Altermagnets represent a distinctive class of antiferromagnetic materials characterized by non-overlapping spin bands and attract extensive research efforts. Herein, we investigate the interplay among electronic, magnetic, and spin transport phenomena of the Janus V2SeTeO monolayer. The Janus monolayer has a direct band gap of 0.32 eV. The Janus V2SeTeO layer has an in-plane magnetic anisotropy along (110) direction. The incorporation of spin-orbit coupling (SOC) induces a Rashba-type band structure with a Rashba coefficient of 1.02 eV angstrom. The Rashba coefficient is insensitive to the compressive strain. In contrast, it is suppressed with tensile strain and becomes almost zero at 3 % tensile strain. The maximum SHC of around similar to -65 (h/e)S/cm is achieved with hole doping. The magnitudes of SHC remain comparable to those in typical topological materials. Overall, this investigation provides fundamental insights into the magnetic, Rashba, and spin transport properties of the Janus V2SeTeO altermagnet monolayer.

In this study, we propose a new concept for achieving metastable ternary transition metal nitride solid solutions, focusing on face centered cubic (fcc) structured Ti(N,B) as a model system. Combining non-reactive magnetron sputtering with computational analysis, we develop a microalloying strategy to manipulate the metallic sublattice, thereby influencing the solubility of B in the non-metal sublattice. We show that imposed tensile strain on the fcc-TiN lattice facilitates the solubility of B, with a 1.5 % strain enabling the incorporation of similar to 28.5 at.% B at the non-metal sublattice. Conversely, compressive strain hinders the formation of the fcc-Ti(N,B) solid solution, highlighting the importance of lattice manipulation in controlling solubility. At the same time, our experimental findings reveal that adding larger atoms, such as Zr, to the metal sublattice enhances the solubility of B in fcc-TiN more effectively (similar to 2 at.% Zr proves to be sufficient to solute 10 at.% B in the fcc-TiN lattice) than smaller atoms, like Cr or similar-sized Ti atoms. The size effect of the alloying atoms on the B solubility is further supported by radial distribution function analysis, showing lower local lattice distortions for Zr compared to Cr.

The construction of molecular photogears that can achieve through-space transmission of the unidirectional double-bond rotary motion of light-driven molecular motors onto a remote single-bond axis is a formidable challenge in the field of artificial molecular machines. Here, we present a proof-of-principle design of such photogears that is based on the possibility of using stereogenic substituents to control both the relative stabilities of two helical forms of the photogear and the double-bond photoisomerization reaction that connects them. The potential of the design was verified by quantum-chemical modeling through which photogearing was found to be a favorable process compared to free-standing single-bond rotation ("slippage"). Overall, our study unveils a surprisingly simple approach to realizing unidirectional photogearing. A stereochemical approach to transmitting the directional double-bond rotary motion of light-driven molecular motors through space onto a remote single-bond axis is put forth and successfully tested by means of quantum-chemical modeling. A key result in the assessment of the approach is that the desired photogearing process is favorable compared to the undesired, free-standing single-bond rotation process ("slippage") with which it competes.**image

The Ti-B-N system offers a wide range of possible meta(stable) phases, making it interesting for science and industry. However, the solubility for B within the face-centered cubic (fcc)-TiN lattice is rather limited and less studied, especially without forming B-rich phases. Therefore, we address how chemistries along the TiN-TiB2 or TiN-TiB tie-line influence this B-solubility. The variation between these two tie-lines is realized through non-reactive co-sputtering of a TiN, TiB2, and Ti target. We show that for variations along the TiN-TiB tie-line, even 8.9 at.% B (equivalent to 19.3 at.% non-metal fractions) can fully be incorporated into the fcc-TiNy lattice without forming other B-containing phases. The combination of detailed microstructural characterization through X-ray diffraction and transmission electron microscopy with ab initio calculations of fcc-Ti1-xNBx, fcc-TiN1-xBx, and fcc-TiN1-2xBx solid solutions indicates that B essentially substitutes N. The single-phase fcc-TiB0.17N0.69 (the highest B-containing sample along the TiN-TiB tie-line studied) exhibits the highest hardness H of 37.1 +/- 1.9 GPa combined with the highest fracture toughness K-IC of 3.0 +/- 0.2 MPa<middle dot>m(1/2) among the samples studied. These are markedly above those of B-free TiN0.87 having H = 29.2 +/- 2.1 GPa and K-IC = 2.7 +/-<0.1 MPa<middle dot>m(1/2).

The decoherence of point defect qubits is often governed by the electron spin-nuclear spin hyperfine interaction that can be parameterized by using ab inito calculations in principle. So far most of the theoretical works have focused on the hyperfine interaction of the closest nuclear spins, while the accuracy of the predictions for distinct nuclear spins is barely discussed. Here we demonstrate for the case of the NV center in diamond that the absolute relative error of the computed hyperfine parameters can exceed 100% using an industry standards first-principles code. To overcome this issue, we implement an alternative method and report on significantly improved hyperfine values with O(1%) relative mean error at all distances. The provided accurate hyperfine data for the NV center enables high-precision simulation of NV quantum nodes for quantum information processing and positioning of nuclear spins by comparing experimental and theoretical hyperfine data.

Transition metal nitride alloys possess exceptional properties, making them suitable for cutting applications due to their inherent hardness or as protective coatings due to corrosion resistance. However, the computational demands associated with predicting these properties using ab initio methods can often be prohibitively high at the conditions of their operation at cutting tools, that is, at high temperatures and stresses. Machine learning approaches have been introduced into the field of materials modeling to address the challenge. In this paper, we present an active learning workflow to model the properties of our benchmark alloy system cubic B1 Ti0.5Al0.5N at temperatures up to 1500 K. With a minimal requirement of prior knowledge about the alloy system for our workflow, we train a moment tensor potential (MTP) to accurately model the material's behavior over the entire temperature range and extract elastic and vibrational properties. The outstanding accuracy of MTPs with relatively little training data demonstrates that the presented approach is highly efficient and requires about two orders of magnitude less computational resources than state-of-the-art ab initio molecular dynamics.

