-field phase diagram, indicating the presence of quantum spin fluctuations. The obtained crystal structure in combination with DFT calculations suggests that the system has a coupled two-leg ladder magnetic lattice, explaining the very low ordering temperature.

We present the effect of substitution-induced pressure on the reversibility of the magnetocaloric effect (MCE) in ${\mathrm{Ni}}_2{\mathrm{Cr}}_x{\mathrm{Mn}}_{1.4-x}{\mathrm{In}}_{0.6}$ ($x=0.1$, 0.2, 0.3) alloys, through characterization in pulsed magnetic fields. We measured the adiabatic temperature change $\mathrm{\Delta }{T}_{\text{ad}}$ directly during applied magnetic field pulses of 2 and 6 T. We paid special attention to the reversibility of $\mathrm{\Delta }{T}_{\text{ad}}$. The substitution of Mn by Cr in ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1.4}{\mathrm{In}}_{0.6}$ leads to a negative pressure, as evidence by the increase of the lattice parameters, which shifts the martensitic transition towards lower temperatures and enhances the ferromagnetism of the martensite phase. We found a large value of $\mathrm{\Delta }{T}_{\text{ad}}=-7$ K at $T=270$ K for the sample with $x=0.1$ for a field change of 6 T. We discuss the reversibility of the MCE in these alloys in terms of the Clausius-Clapeyron equation.

The abnormal brittle failure of superhard boron carbide $\left({\mathrm{B}}_4\mathrm{C}\right)$ and other icosahedral solids arises from the shear-induced amorphization. Mitigating the amorphization in these materials remains challenging due to the lack of other deformation mechanisms such as mobile dislocations. This paper illustrates the shear-induced amorphization process of ${\mathrm{B}}_4\mathrm{C}$ from molecular dynamics (MD) simulations using quantum-mechanics-derived machine-learning force field. The amorphization in ${\mathrm{B}}_4\mathrm{C}$ initiates from the disintegration of icosahedral clusters, and then this icosahedral deconstruction propagates and merges to form an amorphous region with 2–3 nm in width, leading to the following cavitation and brittle failure. More interesting, the deformation mechanism transforms from amorphization to stacking fault (SF) formation by microalloying aluminum (Al) into ${\mathrm{B}}_4\mathrm{C}$. This SF formation originates from the enhanced icosahedral slip as the Al is incorporated into the C-B-C chain to form a C-Al-C chain. This paper illustrates a deformation mechanism of superhard icosahedral solids and provides a strategy for suppressing the amorphization and brittle failure of ${\mathrm{B}}_4\mathrm{C}$.

Broken inversion symmetry in atomic structure can lead to the emergence of specific functionalities at the nanoscale. Therefore, realizing 2D materials in Janus form is a growing field, which offers unique features and opportunities. In this paper, we investigate the structural, vibrational, elastic, piezoelectric, and electronic properties of Janus $\mathrm{Bi}XY$ ($X=\mathrm{S}$, Se, Te and $Y=\mathrm{F}$, Cl, Br, I) monolayers based on first-principle methods. The structural optimization and vibrational frequency analysis reveal that all of the proposed structures are dynamically stable. Additionally, ab initio molecular dynamics simulations verify the thermal stability of these structures even at elevated temperatures. The mechanical response of the Janus $\mathrm{Bi}XY$ crystals in the elastic regime is investigated in terms of in-plane stiffness and the Poisson ratio, and the obtained results ascertain their mechanical flexibility. The piezoelectric stress and strain coefficient analysis demonstrates the appearance of strong out-of-plane piezoelectricity, which is comparable with the Janus transition metal dichalcogenide monolayers. The calculated electronic band structures reveal that except for BiTeF, all Janus $\mathrm{Bi}XY$ monolayers are indirect band gap semiconductors, and their energy band gaps span from the infrared to the visible part of the optical spectrum. Subsequently, large Rashba spin splitting is observed in electronic band structures when the spin-orbit coupling is included. The obtained results point out Janus 2D $\mathrm{Bi}XY$ structures as promising materials for a wide range of applications in nanoscale piezoelectric and spintronics fields.

We devise automated workflows for the calculation of Helmholtz and Gibbs free energies and their temperature and pressure dependence and provide the corresponding computational tools. We employ nonequilibrium thermodynamics for evaluating the free energy of solid and liquid phases at a given temperature and reversible scaling for computing free energies over a wide range of temperatures, including the direct integration of $P\text{−}T$ coexistence lines. By changing the chemistry and the interatomic potential, alchemical and upscaling free energy calculations are possible. Several examples illustrate the accuracy and efficiency of our implementation.

We describe a strategy to design ferroelectric heteroanionic materials based on the coupling of anion order and octahedral tilts in $n=1$ Ruddlesden-Popper structures, which allows noncentrosymmetric and polar compounds to arise from centrosymmetric anion-ordered structures. We investigate the relative phase stabilities of the polymorphs of ${\mathrm{Sr}}_2{\mathrm{ScO}}_3\mathrm{F}$ and ${\mathrm{Ca}}_2{\mathrm{ScO}}_3\mathrm{F}$ derived from this coupling using electronic structure calculations. We find that large degrees of octahedral tilting can stabilize different anion orders derived from assembly of $[{\mathrm{ScO}}_5{\mathrm{F}]}^{8-}$ octahedra. To further understand the link between octahedral tilting and anion order, we quantitatively separate the contributions of electrostatics and covalent interactions to the stability of tilts. We find that the tilts are driven primarily by covalent interactions and that local out-of-plane polar displacements, induced from the anion order, further stabilize the octahedral tilts through the pseudo-Jahn-Teller effect.