The lattice vibrational properties of $\alpha \text{−}\mathrm{Sn}$ (gray tin) were investigated experimentally by temperature-dependent x-ray diffraction and theoretically by density functional theory calculations. Similar to the other elements of group IV, $\alpha \text{−}\mathrm{Sn}$ exhibits a lattice anomaly at low temperatures and negative thermal expansion, with a minimum at $\sim 27\mathrm{K}$ and a magnitude three times larger than in Si. The influence of anharmonic effects up to fourth-order potential terms on the phonon dispersion relations, the lattice parameters, and the thermal expansion coefficient have been tested. The performed analysis gives an excellent agreement with experiment when quartic potential terms are included in the theory. We point out that negative thermal expansion in $\alpha \text{−}\mathrm{Sn}$ is not driven by the anharmonicity of the interatomic potential. This resolves the long-standing puzzle in the thermal behavior of $\alpha \text{−}\mathrm{Sn}$.

Mn-based full Heusler compounds possess well-defined local atomic Mn moments, and thus the correlation effects between localized $d$ electrons are expected to play an important role in determining the electronic and magnetic properties of these materials. Employing ab initio calculations in conjunction with the constrained random-phase approximation (cRPA) method, we calculate the strength of the effective on-site Coulomb interaction parameters (Hubbard $U$ and Hund exchange $J$) in the case of ${\mathrm{X}}_2\mathrm{MnZ}$ full Heusler compounds with X being one of Ni, Pd, or Cu, and Z being one of In, Sn, Sb, or Te. We show that the Z element (or $sp$ element) in Heusler compounds significantly reduces the strength of the Hubbard $U$ parameter for Mn $3d$ electrons compared to the elementary bulk Mn. On the contrary, the effect of the sp atom on the strength of the $U$ parameter of Ni, Cu, or Pd valence $d$ electrons is not so substantial with respect to the elementary bulk values. The $U$ values for all transition-metal atoms decrease with increasing $sp$ electron numbers in the In-Sn-Sb-Te sequence. Our cRPA calculations reveal that despite their well-defined local magnetic moments, the Mn-based full Heusler alloys fall into the category of the weakly correlated materials.

We explore the possibility and potential benefit of rolling a ${\mathrm{Si}}_2\mathrm{BN}$ sheet into single-walled nanotubes (NTs). Using density functional theory (DFT), we consider both structural stability and the impact on the nature of chemical bonding and conduction. The structure is similar to carbon NTs and hexagonal boron-nitride (hBN) NTs and we consider both armchair and zigzag ${\mathrm{Si}}_2\mathrm{BN}$ configurations with varying diameters. The stability of these ${\mathrm{Si}}_2\mathrm{BN}$ NTs is confirmed by first-principles molecular dynamics calculations, by exothermal formation, an absence of imaginary modes in the phonon spectra. Also, we find the nature of conduction varies from semiconducting over semimetallic to metallic, reflecting differences in armchair/zigzag-type structures, curvature effects, and the effect of quantum confinement. We present a detailed characterization of how these properties lead to differences in both the bonding nature and electronic structures.

Since advanced silicon-based device components are moderately chemically tunable, doped graphene has emerged as a promising candidate to replace this semiconducting material in flexible miniaturized electronic devices. Indeed, heteroatom codoping (i.e., with boron and/or nitrogen) is an appealing strategy to tune both its structural and electronic properties, possibly inducing a band gap in graphene. However, presently synthesized BN-doped carbon-based materials are randomly doped, leading their electronic properties not to be reproducible. Using first-principles techniques, the present study investigates the periodic doping of graphene with borazine-like rings in order to search for an entirely new class of BNC hybrid 2D materials exhibiting high stabilities and optimized band gaps for optoelectronic applications. Ab initio calculations show that BN-ring doped graphene displays cohesive energies comparable with benchmark ideal periodic BNC systems (such as ${\mathrm{BC}}_3$ and ${\mathrm{BC}}_2\mathrm{N}$) with a decreasing linear trend toward high concentrations of BN rings. Band gaps of BN-ring doped graphene systems are calculated using many-body perturbation techniques and are found to be sensitive to the doping pattern and to be considerably larger for high concentration of BN rings exhibiting the same orientation. These predictions suggest that BN-ring doped graphene materials could be interesting candidates for the next generation of optoelectronic devices and open new opportunities for their synthesis using chemical bottom-up approaches.