Units of measure are central in all areas of science and technology. There are several ontologicalframeworks aiming to improve interoperability and precision in digital data exchange of quantitiesinvolving units. We introduce an ontology that specifically targets challenges for handling units acrossdatabases of computational and experimental data from various sources. The ontology is created usingdefinition files from the community-driven OPTIMADE standard for a common API for materialsdatabases. The resulting ontology allows addressing data integration challenges encountered in thateffort, including (i) referencing both specific and more general instances of units that have changedover time; (ii) the use of unit systems to define short domain-relevant identifiers for a collection of unitsthat make sense within a specific subdomain, rather than having to adopt globally standardized namingschemes; (iii) specifications of relationships between units that enables tools to convert between them;and (iv) units not part of the International System of Units (SI) can be represented without defining themin SI units or using SI system conventions. This paper provides a brief survey of existing ontologiesfor units of measure and then presents the design and discuss features of an ontology based on theOPTIMADE unit definitions.

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interaction between charge carriers and ionic rearrangements. Here, we investigate the interaction of photogenerated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide (BAPI), varying the inorganic lamellae thickness in the 2D single crystals. We determine the directionality of the transition dipole moments (TDMs) of the relevant phonon modes (in the 0.3-3 THz range) by the angle- and polarization-dependent THz transmission measurements. We find a clear anisotropy of the in-plane photoconductivity, with a similar to 10% reduction along the axis parallel with the transition dipole moment of the most strongly coupled phonon. Detailed calculations, based on Feynman polaron theory, indicate that the anisotropy originates from directional electron-phonon interactions.

Breathing discrete vortices are obtained as numerically exact and generally quasiperiodic, localized solutions to the discrete nonlinear Schr & ouml;dinger equation with cubic (Kerr) on-site nonlinearity, on a two-dimensional square lattice with nearest-neighbor couplings. We identify and analyze three different types of solutions characterized by circulating currents and time- periodically oscillating intensity distributions, two of which have been discussed in earlier works while the third being, to our knowledge, presented here for the first time. (i) A vortex-breather, constructed from the anticontinuous limit as a superposition of a single-site breather and a discrete vortex surrounding it, where the breather and vortex are oscillating at different frequencies. (ii) A charge-flipping vortex, constructed from an anticontinuous solution with an even number of sites on a closed loop, with alternating sites oscillating at different frequencies. (iii) A breathing vortex, constructed by continuation of a non-resonating linear internal eigenmode of a stationary discrete vortex. We illustrate by examples, using numerical Floquet analysis for solutions obtained from a Newton scheme, that linearly stable solutions exist from all three categories, at sufficiently strong discreteness.

Long-wavelength near-infrared light-emitting diodes (NIR LEDs) with peak emission wavelengths beyond 900 nm are of critical importance for various applications including night vision, biomedical imaging, sensing and optical communications. However, the low radiance and poor operational stability of state-of-the-art long-wavelength NIR LEDs based on soft materials remain the most critical factors limiting their practical applications. Here we develop NIR LEDs emitting beyond 900 nm with improved performance through the rational manipulation of p doping in all-inorganic tin perovskites (CsSnI3) by retarding and controlling the crystallization process of perovskite precursors in tin-rich conditions. The resulting NIR LEDs exhibit a peak emission wavelength at 948 nm, high radiance of 226 W sr-1 m-2 and long operational half-lifetime of 39.5 h at a high constant current density of 100 mA cm-2. Our demonstration of efficient and stable NIR LEDs operating at high current densities may also open up new opportunities towards electrically pumped lasers. Controlling the intrinsic doping of lead-free perovskites enables near-infrared LEDs emitting at 948 nm with a peak radiance of 226 W sr-1 m-2 and a half-lifetime of 39.5 h.

Considering charge transfer effects and the variability of the bonding between elements with different electronegativity opens up a deeper understanding of the electronic structure and as a result many of the properties in high entropy-related materials. This study investigates the importance of the diverse bonding and chemical environments when discussing multicomponent carbide materials. A combination of ab initio calculations and X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of multicomponent thin films based on the (HfNbTiVZr)C system. The charge transfer was quantified theoretically using relaxed and nonrelaxed multicomponent as well as binary carbide reference structures, employing a fixed sphere model. High-resolution XPS spectra from (HfNbTiVZr)C magnetron-sputtered thin films displayed core-level binding energy shifts and broadening effects as a result of the complex chemical environment. Charge transfer effects and a changed electronic structure in the multicomponent material, compared with the reference binary carbides, are observed both experimentally and in the density functional theory (DFT) simulations. The observed effects loosely follow electronegativity considerations, leading to a deviation from an ideal solid solution structure assuming nondistinguishable chemically equivalent environments.

In this thesis I focus on combining the high accuracy of first-principles calculations with modern machine learning methods to make large scale investigations of industrially relevant nitride systems reliable and computationally viable. I study the electronic, thermodynamic and mechanical properties of two families of compounds: Ti_1−xAl_xN alloys at the operational conditions of industrial cutting tools and ReN_x systems at crushing pres-sures comparable to inner earth core conditions. Standard first-principles simulations of materials are usually carried out at zero temperature and pressure, and while many state-of-the-art approaches can take these effects into account, they are usually accompanied by a substantial increase in computational demand. In this thesis I therefore explore the possiblities of studying materials at extreme conditions using machine learning methods with extraordinary efficiency without loss of calculational accuracy. 

