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In the present work, we numerically and experimentally study the Co 3 - x Ni x O 4 (spinel-like oxides) system. Using the perturbative density functional theory (p-DFT) method, we start the study from the homogeneous sample ( x = 0 ), obtaining the main electronic properties (band structure (BS), density of states (DOS), and Fermi surface (FS)). Subsequently, we doped (x) with Ni atoms in different proportions (0–7% respectively, taking 56 atoms as 100% and the percentage of doping, on this percentage). As we increase the doping (x ≠ 0) , we have found that the forbidden gap decreases and the Fermi energy (FE) decreases, causing the material to exhibit a transition phase for a particular doping value. In addition, we find that more bands are generated when the system is doped, which would be responsible for the phase transition. The data from the theoretical analysis carried out in this paper was compared with the experimental data of various widely accepted works. Some of the results, when compared with the information available from the experimental ones, show good agreement. [ABSTRACT FROM AUTHOR]

In this paper, we study the electrical conductance of a molecular bridge consisting of a graphyne ring with the chemical formula C18H6 connected to two cumulene electrodes using the tightbinding model. We then compare its conductance to a similar system in which the graphyne ring has been replaced by a benzene molecule. To do this, we first obtain the structural characteristics of the studied rings using density functional theory, and then use the method of matching levels, to obtain the tight-binding parameters, i.e., the on-site and hopping energies for the benzene and graphyne rings. Using these parameters, we study the electrical conductance for these two molecules, the benzene and the graphyne rings. We conclude that when these two molecules are in the same position between two cumulene electrodes, they exhibit the same electrical properties. However, in the graphyne ring, a phase transition from metal to semiconductor or vice versa can be created with less energy than in a benzene ring. [ABSTRACT FROM AUTHOR]

This paper presents a simulation of the structural and optoelectronic properties of Scandium Arsenide (ScAs) and Aluminum Arsenide (AlAs) compounds. Theoretical modeling was performed using ab initio first-principles calculations, specifically the density functional theory (DFT), and the Mindlab numerical software. The software used two methods: the full-potential muffin-tin orbital method (FP-LMTO) and the full-potential plane-wave method (FP-LAPW). These two methods are employed to solve the Schrödinger equation. The exchange correlation effects have been computed using two different approximations: the generalized gradient approximation (GGA) and the local density approximation (LDA). Our findings indicate that the zinc blende structure (B3) is the stable phase, while the Wurtzite phase (B4) is metastable for the AlAs compound. On the other hand, the ScAs compound crystallizes in the NaCl phase (B1). The AlAs compounds undergo three phase transitions: B3→B4, B3→B2 and B3→B1. In contrast, ScAs does not undergo any transition. The obtained results for equilibrium energies, lattice parameters and gap energies are in closer agreement with the experimental and theoretical data. The AlAs compound exhibits a semiconducting character, while ScAs exhibits a semi-metallic character. Additionally, the refractive index of these two compounds is similar to that of silicon, which is crucial for their application in photovoltaic cells. [ABSTRACT FROM AUTHOR]

An attempt to compare and describe the differences in the electron density distribution between two phase structures of AlOOH has been made. High-resolution, high-pressure experiments with α-AlOOH diaspore were conducted using single-crystal synchrotron X-ray diffraction data. A multipole model of experimental electron density in the α-AlOOH single crystal was refined. Simultaneously, similar multipole refinement was conducted for another phase of diaspore (δ-AlOOH), this time based on a previously published data set. Both results were compared and supported by density functional theory (DFT) calculations. Although the results are affected by the limited quality of the data, it is clear that the phase transition caused significant changes in the shape and arrangement of the atomic basins. Atomic basins are a much better tool to present subtle electron density distribution changes than traditional polyhedra. Straightforward comparison of datasets available in older scientific papers and current datasets is challenging because of differences in data quality and collection parameters. However, augmenting experimental data with computational results can help reveal important information in even incomplete datasets. [ABSTRACT FROM AUTHOR]