Layered oxide intercalation compounds continue to attract interest as electrode materials for Na-ion batteries. However, many of these materials undergo complex phase transitions during cycling that influence battery performance but are still not completely understood. We have conducted a first-principles study of layered ${\mathrm{Na}}_x{\mathrm{CrO}}_2$ $(0\le x\le 1)$ to assess phase stability between various Na-vacancy ordered phases in the O3 and P3 host structures. We predict that many of the low-energy phases belong to families of Na orderings that follow specific patterns. At high $x$, we identify families of vacancy row orderings in O3, which may also couple to magnetic orderings of the Cr spins. We predict similar orderings at intermediate $x$ in P3 that contain antiphase boundaries between regions of the $x=1/2$ ordering. In both cases, the average spacing between rows/boundaries is set by the overall composition. At $x=0$, we find a strong preference for charge disproportionation and migration of Cr to tetrahedral sites in the intercalation layers. We obtain generally good agreement with experimental observations and rationalize key discrepancies.

Lomer (L) and Lomer-Cottrell (LC) dislocations have long been considered to be central to work hardening in face-centered cubic (FCC) metals and alloys. These dislocations act as barriers of motion for other dislocations, and can serve as sites for twin nucleation. Recent focus on multicomponent concentrated FCC solid solution alloys has resulted in many reported observations of LC dislocations. While these and L dislocations are expected to have a role in the mechanical behavior of these alloys, little is understood about how variations in composition and associated fault energies change the response of these dislocations under stress. We present atomistic simulations of L and LC dislocations in a model Cu-Ni system and find that changes in composition and applied stress conditions result in a wide variety of responses, including changes in core configuration and (100) glide. The results are compared to and extend previous literature related to the nature of L/LC core structures and how they vary with respect to intrinsic materials properties and stress states. This study also provides insights into mechanisms such as twin nucleation that could have important implications for work hardening in FCC solid-solution alloys.

We demonstrate the presence of an emergent magnetic anisotropy in square lattices of circular mesospins. An external field is used to saturate the magnetization along the [10] and [11] directions before quantifying the magnetic textures at remanence. A clear directional dependence was obtained. The concomitant changes in the interactions are argued to cause the observed anisotropy and, thereby, the directional dependence in the transition temperature of the mesospins.

The electrochemical performance of potassium titanyl arsenate (${\mathrm{KTiOAsO}}_4$, KTA) as the cathode and anode in K-ion batteries is calculated within density-functional theory. Cathodes and anodes are modeled using K-deficient ${\mathrm{K}}_{1-x}{\mathrm{TiOAsO}}_4$ ($x=0.0\text{–}1.0$) and K-doped ${\mathrm{KTiOAsO}}_4{\mathrm{K}}_x$ ($x=0.0\text{–}0.5$), respectively. For KTA cathodes/anodes a slightly larger/smaller open circuit voltage is found than predicted for potassium titanyl phosphate. The present results suggest that a solely KTA-based K-ion battery can reach an average working voltage of about $3.8\mathrm{V}$. The migration barriers of K vacancies and K dopants are found to be smaller than 750 meV. In addition, the present calculations show a very moderate volume expansion and shrinkage upon K intercalation and deintercalation. Our results suggest KTA to be a promising electrode material for K-ion batteries.

We report a simple technique for the synthesis of uniaxially textured, metastable hexagonal close-packed-like uranium thin films with thicknesses between 175 and 2800 Å. The initial structure and texture of the layers have been studied via x-ray diffraction and reflectivity, and the time-dependent transitions of the samples into various orientations of orthorhombic $\alpha \text{−}\mathrm{U}$ have been mapped by similar techniques. The final crystallographic orientations of each system and the timescales on which the transitions occur are found to depend on the lattice parameters of the original layer. The absence of the $\alpha \text{−}\mathrm{U}$ (001) orientation in the transition products suggests that the transitions in these layers are mediated by mechanisms other than the [110] transverse acoustic phonon mode previously suggested for the cubic $\gamma \text{−}\mathrm{U}\left(110\right)$ to hcp-U(00.1) to $\alpha \text{−}\mathrm{U}\left(001\right)$ displacive phase transition. Alternative transition pathways are discussed.