Ti_1−xAl_xN alloy coatings exhibit exceptional properties due to their inherent ability to spinodally decompose at elevated temperature, leading to age-hardening. Since the cubic B1 phase of Ti_1−xAl_xN is well-studied, available high-accuracy first-principles data served as both a benchmark and data set on which to train a machine learning interatomic potential. Using the reliable moment tensor potentials, an investigation of the accuracy and efficiency of this approach was carried out in a machine learning study. Building upon the success of this technique, implementation of a learning-on-the-fly (active learning) methodology into a workflow to determine accurate material properties with minimal prior knowledge showed great promise, while maintaining a computational demand up to two orders of magnitude lower than comparable first-principles approaches. Investigations of properties of industrially lesser desired, but sometimes present hexagonal alloy phases of Ti_1−xAl_xN are also included in this thesis, since knowledge and understanding of all competing phases can help guide development toward improving cutting tool lifetime and performance. Furthermore, while w-Ti_1−xAl_xN may not be able to compete with its cubic counterpart in terms of hardness, it shows promise for other applications due to its electronic and elastic properties. 

Metastable ReN_x phases are high energy materials due to their covalent N-N and Re-N bonds, leading to exceptional mechanical and electronic properties. Just like diamond, the hardest and arguably most famous metastable mate-rial naturally occurring on earth, they are stabilized by extreme pressures and high temperatures, but can be quenched to ambient conditions. Understanding the formation and existence of these non-equilibrium compounds may hold the key to unlocking a new generation of hard materials. In this thesis, all currently known phases of ReN_x compounds have been investigated, encompassing both experimentally observed and theoretically suggested structures. Investigations of the convex hulls across a broad pressure range were carried out, coupled with calculations of phonons in the proposed crystals to determine both energetic and dynamical stability. Overall, the studies included in this thesis focused mainly on investigation of the ground state of ReN₂ at higher pressure, where experimental results were deviating from earlier theoretical predictions. Additional research focused on specifically exploring properties and stability of novel ReN₆ at synthesis conditions using the active learning workflow to train an interatomic potential. 

Transition metal carbides are valued for high hardness, thermal and mechanical stability, but fall short in fracture toughness. Contrarily, their less hard transition metal nitride counterparts offer more favorable fracture characteristics. Here, we use magnetron-sputtering to synthesize nitrides and carbides-TiC/TaN, TiN/TaC-in a nanolaminate superlattice (SL) architecture and compare their properties (hardness, fracture toughness, thermal stability) with that of their layer materials, as well as of carbide SLs, TiC/TaC. Except for the monolithically grown TaN and TiC/TaN SLs with nominal bilayer periods above 14 nm, all other coatings are purely fccstructured and feature close-to-stoichiometric compositions, revealed by EBS-ERDA and XRF measurements. In-situ X-ray diffraction investigations indicate that the monolithically grown coatings have poor thermal stability compared to the SLs, which remain stable up until well over 1000-degrees C. While the TiC/TaC superlattices retain the highest hardness of all three systems, with 44.1 +/- 3.4 GPa at a bilayer period (Lambda) of 2 nm, the TiN/TaC system exhibits significantly higher fracture toughness values with up to 4.75 +/- 0.33 MPa root m for the Lambda = 14 nm coating. The TiC/TaN system exhibits neither hardness nor fracture toughness enhancement, as explained by the formation of a secondary hexagonal Ta2N phase.

The Open Databases Integration for Materials Design (OPTIMADE) application programming interface (API) empowers users with holistic access to a growing federation of databases, enhancing the accessibility and discoverability of materials and chemical data. Since the first release of the OPTIMADE specification (v1.0), the API has undergone significant development, leading to the v1.2 release, and has underpinned multiple scientific studies. In this work, we highlight the latest features of the API format, accompanying software tools, and provide an update on the implementation of OPTIMADE in contributing materials databases. We end by providing several use cases that demonstrate the utility of the OPTIMADE API in materials research that continue to drive its ongoing development. The Open Databases Integration for Materials Design (OPTIMADE) application programming interface (API) empowers users with holistic access to a federation of databases, enhancing the accessibility and discoverability of materials and chemical data.

We report a comparative study of temperature-dependent photoluminescence and structural dynamics of two perovskite semiconductors, the chalcogenide BaZrS3 3 and the halide CsPbBr3. 3 . These materials have similar crystal structures and direct band gaps, but we find that they have quite distinct optoelectronic and vibrational properties. Both materials exhibit thermally activated nonradiative recombination, but the nonradiative recombination rate in BaZrS3 3 is four orders of magnitude faster than in CsPbBr3, 3 , for the crystals studied here. Raman spectroscopy reveals that the effects of phonon anharmonicity are far more pronounced in CsPbBr3 3 than in BaZrS3. 3 . Further, although both materials feature a large dielectric response due to low-energy polar optical phonons, the phonons in CsPbBr3 3 are substantially lower in energy than in BaZrS3. 3 . Our results suggest that electron-phonon coupling in BaZrS3 3 is more effective at nonradiative recombination than in CsPbBr3 3 and that BaZrS3 3 may also have a substantially higher concentration of nonradiative recombination centers than CsPbBr3. 3 . The low defect concentration in CsPbBr3 3 may be related to the ease of lattice reconfiguration, typified by anharmonic bonding. It remains to be seen to what extent these differences are inherent to the chalcogenide and halide perovskites and to what extent they can be affected by materials processing.

The present work models temperature-dependent ( 500-1300K) diffusion dynamics of Ag, Au, and Cu adatoms on MoS2 as well as electronic and magnetic properties of adatom (Ag, Au, and Cu)/MoS2 systems. Modeling is done by means of ab initio molecular dynamics (AIMD) simulations that account for van der Waals corrections and electronic spin degrees of freedom in the framework of density functional theory. It is found that Ag and Au adatoms exhibit super-diffusive motion on MoS2 at all temperatures, while Cu adatoms follow a random walk pattern of uncorrelated surface jumps. The observed behavior is consistent with AIMD-calculated effective migration barriers Ea ( EaAg=190 +/- 50meV, EaAu=67 +/- 7meV, and EaCu=300 +/- 100meV) and can be understood on the basis of the considerably flatter potential energy landscapes encountered by Ag and Au adatoms on the MoS2 surface (corrugation of the order of tens of meV), as compared to Cu adatoms (corrugation >100meV). Moreover, evaluation of the electronic and magnetic properties of AIMD configurations suggest that Ag, Au, and Cu monomer adsorption induces semimetallic features in at least one spin channel of the adatom/MoS2 electronic structure at elevated temperatures. The overall results presented herein may provide insights into fabricating 2D-material-based heterostructure devices beyond graphene