Density functional theory plus Hubbard U (DFT+U) methodology was used to calculate the structures and energetic landscapes of CeSiO4, including its stetindite and scheelite phases from ambient pressure to ~24 GPa. To ensure accurate simulations of the high-pressure structures, assessments of strain-stress methods and stress-strain methods were conducted in prior, with the former found to have a better agreement with the experimental result. From DFT calculations the equation of states (EOS) of both stetindite and scheelite were further obtained, with the fitted bulk moduli being 182(2) GPa and 190.0(12) GPa, respectively. These results were found to be consistent with the experimental values of 177(5) GPa and 222(40) GPa. Furthermore, the calculated energetics suggest that the stetindite structure is more thermodynamically stable than the scheelite structure at a pressure lower than 8.35 GPa. However, the stetindite → scheelite phase transition was observed experimentally at a much higher pressure of ~15 GPa. A further phonon spectra investigation by the density functional perturbation theory (DFPT) indicated the Eg¹ mode is being softened with pressure and becomes imaginary after 12 GPa, which is a sign of the lattice instability. Consequently, it was concluded that the stetindite → scheelite transition is predominantly initiated by the lattice instability under high-pressure. [ABSTRACT FROM AUTHOR]

The paper is aimed to understand the key phonon modes that are responsible for the temperature dependent structural phase transition and negative thermal expansion of Hg2Cl2 compound for the first time with the aid of density functional theory and Born-Oppenheimer on the fly molecular dynamics calculations. The phonon dispersion spectra, phonon density of states and Grüneisen parameters for the body-centered tetragonal and base-centered orthorhombic phases of the compound have been explored in detail. The order parameter associated with the phase transition and its nature has also been reported herewith. We believe that the present study will not only help for futuristic designs of improved functionalized systems with Hg2Cl2 compound but also can augment their applications in thermoelectric conversion systems, fibre-optic communications, thermal expansion compensators and in fuel cells. [ABSTRACT FROM AUTHOR]

This work is motivated by the recent fabrication of a new four-atom-thick hexagonal polymorph from group IV monochalcogenide, so-called γ -GeSe (Lee et al 2021 Nano Lett. 21 4305). In this paper, we propose and examine the structural characteristics, electronic properties, and carrier mobility of monolayers Janus γ -Ge2 XY ( X / Y = S, Se, or Te) based on comprehensive first-principles calculations. Monolayers γ -Ge2 XY are confirmed to be structurally stable. Our calculations reveal that γ -Ge2 XY monolayers are indirect semiconductors with Mexican-hat-like dispersions in the top valence band. While the effect of the electric field on the energy band dispersions of γ -Ge2 XY monolayers is weak, the energy band dispersions are changed drastically in the presence of strain, especially compressive strain. Interestingly, a structural phase transition from semiconductor to metal is observed in γ -Ge2 XY under compressive strain. γ -Ge2STe and γ -Ge2SeTe possess high electron mobility with values of 3.22 × 10 3 and 8.33 × 10 3 cm2 V−1 s−1, respectively. Our findings not only explore the fundamental physical properties of γ -Ge2 XY but also open up new opportunities in the design of high-performance electronic nanodevices based on layered nanomaterials with Mexican-hat-like dispersions. [ABSTRACT FROM AUTHOR]

The recent development of high‐energy LiCoO2 (LCO) and progress in the material recycling technology have brought Co‐based materials under the limelight, although their capacity still suffers from structural instability at highly delithiated states. Thus, in this study, a secondary doping ion substitution method is proposed to improve the electrochemical reversibility of LCO materials for Li‐ion batteries. To overcome the instability of LCO at highly delithiated states, Na ions are utilized as functional dopants to exert the pillar effect at the Li sites. In addition, Fe‐ion substitution (secondary dopant) is performed to provide thermodynamically stable surroundings for the Na‐ion doping. Density functional theory calculations reveal that the formation energy for the Na‐doped LCO is significantly reduced in the presence of Fe ions. Na and Fe doping improve the capacity retention as well as the average voltage decay at a cutoff voltage of 4.5 V. Furthermore, structural analysis indicates that the improved cycling stability results from the suppressed irreversible phase transition in the Na‐ and Fe‐doped LCO. This paper highlights the fabrication of high‐energy Co‐rich materials for high voltage operations, via a novel ion substitution method, indicating a new avenue for the manufacturing of layered cathode materials with a long cycle life. [ABSTRACT FROM AUTHOR]