Calcium-containing oxides are fundamental components of the Earth's crust and mantle. Analysis of their structural behavior contributes to our understanding of the Earth's interior, which needs a reliable interatomic potential. Here, we present an interatomic potential for the Ca-O system using the deep neural network method. The initial training data set for the deep interatomic potential (DP) consists of snapshots of prototype binary structures from materials projects and crystal-derived structures based on Ca-O binary compounds, as well as their molecular dynamic trajectories. The accuracy of the DP is evaluated by predicting forces and energies in comparison with those from ab initio calculations. We demonstrate that the vibrational and thermodynamic properties based on DP calculations are in excellent agreement with those from ab initio calculations. Besides, we also construct a temperature-pressure phase diagram of Ca-O compounds with DP at a lower cost compared to ab initio methods. Finally, we use the DP to explore Ca-O structures by combining it with the genetic algorithm, the accuracy of which is validated by a principal component analysis for the local atomic environments. The DP training procedure used in this work is equally applicable to other systems for accurate atomistic simulations as an effective method.

Over the years, designing and establishing new materials with good stability, a suitable band gap, and high carrier mobility have become the focus in researching two-dimensional (2D) materials. In this paper, we predict two 2D semiconductors, $C2/m\text{−}\mathrm{Sn}X$ ($X=\text{P}$, As), using first-principles simulation. They possess excellent dynamical and thermal stability, low structural symmetry, high cohesive energy, and good flexibility for the structural properties. Regarding electronic properties, SnP and SnAs are direct band gap semiconductors with suitable band gap values. In addition, they exhibit a high carrier mobility (about $\sim {10}^3\text{–}{10}^4{\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$). Their optical properties reveal good transparency in the infrared and visible light regions, but the primary response region of light locates in the ultraviolet area. In addition, their electronic and optical properties show different characteristics in different directions. Therefore, $C2/m\text{−}\mathrm{Sn}X$ ($X=\text{P}$, As) are 2D direct band gap semiconductors with excellent stability, a suitable band gap value, high carrier mobility, and good IR-VIS light transparency. We suggest that the group IV-V 2D semiconductor $C2/m\text{−}\mathrm{Sn}X$ ($X=\text{P}$, As) will have bright prospects in field-effect transistors, integrated circuits, the hole transport layer of solar cells, blue LEDs, and other nanoelectronic and nano-optoelectronic devices.

Various defects in ZnO, focused on substitutional N on the O site (${\mathrm{N}}_{\mathrm{O}}$) and ${\mathrm{N}}_2$ at various sites—the O site, interstitial sites, and the Zn site—are studied using first-principles calculations with the goal of understanding the electron paramagnetic resonance (EPR) center reported for ${\mathrm{N}}_2$ in ZnO and substitutional N on the O site. The $g$ tensors are calculated using the gauge-including projector augmented wave (GIPAW) method and compared with experiments. The $g$ tensors of the free ${\mathrm{N}}_2^{+}$ and ${\mathrm{N}}_2^{-}$ radicals and their various contributions within the GIPAW theory are analyzed first to provide a baseline reference for the accuracy of the method and for understanding the ${\mathrm{N}}_2$ behavior in ZnO. Previous controversies regarding the site location of ${\mathrm{N}}_2$ in ZnO for this EPR center and regarding the shallow or deep nature and donor or acceptor nature of this center are resolved. We find that the ${\mathrm{N}}_2$ on the Zn site is mostly zinc-vacancy-like in its spin density and $g$ tensor, while for the O site, a model with the ${\mathrm{N}}_2$ axis lying in the basal plane and the singly occupied ${\pi }_g$ orbital along the c axis provides good agreement with experiment. For interstitial locations, if the ${\mathrm{N}}_2$ is not strongly interacting with the surroundings, no levels in the gap are found and hence also no possible EPR center. The calculated $g$ tensors for ${\mathrm{N}}_{\mathrm{O}}$ and zinc vacancy are also found to be in good agreement with experiment. The use of different functionals affecting the localization of the spin density is shown to affect the $g$-tensor values.