Virtually noiseless due to the scarcity of spinful nuclei in the lattice, simple oxides hold promise as hosts of solid-state spin qubits. However, no suitable spin defect has yet been found in these systems. Using high-throughput first-principles calculations, we predict spin defects in calcium oxide with electronic properties remarkably similar to those of the NV center in diamond. These defects are charged complexes where a dopant atom - Sb, Bi, or I - occupies the volume vacated by adjacent cation and anion vacancies. The predicted zero phonon line shows that the Bi complex emits in the telecommunication range, and the computed many-body energy levels suggest a viable optical cycle required for qubit initialization. Notably, the high-spin nucleus of each dopant strongly couples to the electron spin, leading to many controllable quantum levels and the emergence of atomic clock-like transitions that are well protected from environmental noise. Specifically, the Hanh-echo coherence time increases beyond seconds at the clock-like transition in the defect with 209Bi. Our results pave the way to designing quantum states with long coherence times in simple oxides, making them attractive platforms for quantum technologies. Recently, long spin coherence times have been predicted for spin defects in simple oxides. Here, by using high-throughput first-principles calculations, the authors identify promising spin defects in CaO, with electronic properties similar to those of NV centers but with longer coherence times.

The yttrium-hydrogen system has gained attention because of near-ambient temperature superconductivity reports in yttrium hydrides at high pressures. We conducted a study using synchrotron single-crystal x-ray diffraction (SCXRD) at 87 to 171 GPa, resulting in the discovery of known (two YH3 phases) and five previously unknown yttrium hydrides. These were synthesized in diamond anvil cells by laser heating yttrium with hydrogen-rich precursors-ammonia borane or paraffin oil. The arrangements of yttrium atoms in the crystal structures of new phases were determined on the basis of SCXRD, and the hydrogen content estimations based on empirical relations and ab initio calculations revealed the following compounds: Y3H11, Y2H9, Y4H23, Y13H75, and Y4H25. The study also uncovered a carbide (YC2) and two yttrium allotropes. Complex phase diversity, variable hydrogen content in yttrium hydrides, and their metallic nature, as revealed by ab initio calculations, underline the challenges in identifying superconducting phases and understanding electronic transitions in high-pressure synthesized materials.

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behavior. In the higher temperature, higher symmetry phases of these materials, several complex structural features are observed. The local structure can differ greatly from the average structure and there is evidence that dynamic 2D structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI3 is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, it is revealed that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and the length and timescales of the motion are reported. Finally, the spatial arrangement of these features and their interactions are investigated and visualized, providing a comprehensive picture of local structure in the higher symmetry phases.

CrN-based alloy thin films are of interest as thermoelectric materials for energy harvesting. Ab initio calculations show that dilute alloying of CrN with 3 at. % W substituting Cr induce flat electronic bands and push the Fermi level E-F into the conduction band while retaining dispersive Cr 3d bands. These features are conducive for both high electrical conductivity sigma and high Seebeck coefficient alpha and, hence, a high thermoelectric power factor alpha(2)sigma. To investigate this possibility, epitaxial CrWxNz films were grown on c-sapphire by dc-magnetron sputtering. However, even films with the lowest W content (x = 0.03) in our study contained metallic h-Cr2N, which is not conducive for a high alpha. Nevertheless, the films exhibit a sizeable power factor of alpha(2)sigma similar to 4.7 x 10(-4) W m(-1) K-2 due to high sigma similar to 700 S cm(-1), and a moderate alpha similar to - 25 mu V/K. Increasing h-Cr2N fractions in the 0.03 < x <= 0.19 range monotonically increases sigma, but severely diminishes alpha leading to two orders of magnitude decrease in alpha(2)sigma. This trend continues with x > 0.19 due to W precipitation. These findings indicate that dilute W additions below its solubility limit in CrN are important for realizing a high thermoelectric power factor in CrWxNz films.

The advent of machine learning in materials science opens the way for exciting and ambitious simulations of large systems and long time scales with the accuracy of ab initio calculations. Recently, several pretrained universal machine learned interatomic potentials (UPMLIPs) have been published, i.e., potentials distributed with a single set of weights trained to target systems across a very wide range of chemistries and atomic arrangements. These potentials raise the hope of reducing the computational cost and methodological complexity of performing simulations compared to models that require for-purpose training. However, the application of these models needs critical evaluation to assess their usability across material types and properties. In this work, we investigate the application of the following UPMLIPs: MACE, CHGNET, and M3GNET to the context of alloy theory. We calculate the mixing enthalpies and volumes of 21 binary alloy systems and compare the results with DFT calculations to assess the performance of these potentials over different properties and types of materials. We find that the small relative energies necessary to correctly predict mixing energies are generally not reproduced by these methods with sufficient accuracy to describe correct mixing behaviors. However, the performance can be significantly improved by supplementing the training data with relevant training data. The potentials can also be used to partially accelerate these calculations by replacing the ab initio structural relaxation step.

Metal carbides are known to contain small carbon units similar to those found in the molecules of methane, acetylene, and allene. However, for numerous binary systems ab initio calculations predict the formation of unusual metal carbides with exotic polycarbon units, [C-6] rings, and graphitic carbon sheets at high pressure (HP). Here we report the synthesis and structural characterization of a HP-CaC2 polymorph and a Ca3C7 compound featuring deprotonated polyacene-like and para-poly(indenoindene)-like nanoribbons, respectively. We also demonstrate that carbides with infinite chains of fused [C-6] rings can exist even at conditions of deep planetary interiors (similar to 140GPa and similar to 3300K). Hydrolysis of high-pressure carbides may provide a possible abiotic route to polycyclic aromatic hydrocarbons in Universe.