In this paper, a new version of the Stillinger–Weber (SW) potential for wurtzite GaN is presented, by which we systematically explore the structural and thermodynamical properties of native point defects and their complexes. In parallel, the semi-empirical Modified Embedded-Atom Method (MEAM) potential is selected for comparison. The SW and MEAM potentials are assessed by the reproduction of the fundamental properties of wurtzite GaN and by the ability to describe the inversion domain boundaries and the wurtzite–rocksalt phase transition. Then the structural search of native point defects and their complexes in GaN is implemented using both SW and MEAM potentials with the benchmark of Density Functional Theory (DFT) calculations. Besides vacancies and antisites, four N and five Ga interstitials are confirmed by refining the DFT calculations, among which two N split interstitials N + − N ⟨ 2 1 ̄ 1 ̄ 0 ⟩ and N + − Ga ⟨ 01 1 ̄ 0 ⟩ , and two Ga split interstitials, Ga + − Ga ⟨ 01 1 ̄ 0 ⟩ − g and Ga + − N ⟨ 01 1 ̄ 0 ⟩ , are observed for the first time. The SW potential correctly predicts the octahedral occupation GaOct to be the most stable Ga interstitial, while the MEAM potential predicts the ground state of the N + − N ⟨ 01 1 ̄ 0 ⟩ split interstitial ( N + − N ⟨ 01 1 ̄ 0 ⟩ − g) as the most stable N interstitial. However, neither of the two potentials could simultaneously generate the most stable configurations of N and Ga interstitials. The investigations of point defect complexes reveal that N octahedral Frenkel [FrenkelOct(N)] and paired antisite (NGaGaN) defects are unstable and get converted into V N ⊕ N + − N ⟨ 01 1 ̄ 0 ⟩ − g configurations with different separations between VN and N + − N ⟨ 01 1 ̄ 0 ⟩ − g point defects based on the DFT calculations. The formation energies calculated by the DFT and SW potential demonstrate that Schottky, Ga octahedral Frenkel [FrenkelOct(Ga)], and V N ⊕ N + − N ⟨ 01 1 ̄ 0 ⟩ − g point defect complexes are energetically feasible and that they should not dissociate into two isolated point defects. In contrast, the MEAM potential predicts the dissociation to be exothermic for Schottky and V N ⊕ N + − N ⟨ 01 1 ̄ 0 ⟩ − g. Overall, the structural features concerned with N–N or Ga–Ga bonds relaxed by the SW potential are more consistent with DFT calculations than the MEAM counterpart. [ABSTRACT FROM AUTHOR]

High-pressure single-crystal X-ray diffraction, intermolecular interaction energy calculations, and density functional theory are used to examine the structure and emission properties of the classical organic rotor hexaphenylbenzene (C6Ph6) during hydrostatic compression to 4.14 GPa. Under applied pressure, the inter-phenyl distance in intermolecular dimers with displaced-stacked conformations gradually shortens, indicating the likelihood for hexaphenylbenzene to undergo mechanofluorochromism, while the stability of the crystal is governed by T-shaped intermolecular phenyl dimers. The theoretical band gaps and UV absorption spectra of hexaphenylbenzene at ambient pressure and 1.05 GPa predict an absorption red-shift by about 50 nm, in agreement with shortening of the intermolecular displaced-stacked interactions in the crystal under pressure. Compression of the intermolecular interactions in hexaphenylbenzene prompts a non-centrosymmetric to centrosymmetric phase transition from orthorhombic Pna21 to monoclinic P21/c at 1.05 GPa, as the approximately hexagonally-packed layers of hexaphenylbenzene molecules adopt a perfect hexagonal arrangement. This pressure-stimulated phase transition involving gain of centrosymmetry is an interesting structural phenomenon that is previously unreported in organic crystals. [ABSTRACT FROM AUTHOR]

Iron phosphate materials have attracted a lot of attention due to their potential as cathode materials for lithium-ion rechargeable batteries. It has been shown that lithium insertion or extraction depends on the Fe mixed valence and reduction or oxidation of the Fe ions' valences. In this paper, we report a new synthesis method for the Fe3(PO3OH)4(H2O)4 mixed valence iron phosphate. In addition, we perform temperature-dependent measurements of structural and physical properties in order to obtain an understanding of electronic–structural interplay in this compound. Scanning electron microscope images show needle-like single crystals of 50 μm to 200 μm length which are stable up to approximately 200 °C, as revealed by thermogravimetric analysis. The crystal structure of Fe3(PO3OH)4(H2O)4 single crystals has been determined in the temperature range of 90 K to 470 K. A monoclinic isostructural phase transition was found at ~213 K, with unit cell volume doubling in the low temperature phase. While the local environment of the Fe2+ ions does not change significantly across the structural phase transition, small antiphase rotations occur for the Fe3+ octahedra, implying some kind of electronic order. These results are corroborated by first principle calculations within density functional theory, which also point to ordering of the electronic degrees of freedom across the transition. The structural phase transition is confirmed by specific heat measurements. Moreover, hints of 3D antiferromagnetic ordering appear below ~11 K in the magnetic susceptibility measurements. Room temperature visible light absorption is consistent with the Fe2+/Fe3+ mixed valence. [ABSTRACT FROM AUTHOR]