A material's properties and functionalities are determined by its chemical constituents and the atomic arrangement in which they crystallize. For the recently discovered pigment ${\mathrm{YIn}}_{1-x}{\mathrm{Mn}}_x{\mathrm{O}}_3$, for instance, it had been surmised that its bright blue color owes to an unusual, trigonal bipyramidal, oxygen coordination of the manganese impurities. Here, we demonstrate that, indeed, a direct correspondence between details of the local Mn environment and the pigment's blue color holds: Combining realistic many-body calculations (dynamical mean-field theory to treat the quasi-atomic Mn-multiplets at low doping $x=8\%$) with an effective medium description (Kubelka-Munk model to describe scattering in a milled pigment sample), we find that only a Mn-coordination polyhedra consisting of two distorted oxygen pyramids results in a diffuse reflectance commensurate with the experimental blue color. We motivate that the distortion of the bipyramid helps circumventing atomic selection rules, allowing for dipolar $d\text{−}d$ transitions and creating the desired two-peak absorption profile.

Magnetic topological insulators $\left(\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4\right){\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ were anticipated to exhibit magnetic energy gaps, while recent spectroscopic studies did not observe them. Thus, magnetism on the surface is under debate. In this work, we propose another symmetry criterion to probe surface magnetism. Because of both time-reversal symmetry breaking and inversion symmetry breaking, we demonstrate that the surface band structure violates momentum-inversion symmetry and leads to a threefold rather than sixfold rotational symmetry on the Fermi surface if corresponding surface states couple strongly to the surface magnetism. Such a momentum-inversion symmetry violation is significant along the $\mathrm{\Gamma }-K$ direction for surface bands on the (0001) plane, which serves as a criterion for determining the surface magnetism.

The combination of geometric frustration, extended hopping, spin-orbit coupling, and a disordered magnetic ground state make ${\mathrm{NaRuO}}_2$ an attractive Heisenberg-Kitaev candidate material. Historically, ${\mathrm{NaRuO}}_2$ has been a challenging material to produce, even in polycrystalline form. Here we present synthetic efforts that identify a propensity for ${\mathrm{Na}}_{\text{Ru}}$ defects to form in ${\mathrm{NaRuO}}_2$, revealing a full solid-solution between ${\mathrm{NaRuO}}_2$ and disordered ${\mathrm{Na}}_2{\mathrm{RuO}}_3$. We report the synthesis of alloys along the ${\mathrm{Na}}_{3+x}{\mathrm{Ru}}_{3-x}{\mathrm{O}}_6$ solid solution and characterize changes in the bulk magnetization and electron transport as a function of Na loading. Our results highlight the importance of stoichiometry control in ${\mathrm{NaRuO}}_2$ when investigating and interpreting this material's physical properties.

Despite the high attendance of shear banding in metallic glasses and other disordered materials, the nature of the emergence of shear band is still mysterious. Using molecular dynamics simulations, a set of detailed characterizations of shear band in a typical ${\mathrm{Cu}}_{50}{\mathrm{Zr}}_{50}$ metallic glass is obtained. Then we uncover a large number of robust and intriguing commonalities between the emergence of shear bands and the glass-to-liquid transition, including strong similarities on viscosity drop, enthalpy discontinuity, breakdown of hard backbone network, as well as relaxation process. Such observations indicate that shear banding in metallic glasses is a consequence of deformation-controlled glass transition, as further quantitatively validated via the compelling overlap between the venerable Vogel-Fulcher-Tammann law (and Adam-Gibbs relation) and the evolving glass state of shear band controlled by configurational temperature. These results provide a direct bridge between shear banding and glass-to-liquid transition and are instrumental to build the unified framework of flow behavior induced either by thermal or stressed stimuli in disordered materials.