Quantum defects are atomic defects in materials that provide resources to construct quantum information devices such as single-photon emitters and spin qubits. Recently, two-dimensional (2D) materials gained prominence as a host of quantum defects with many attractive features derived from their atomically thin and layered material formfactor. In this Perspective, we discuss first-principles computational methods and challenges to predict the spin and electronic properties of quantum defects in 2D materials. We focus on the open quantum system nature of the defects and their interaction with external parameters such as electric field, magnetic field, and lattice strain. We also discuss how such prediction and understanding can be used to guide experimental studies, ranging from defect identification to tuning of their spin and optical properties. This Perspective provides significant insights into the interplay between the defect, the host material, and the environment, which will be essential in the pursuit of ideal two-dimensional quantum defect platforms.

The effects of B-site substitution (BMn, Fe, and Co) in La-based perovskite oxides (LPOs); LaMnO3, LaFeO3, LaCoO3, as bifunctional electrocatalysts during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in metal-air batteries (MABs) under an alkaline electrolyte (pH = 13) are investigated using density functional theory (DFT). It is found that LaMnO3 exhibits higher ORR activity than others with ORR overpotential (eta ORR) of 0.57 V, but its OER activity is poor with OER overpotential (eta OER) of 1.12 V. The eta ORR (0.59 V) and eta OER (1.13 V) of LaMn0.75Fe0.25O3 closely resemble those of LaMnO3, suggesting that Fe substitution does not yield appreciable enhancements in activity. Fe substitution reduces the ORR and OER activity because the adsorption energies of intermediate species on Fe-substituted LPOs surfaces are too strong to obtain a potential determining step for ORR and OER. According to Sabatier's principle, the LaMn0.25Co0.75O3 demonstrates superior OER activity compared to the other composition, while ORR activity approximates that of LaMnO3, evidenced by eta ORR of 0.65 V and eta OER of 0.53 V. The Co-terminated LaMn0.25Co0.75O3 shows bifunctional activity higher than Mn/Co termination, indicating that Co is an active site for OER and Mn is a promoter for improved ORR activity. The effects of B-site substitution (BMn, Fe, and Co) on La-based perovskite oxides during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are investigated via density functional theory. LaMn0.25Co0.75O3 as a promising bifunctional electrocatalyst exhibits ORR/OER overpotentials of 0.65 V/0.53 V due to the presence of Mn and Co promoting electron transfer. image

Color centers in hexagonal boron nitride (hBN) have attracted considerable attention due to their remarkable optical properties enabling robust room temperature photonics and quantum optics applications in the visible spectral range. On the other hand, identification of the microscopic origin of color centers in hBN has turned out to be a great challenge that hinders the in-depth theoretical characterization, on-demand fabrication, and development of integrated photonic devices. This is also true for the blue emitter, which is a result of irradiation damage in hBN, emitting at 436 nm wavelength with desirable properties. Here, we propose the negatively charged nitrogen split interstitial defect in hBN as a plausible microscopic model for the blue emitter. To this end, we carried out a comprehensive first-principles theoretical study of the nitrogen interstitial. We carefully analyzed the accuracy of first-principles methods and showed that the commonly used HSE hybrid exchange-correlation functional fails to describe the electronic structure of this defect. Using the generalized Koopman's theorem, we fine-tuned the functional and obtained a zero-phonon photoluminescence (ZPL) energy in the blue spectral range. We showed that the defect exhibits a high emission rate in the ZPL line and features a characteristic phonon side band that resembles the blue emitter's spectrum. Furthermore, we studied the electric field dependence of the ZPL and numerically showed that the defect exhibits a quadratic Stark shift that is perpendicular to plane electric fields, making the emitter insensitive to electric field fluctuations in the first order. Our work emphasizes the need for assessing the accuracy of common first-principles methods in hBN and exemplifies a workaround methodology. Furthermore, our work is a step towards understanding the structure of the blue emitter and utilizing it in photonics applications. We elaborate on the methodology of computing color centers in periodic and cluster models of hBN. Using first-principles methods, we determined various optical properties of the nitrogen interstitial defect and compared them to experimental results.

Wurtzite TmAlN (T-m = transition metal) themselves are of interest as semiconductors with tunable band gap, insulating motifs to superconductors, and piezoelectric crystals. Characterization of wurtzite TmAlN is challenging because of the difficulty to synthesize them as single-phase solid solution and such thermodynamic, elastic properties, and high temperature behavior of wurtzite Ti1-xAlxN is unknown. Here, we investigated the high temperature decomposition behavior of wurtzite Ti1-xAlxN films using experimental methods combined with first-principles calculations. We have developed a method to grow single-phase metastable wurtzite Ti1-xAlxN (x = 0.65, 0.75, 085, and 0.95) solid-solution films by cathodic arc deposition using low duty-cycle pulsed substrate-bias voltage. We report the full elasticity tensor for wurtzite Ti1-xAlxN as a function of Al content and predict a phase diagram including a miscibility gap and spinodals for both cubic and wurtzite Ti1-xAlxN. Complementary high-resolution scanning transmission electron microscopy and chemical mapping demonstrate decomposition of the films after high temperature annealing (950 degrees C), which resulted in nanoscale chemical compositional modulations containing Ti-rich and Al-rich regions with coherent or semicoherent interfaces. This spinodal decomposition of the wurtzite film causes age hardening of 1-2 GPa.