In this paper, the first‐principles calculations based on the Density Functional Theory (DFT) have been used to study the effect of strain on the structural, electronic, optical and thermoelectric properties of the puckered Si2SeS monolayer. Our calculations show that the puckered Si2SeS monolayer has an indirect band gap of 1.30 eV at the equilibrium state, which can be tuned by biaxial strain and the semiconductor–metal phase transition occurs at −10%. Interestingly, a high absorption coefficient exceeds 107 cm−1 in the visible light region under compression biaxial strain was predicted. The electronic Figure of merit (ZTe) of puckered Si2SeS monolayer reaches 2.55 under the tensile biaxial strain of +6%. The presented results show the promising potential of puckered Si2SeS monolayer for optoelectronic and energy conversion applications. [ABSTRACT FROM AUTHOR]

This paper reports for the first time an in-depth study based on first-principle calculations to unveil the underlying physics that governs the pressure induced structural phase transitions of Hg2Cl2 compound at room temperature. The phonon dispersion relations and phonon density of states have been critically explored for the tetragonal and orthorhombic phases of the compound to unveil the phonon modes associated with the phase transitions. The nature of the phase transition whether it is "displacive" or of "order–disorder" type has also been explored. We believe that the present study based on density functional theory and Born–Oppenheimer molecular dynamics calculations will help understand the underlying physics behind the above referred phase transitions and the anisotropic behavior of the compound, which in turn bears technologically significant relevance for its applications in optoelectronic devices, acousto-optic tunable filters, and in fiber-optic communication systems. [ABSTRACT FROM AUTHOR]

The phase-field crystal (PFC) method is known as a relatively new continuum approach for describing the microstructural dynamics of materials during structural and phase transitions. In contrast to molecular dynamics (MD) or density functional theory (DFT) methods, PFC allows description processes on diffusion time scales while maintaining an atomistic spatial resolution which explains its attraction to scientists over the last 20 years. After certain development, PFC serves today as a solution of a wide variety of new and increasingly complex problems. As a result, dozens of various modifications of the original PFC model are already helping to study special features in material science. New possibilities and challenges arise when extended spatio-temporal correlations are introduced in PFC model. The current paper briefly systematizes and characterizes PFC model's applications mainly focused on spatio-temporal correlations, systems with memory and models suitable for the description of non-equilibrium phase transformations. [ABSTRACT FROM AUTHOR]

Electrocatalyst design is an important approach to prompt the commercialization of water electrolysis technologies. In this work, a ruthenium doped MoS2/AB heterostructure is synthesized as an electrocatalyst for the hydrogen evolution reaction (HER) through hydrothermal and annealing processes. The physical–chemical characterization studies show that the MoS2/AB heterostructure and the incorporation of Ru effectively induce a phase transition from 2H to 1T-MoS2. The as-prepared Ru-MoS2/AB exhibits an excellent HER performance with a very low overpotential of 13 mV at 10 mA cm−2 and a Tafel slope of 31 mV dec−1 in 0.5 M H2SO4, remarkably higher than those of Pt/C (overpotential of 28 mV at 10 mA cm−2, 41 mV dec−1). Density functional theory calculations suggest that the H absorption on Ru bonding to S exhibits a rather low binding energy (−0.22 eV), indicating the optimum active sites of Ru near S for HER. Significantly, the Ru-MoS2/AB also demonstrates high stability under long-term discharge and elevated temperature conditions. These results suggest that the as-prepared Ru-MoS2/AB can be a promising alternative to Pt/C for water electrolysis, due to its high HER activity, easy synthesis, and good stability. [ABSTRACT FROM AUTHOR]