Scandium is well known to produce grain refinement in Al-based alloys, and its potency is generally attributed to intermetallic ${\mathrm{Al}}_3\mathrm{Sc}$ formation within liquid phase. However, the influence of Sc atoms and ${\mathrm{Al}}_3\mathrm{Sc}$ on the local structure of the surrounding melt, and subsequent nucleation remains unclear. Towards that end, we have probed structural changes in three bulk compositions, i.e., $\mathrm{Al}\text{−}x\mathrm{Sc}$ $(x=0, 0.4, 1.0 \mathrm{at}.\%)$, and near liquid-$\mathrm{Al}/{\mathrm{Al}}_3\mathrm{Sc}$ interfacial regions using ab initio molecular dynamics. In-liquid ordering was determined using unsupervised learning techniques, i.e., structural fingerprinting, dimensionality reduction, and cluster analysis. Sc atoms ordered the surrounding liquid Al atoms by forming Sc-centered polyhedrons, while liquid-$\mathrm{Al}/{\mathrm{Al}}_3\mathrm{Sc}$ interface manifested planar ordering that resembled ${\left\{100\right\}}_{fcc-\mathrm{Al}}$. Both structures were geometrically persistent but constitutionally transient, i.e., they exchanged Al atoms with the surrounding liquid. This behavior was rationalized on the basis of their mixed metallic and covalent bond character. At a lower temperature, ${\left\{100\right\}}_{fcc-\mathrm{Al}}$ interfacial planes heteroepitaxially nucleated equilibrium $fcc\text{−}\mathrm{Al}$, while Sc-centered polyhedrons sequentially formed metastable $hcp$- and $bcc\text{−}\mathrm{Al}$. Using our simulations and extant experimental reports, we postulate two transformation pathways during Al-Sc solidification: (i) $\mathrm{Sc}-centeredpolyhedrons\to {\mathrm{Al}}_3\mathrm{Sc}\to liquid-Al/A{l}_{3}Scinterfacialordering\to fcc-\mathrm{Al}$; and (ii) $\mathrm{Sc}-centeredpolyhedrons\to hcp-\mathrm{Al}\to bcc-\mathrm{Al}\to fcc-\mathrm{Al}$. The two in-liquid ordered structures provide an atomistic basis for the potency of Sc element, and, also, serve as possible structural metrics for designing novel Al-based alloys.

High entropy alloys (HEA) represent a class of materials with promising properties, such as high strength and ductility, radiation damage tolerance, etc. At the same time, a combinatorially large variety of compositions and a complex structure render them quite hard to study using conventional methods. In this work, we present a computationally efficient methodology based on ab initio calculations within the coherent potential approximation. To make the methodology predictive, we apply an exchange-correlation correction to the equation of state and take into account thermal effects on the magnetic state and the equilibrium volume. The approach shows good agreement with available experimental data on bulk properties of solid solutions. As a particular case, the workflow is applied to a series of iron-group HEA to investigate their solid solution strengthening within a parameter-free model based on the effective medium representation of an alloy. The results reveal intricate interactions between alloy components, which we analyze by means of a simple model of local bonding. Thanks to its computational efficiency, the methodology can be used as a basis for an adaptive learning workflow for optimal design of HEA.

Two-dimensional heterostructures are an emerging class of materials for novel applications because of extensive engineering potential by tailoring intriguing properties of different layers as well as the ones arising from their interface. A systematic investigation of mechanical, electronic, and optical properties of possible heterostructures formed by bilayer structures graphenelike ZnO and MgO monolayers is presented. Different functionality of each layer makes these heterostructures very appealing for device applications. ZnO layer is convenient for electron transport in these structures, while MgO layer improves electron collection. At the outset, all of the four possible stacking configurations across the heterostructure are mechanically stable. In addition, stability analysis using phonon dispersion reveals that the AB stacking formed by placing the Mg atom on top of the O atom of the ZnO layer is also dynamically stable at zero temperature. Henceforth, we have investigated the optical properties of these stable heterostructures by applying many-body perturbation theory within the framework of GW approximation and solving the Bethe-Salpeter equation. It is demonstrated that strong excitonic effects reduce the optical band gap to the visible light spectrum range. These results show that this new two-dimensional form of ZnO/MgO heterostructures open an avenue for novel optoelectronic device applications.

We performed angle-resolved photoemission spectroscopy with microfocused beam on a topological line-nodal compound ${\mathrm{CaSb}}_2$ which undergoes a superconducting transition at the onset $T_{\mathrm{c}}\sim$ 1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the $\mathrm{\Gamma }$ point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the $Y\left(C\right)$ point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar $T_{\mathrm{c}}$'s. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in ${\mathrm{CaSb}}_2$.