Exploring the chemistry of materials at high pressure leads to discoveries of previously unknown compounds and phenomena. Here chemical reactions between elemental dysprosium and carbon were studied in laser-heated diamond anvil cells at pressures up to 95 GPa and temperatures of similar to 2800 K. In situ single-crystal synchrotron X-ray diffraction (SCXRD) analysis of the reaction products revealed the formation of novel dysprosium carbides, gamma-DyC2, Dy5C9, and gamma-Dy4C5, along with previously reported Dy3C2 and Dy4C3 . The crystal structures of gamma-DyC (2) and Dy (5) C (9) feature infinite flat carbon polyacene-like ribbons and cis-polyacetylene-type chains, respectively. In the structure of gamma-Dy4C5, carbon atoms form dimers and non-linear trimers. Dy3C2 contains ethanide-type carbon dumbbells, and Dy4C3 is methanide featuring single carbon atoms. Density functional theory calculations reproduce well the crystal structures of high-pressure dysprosium carbides and reveal conjugated pi-electron systems in novel infinite carbon polyanions. This work demonstrates that complex carbon homoatomic species previously unknown in organic chemistry can be synthesized at high pressures by direct reactions of carbon with metals.

Exfoliation of graphite and the discovery of the unique properties of graphene-graphite's single layer-have raised significant attention to layered compounds as potential precursors to 2D materials with applications in optoelectronics, spintronics, sensors, and solar cells. In this work, a new orthorhombic polymorph of yttrium bromide, oC16-YBr3 was synthesized from yttrium and CBr4 in a laser-heated diamond anvil cell at 45 GPa and 3000 K. The structure of oC16-YBr3 was solved and refined using in situ synchrotron single-crystal X-ray diffraction. At high pressure, it can be described as a 3D framework of YBr9 polyhedra, but upon decompression below 15 GPa, the structure motif changes to layered, with layers comprising edge-sharing YBr8 polyhedra weakly bonded by van der Waals interactions. The layered oC16-YBr3 material can be recovered to ambient conditions, and according to Perdew-Burke-Ernzerhof-density functional theory calculations, it exhibits semiconductor properties with a band gap that is highly sensitive to pressure. This polymorph possesses a low exfoliation energy of 0.30 J/m(2). Our results expand the list of layered trivalent rare-earth metal halides and provide insights into how high pressure alters their structural motifs and physical properties.

The rhenium carbide Re3C was predicted to be stable under high pressure and expected to have high hardness and low compressibility. In this study, we realise the synthesis of Re3C at megabar pressures of 105(3) and 140(5) GPa in laser-heated diamond anvil cells and characterise its structure using synchrotron single-crystal X-ray diffraction. The structure of Re3C has the monoclinic space group C2/m and is built of CRe7 capped octahedra. Our combined ab initio calculations and quantitative topological analysis support experimental structural data and further deepen the understanding of the chemical bonding in the newly synthesized compound.

Here, we present the synthesis of a novel quaternary compound, iodide carbonate Na-5(CO3)(2)I, at 18(1) and 25.1(5) GPa in laser-heated diamond anvil cells. Single-crystal synchrotron X-ray diffraction provides accurate structural data for Na-5(CO3)(2)I and shows that the structure of the material can be described as built of INa8 square prisms, distorted NaO6 octahedra, and trigonal planar CO32- units. Decompression experiments show that the novel iodide carbonate is recoverable in the N-2 atmosphere to ambient conditions. Our ab initio calculations agree well with the experimental structural data, provide the equation of state, and shed light on the chemical bonding and electronic properties of the new compound.

Transition metal nitrides, particularly those of 5d metals, are known for their outstanding properties, often relevant for industrial applications. Among these metal elements, tungsten is especially attractive given its low cost. In this high-pressure investigation of the W-N system, two novel ultra-incompressible tungsten nitride superconductors, namely W2N3 and W3N5, are successfully synthesized at 35 and 56 GPa, respectively, through a direct reaction between N2 and W in laser-heated diamond anvil cells. Their crystal structure is determined using synchrotron single-crystal X-ray diffraction. While the W2N3 solid's sole constituting nitrogen species are N3- units, W3N5 features both discrete N3- as well as N24- pernitride anions. The bulk modulus of W2N3 and W3N5 is experimentally determined to be 380(3) and 406(7) GPa, and their ultra-incompressible behavior is rationalized by their constituting WN7 polyhedra and their linkages. Importantly, both W2N3 and W3N5 are recoverable to ambient conditions and stable in air. Density functional theory calculations reveal W2N3 and W3N5 to have a Vickers hardness of 30 and 34 GPa, and superconducting transition temperatures at ambient pressure (50 GPa) of 11.6 K (9.8 K) and 9.4 K (7.2 K), respectively. Additionally, transport measurements performed at 50 GPa on W2N3 corroborate with the calculations. Two recoverable tungsten nitrides, namely W2N3 and W3N5, are synthesized using laser-heated diamond anvil cells. Both compounds exhibit a high bulk modulus, hardness, and superconducting transition temperature. image

Density functional theory is used to compare the catalytic performance of PtPdRhFeCo(100) high entropy alloy (HEA) three-way catalyst (TWC) to the conventional Pt(100) in the NO reduction step during NH3 production that supplies to passive NH3-SCR. Stronger adsorption of NO on the HEA(100) surface is beneficial to capture NO. During adsorption, the catalyst surface acts as an electron donor while the adsorbate is the acceptor on both HEA(100) and Pt(100) systems. Herein, the reaction mechanism of NO reduction can be classified into two steps: 1) NO activation and 2) product formation. During NO activation, direct NO dissociation is the preferable pathway on both HEA(100) and Pt(100) surfaces with the same Ea, whereas HNO and NOH pathways on HEA(100) are suppressed. For NH3, N2, and N2O production on HEA(100) is found to be more difficult than on Pt(100). However, the thermodynamic driving force of all reactions on HEA(100) is more spontaneous than on Pt(100). Also, the rate-determining step on HEA(100) is found to be NH3 formation different from the Pt(100), while difficult H diffusion on HEA(100) is the key factor that reduces NH3 production.