Two polytypic heterolayered salt-inclusion chalcogenides, o-[Na2Cl]GaQ2 and t-[Na2Cl]GaQ2, were obtained via a NaCl/NaI flux-assisted synthesis, as part of an investigation of the Na–Ga–Q (Q = S and Se) system. The use of a different flux, NaBr/NaI, in the Na–Ga–Se system did not lead to the formation of salt-inclusion phases, but instead the novel Na2GaSe3 and Na4Ga2Se5 phases were obtained. Thermal and electronic properties of [Na2Cl]GaS2 materials were investigated with differential scanning calorimetry, post-quenching ex situ powder X-ray diffraction (PXRD), high-temperature PXRD, and enthalpy and electronic structure calculations via density functional theory. Those studies determined the absence of any temperature-induced phase transition between the o-[Na2Cl]GaS2 and t-[Na2Cl]GaS2 polytypic compounds. Moreover, we probed the suitability of the heterolayered [Na2Cl]GaS2 single crystals as a sorbent for UO22+ uptake and monitored this process by energy-dispersive and infrared spectroscopies and PXRD, which revealed that the [Na2Cl]+ insert could be exchanged with UO22+ on the surface while the UO22+ intercalation decomposes the [Na2Cl]GaS2 structure. [ABSTRACT FROM AUTHOR]

Despite the pivotal role of stannum doping in achieving ultrahigh piezoelectric performance in barium titanate-based ceramics, the fundamental mechanisms underlying this enhancement remain elusive. Here, we introduce a single variable nonstoichiometric stannum strategy in lead-free barium titanate-based ceramics with giant piezoelectricity, revealing that stannum doping contributes intrinsically and extrinsically to enhance piezoelectricity. Density functional theory calculations elucidate the intrinsic enhancement of polarization arising from lattice distortion and increased space for titanium-oxygen bonds induced by optimal stannum doping, which is corroborated by Rayleigh analysis. A phase transition from ferroelectric multiphase coexistence to paraelectric phase is observed, alongside a rapid miniaturized and eventually disappeared domains with increasing stannum doping. This evolution in phase structure and domain configuration induces a nearly vanishing polarization anisotropy and low domain wall energy, facilitating easy polarization rotation and domain wall motion, thereby significantly contributing to the extrinsic piezoelectric response. Consequently, the origins of ultrahigh performance can be attributed to the synergistic effect of stannum-induced intrinsic and extrinsic contributions in barium titanate-based ceramics. This study provides fundamental insights into the role of doping elements and offers guidance for the design of high-performance piezoelectrics. The mechanisms of the enhancement of stannum doping in achieving high piezoelectric performance in barium titanate-based ferroelectric ceramics remain elusive. Here, the authors introduce a single variable nonstoichiometric stannum strategy to reveal the intrinsic and extrinsic contributions for enhancing piezoelectricity after stannum doping. [ABSTRACT FROM AUTHOR]

Extensive ab initio density functional theory molecular dynamics calculations were used to evaluate stability conditions for relevant phases of InN. In particular, the p-T conditions of the thermal decomposition of InN and pressure-induced wurtzite–rocksalt solid–solid phase transition were established. The comparison of the simulation results with the available experimental data allowed for a critical evaluation of the capabilities and limitations of the proposed simulation method. It is shown that ab initio molecular dynamics can be used as an efficient tool for simulations of phase transformations of InN, including solid–solid structural transition and thermal decomposition with formation of N2 molecules. It is of high interest, because InN is an important component of epitaxial quantum structures, but it has not been obtained as a bulk single crystal. This makes it difficult to determine its basic physical properties to develop new applications. [ABSTRACT FROM AUTHOR]