The large amount of powder diffraction data for which the corresponding crystal structures have not yet been identified suggests the existence of numerous undiscovered, physically relevant crystal structure prototypes. In this paper, we present a scheme to resolve powder diffraction data into crystal structures with precise atomic coordinates by screening the space of all possible atomic arrangements, i.e., structural prototypes, including those not previously observed, using a pre-trained machine learning (ML) model. This involves (i) enumerating all possible symmetry-confined ways in which a given composition can be accommodated in a given space group, (ii) ranking the element-assigned prototype representations using energies predicted using and perturbing atoms along the degree of freedom allowed by the Wyckoff positions to match the experimental diffraction data, and (iv) validating the thermodynamic stability of the material using density-functional theory. An advantage of the presented method is that it does not rely on a database of previously observed prototypes and is, therefore capable of finding crystal structures with entirely new symmetric arrangements of atoms. We demonstrate the workflow on unidentified x-ray diffraction spectra from the ICDD database and identify a number of stable structures, where a majority turns out to be derivable from known prototypes. However, at least two are found not to be part of our prior structural data sets.

Transition metal diboride thin films are promising functional materials for their outstanding mechanical properties and thermal stability. By combining experiment and simulations, we discuss angular distribution of the sputtered species, their scattering in the gas phase, re-sputtering and potential evaporation from the grown films for the complex evolution of film compositions, as well as energetic preference for vacancy formation and competing phases as factors for governing the phase constitution. By co-sputtering from two compound targets, we developed phase-pure crystalline (Ta,Al)B2 solid solution thin films and correlate the stoichiometry changes with the evolution of their microstructure, hardness, and elastic modulus. {GRAPHICAL ABSTRACT}

Point defect qubits in semiconductors have demonstrated their outstanding capabilities for high spatial resolution sensing generating broad multidisciplinary interest. Hexagonal boron nitride (hBN) hosting point defect qubits have recently opened up new horizons for quantum sensing by implementing sensing foils. The sensitivity of point defect sensors in hBN is currently limited by the linewidth of the magnetic resonance signal, which is broadened due to strong hyperfine couplings. Here, we report on a vacancy-related spin qubit with an inherently low symmetry configuration, the VB2 center, giving rise to a reduced magnetic resonance linewidth at zero magnetic fields. The VB2 center is also equipped with a classical memory that can be utilized for storing population information. Using scanning transmission electron microscopy imaging, we confirm the existence of the VB2 configuration in free-standing monolayer hBN.

Machine-learning interatomic potentials (MLIPs) offer a powerful avenue for simulations beyond length and timescales of ab initio methods. Their development for investigation of mechanical properties and fracture, however, is far from trivial since extended defects-governing plasticity and crack nucleation in most materials-are too large to be included in the training set. Using TiB2 as a model ceramic material, we propose a training strategy for MLIPs suitable to simulate mechanical response of monocrystals until failure. Our MLIP accurately reproduces ab initio stresses and fracture mechanisms during room-temperature uniaxial tensile deformation of TiB2 at the atomic scale ( approximate to 103 atoms). More realistic tensile tests (low strain rate, Poisson's contraction) at the nanoscale ( approximate to 104-106 atoms) require MLIP up-fitting, i.e., learning from additional ab initio configurations. Consequently, we elucidate trends in theoretical strength, toughness, and crack initiation patterns under different loading directions. As our MLIP is specifically trained to modelling tensile deformation, we discuss its limitations for description of different loading conditions and lattice structures with various Ti/B stoichiometries. Finally, we show that our MLIP training procedure is applicable to diverse ceramic systems. This is demonstrated by developing MLIPs which are subsequently validated by simulations of uniaxial strain and fracture in TaB2, WB2, ReB2, TiN, and Ti2AlB2.

Face -centered cubic transition metal carbides exhibit high melting points and hardness, making them prominent candidates for protective coating applications. Vanadium carbide (VC) has typical characteristics of transition metal carbides. It serves as model material in this study, in which we showcase the effect of superlattice architecture on mechanical properties and fracture toughness. While beneficial effects of superlattice arrangement have been demonstrated for a series of nitride -based coatings, transition metal carbides remain largely unexplored territory. Following density functional theory based ab initio predictions on lattice and shear modulus mismatch, we develop VC/ZrC and VC/HfC superlattice coatings synthesized via pulsed DC sputter deposition. The bilayer periods ( Lambda ) of our fully fcc-structured polycrystalline coatings range between 2 - 50 nm (indicated by X-ray diffraction, transmission and scanning electron microscopy). The chemical composition is close to 1:1 stoichiometry (from X-ray fluorescence, elastic back -scattering spectrometry and elastic recoil detection analysis). Both superlattice series exhibit a strong dependence of hardness, elastic modulus, and fracture toughness on their bilayer periods, which can only be correlated with the in -plane stress variations for VC/ZrC. The VC/HfC superlattices provide their peak -hardness of 36.0 +/- 1.6 GPa for Lambda = 6 nm and their peak in fracture toughness of 3.5 +/- 0.5 MPa root m for Lambda = 10 nm.

Color centers in diamond are at the forefront of the second quantum revolution. A handful of defects are in use, and finding ones with all the desired properties for quantum applications is arduous. By using high-throughput calculations, we screen 21,607 defects in diamond and collect the results in the ADAQ database. Upon exploring this database, we find not only the known defects but also several unexplored defects. Specifically, defects containing sodium stand out as particularly relevant because of their high spins and predicted improved optical properties compared to the NV center. Hence, we studied these in detail, employing high-accuracy theoretical calculations. The single sodium substitutional (NaC) has various charge states with spin ranging from 0.5 to 1.5, ZPL in the near-infrared, and a high Debye-Waller factor, making it ideal for biological quantum applications. The sodium vacancy (NaV) has a ZPL in the visible region and a potential rare spin-2 ground state. Our results show sodium implantation yields many interesting spin defects that are valuable additions to the arsenal of point defects in diamond studied for quantum applications.

Solid-state sodium batteries require effective electrolytes that conduct at room temperature. The Na(3)PnCh(4) (Pn = P, Sb; Ch = S, Se) family has been studied for their high Na ion conductivity. The population of Na vacancies, which mediate ion diffusion in these materials, can be enhanced through aliovalent doping on the pnictogen site. To probe the microscopic role of extrinsic doping and its impact on diffusion and phase stability, we trained a machine learning force field for Na3-xWxSb1-xS4 based on an equivariant graph neural network. Analysis of large-scale molecular dynamics trajectories shows that an increased Na vacancy population stabilizes the global cubic phase at lower temperatures with enhanced Na ion diffusion and that the explicit role of the substitutional W dopants is limited. In the global cubic phase, we observe large and long-lived deviations of atoms from the averaged symmetry, echoing recent experimental suggestions. Evidence of correlated Na ion diffusion is also presented that underpins the suggested superionic nature of these materials.

We study cross -relaxation features near zero magnetic field with ensembles of nitrogen -vacancy (NV) centers in diamond and examine their properties in samples with a range of 0.9-16.0 ppm of NV concentrations. The observed NV -NV cross -relaxation features between differently oriented NV centers in high (>= 0.9 ppm)NV-density samples hold promise for a variety of magnetometry applications where microwave fields (or any bias field) disturb the system under study. We theoretically determine the values of the bias magnetic fields corresponding to cross relaxations between different axes and experimentally validate them. The behavior of zero -field cross -relaxation features as a function of temperature is also investigated.

Hexagonal boron nitride (hBN) has recently gained attention as a solid state host of quantum emitters. In particular, the recently discovered B centers show promise for integration in scalable quantum technologies thanks to site-specific defect generation and a reproducible wavelength. Here we employ spectral hole burning spectroscopy and resonant polarization measurements to observe nearly coherent hBN quantum emitters, both as singles and in ensembles, with three discrete polarization axes indicative of a C (2v) symmetry defect. Our results constitute an important milestone toward the implementation of hBN quantum emitters in integrated quantum photonics.

We study the fluorescence of nanodiamond ensembles as a function of static external magnetic field and observe characteristic dip features close to the zero field with potential for magnetometry applications. We analyze the dependence of the feature's width and the contrast of the feature on the size of the diamond (in the range 30 nm-3000 nm) and on the strength of a bias magnetic field applied transversely to the field being scanned. We also perform optically detected magnetic resonance (ODMR) measurements to quantify the strain splitting of the zero-field ODMR resonance across various nanodiamond sizes and compare it with the width and contrast measurements of the zero-field fluorescence features for both nanodiamonds and bulk samples. The observed properties provide compelling evidence of cross-relaxation effects in the NV system occurring close to zero magnetic fields. Finally, the potential of this technique for use in practical magnetometry is discussed.

In a GraphQL Web API, a so-called GraphQL schema defines the types of data objects that can be queried, and so-called resolver functions are responsible for fetching the relevant data from underlying data sources. Thus, we can expect to use GraphQL not only for data access but also for data integration, if the GraphQL schema reflects the semantics of data from multiple data sources, and the resolver functions can obtain data from these data sources and structure the data according to the schema. However, there does not exist a semantics-aware approach to employ GraphQL for data integration. Furthermore, there are no formal methods for defining a GraphQL API based on an ontology.In this work, we introduce a framework for using GraphQL in which a global domain ontology informs the generation of a GraphQL server that answers requests by querying heterogeneous data sources.The core of this framework consists of an algorithm to generate a GraphQL schema based on an ontology and a generic resolver function based on semantic mappings. We provide a prototype, OBG-gen, of this framework, and we evaluate our approach over a real-world data integration scenario in the materials design domain and two synthetic benchmark scenarios (Linköping GraphQL Benchmark and GTFS-Madrid-Bench). The experimental results of our evaluation indicate that: (i) our approach is feasible to generate GraphQL servers for data access and integration over heterogeneous data sources, thus avoiding a manual construction of GraphQL servers, and (ii) our data access and integration approach is general and applicable to different domains where data is shared or queried via different ways.

Lead-free halide double perovskite (HDP) Cs2AgBiBr6 has set a benchmark for research in HDP photoelectric applications due to its attractive optoelectronic properties. However, its narrow absorption range is a key limitation of this material. Herein, a novel dopant, palladium (Pd), is doped into Cs2AgBiBr6 and significantly extends the absorption to approximate to 1400 nm. Pd2+ ions are partially doped in the host lattice, most probably replacing Ag atoms and introducing a sub-bandgap state within the host bandgap, as indicated by the combination of spectroscopical measurements and theoretical calculations. Importantly, this sub-bandgap state extends the photoresponse of Cs2AgBiBr6 up to the NIR-II region of 1300 nm, setting a new record for HDPs. This work demonstrates a novel and efficient dopant for HDPs and highlights the effectiveness of employing a sub-bandgap to broaden the absorption of HDPs, shedding new light on tailoring large bandgap HDPs for NIR optoelectronic applications.

MAB phases (MABs) are atomically-thin laminates of ceramic/metallic-like layers, having made a breakthrough in the development of 2D materials. Though offering a vast chemical and phase space, relatively few MABs have been synthesised. To guide experiments, we perform high-throughput ab initio screening of MABs that combine group 4-7 transition metals (M); Al, Si, Ga, Ge, or In (A); and boron (B) focusing on their phase stability trends and mechanical properties. Considering the 1:1:1, 2:1:1, 2:1:2, 3:1:2, 3:1:3, and 3:1:4 M:A:B ratios and 10 phase prototypes, synthesisability of a single-phase compound for each elemental combination is estimated through formation energy spectra of competing dynamically stable MABs. Based on the volumetric proximity of energetically-close phases, we identify systems in which volume-changing deformations may facilitate transformation toughening. Subsequently, chemistry- and phase-structure-related trends in the elastic stiffness and ductility are predicted using elastic-constants-based descriptors. The analysis of directional Cauchy pressures and Young's moduli allows comparing mechanical response parallel and normal to M-B/A layers. The suggested promising MABs include Nb 3 AlB 4 , Cr 2 SiB 2 , Mn 2 SiB 2 or the already synthesised MoAlB.