While the surface-bulk correspondence has been ubiquitously shown in topological phases, the relationship between surface and bulk in Landau-like phases is much less explored. Theoretical investigations since 1970s for semi-infinite systems have predicted the possibility of the surface order emerging at a higher temperature than the bulk, clearly illustrating a counterintuitive situation and greatly enriching phase transitions. But experimental realizations of this prediction remain missing. Here, we demonstrate the higher-temperature surface and lower-temperature bulk phase transitions in CrSBr, a van der Waals (vdW) layered antiferromagnet. We leverage the surface sensitivity of electric dipole second harmonic generation (SHG) to resolve surface magnetism, the bulk nature of electric quadrupole SHG to probe bulk spin correlations, and their interference to capture the two magnetic domain states. Our density functional theory calculations show the suppression of ferromagnetic-antiferromagnetic competition at the surface is responsible for this enhanced surface magnetism. Our results not only show counterintuitive, richer phase transitions in vdW magnets, but also provide viable ways to enhance magnetism in their 2D form. Surface magnetic order could precede bulk order, but detecting such a separation necessitates experimental probes sensitive to both surface and bulk phase transitions. Here, using second harmonic generation, the authors propose that this situation is realized in a van der Waals antiferromagnet CrSBr. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS2)-based transition-metal chalcogenides are considered to be cost-efficient, environmentally-friendly, and stable materials in the application of electrocatalytic hydrogen production. However, the low density and poor reactivity of active sites within the basal plane of MoS2 hinder hydrogen evolution reaction (HER). Herein, we reported that N, P co-doped in the basal plane of MoS2 (N, P-MoS2) can convert the parts of 2H MoS2 to 1T MoS2 and enlarge interlayer spacing for an efficient HER. Subsequently, electrochemical tests revealed that N, P-MoS2 showed an overpotential of 179 mV at 10 mA cm−2, a corresponding Tafel slope of 143 mV dec−1, and remarkable long-term stability over 20 h. In addition, we combined these findings with theoretical results from density functional theory (DFT) simulations. The results confirmed that the synergistic effect of N and P activated the HER catalytic activity at the edge of N-MoS2. In short, the synergy improved the conductivity of N-MoS2 and promoted the rapid transfer of charge to achieve high-performance HER activity. The work provides new insight into the synergistic effect of N and P co-doped in the basal plane of MoS2, as well as a new pathway for the rational design of efficient HER electrocatalysts. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) Si-based materials are very attractive for use in electronic devices due to their main advantages, such as hardness, stability and non-toxicity. In this work, we study the electronic and optical properties of the penta-siligraphene (p-Si2C4) monolayer with/without effects of biaxial strain (εb) and external electric field (E) using density functional theory (DFT). The structural, dynamic and thermal stabilities of the p-Si2C4 monolayer were confirmed by its negative formation energy, absence of soft modes in the phonon dispersion curve and small drift in the total energy at standard temperature, respectively. Our results showed that the p-Si2C4 monolayer is an indirect semiconductor with a bandgap of 1.39 eV. We found that under the effect of biaxial strain and electric field, a semiconductor-to-metal phase transition occurs and the bandgap depends on both εb and E. Indeed, the bandgap decreases under compressive strain and both negative and positive electric fields, while the bandgap increases slightly with a tensile strain of up to 2%. For the optical properties, the p-Si2C4 monolayer has a high optical absorption of the order of ∼105/106 cm−1 in both visible/UV ranges and it can be enhanced by biaxial strain. This improvement in optical absorption can increase the range of applicability of p-Si2C4 in optoelectronics. [ABSTRACT FROM AUTHOR]

The generalized gradient approximation (GGA) density functional theory (DFT), Perdew–Burke–Ernzerh (PBE), and long-range corrected DFT (ωB97XD) were used to investigate the interaction between lithium polysulfides (Li2Sx, x = 2–8) and N-doped graphene to understand the adsorption mechanism and effect of catalytic conversion, including the reduction from S8 to Li2S and Li2S decomposition. The binding of Li2Sx to N-doped graphene was significantly stronger than that to solvents, such as 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME), approximately by 0.5–3.0 eV, indicating that N-doped graphenes possessed the ability to trap soluble Li2Sx. The binding of Li2Sx to N-doped graphenes followed the order of pyrrolic N > pyridinic N > graphene > graphitic N. Among them, pyrrolic N exhibited the strongest anchoring effect mainly via forming the coordination bond of LiN3. In addition, in all the investigated systems, the van der Waals force also played an important role in binding. We comprehensively analyzed the micro-nano-scale theoretical data, including charge, bond distance, bond order, electron density difference, the electron localization function (ELF), and the independent gradient model (IGM) of the Li ion and N atom. The strength of the Li–N bond confirmed that the strongest anchoring occurred when the Li ion was trapped above the vacancy-forming coordination bond rather than a regular covalent bond. Although the strength of Li–N binding had a direct effect on the binding energy, other thermodynamic parameters (free energy and dissociation energy) of catalytic conversion and the energy barrier of Li2S phase transformation also showed a similar capacity. [ABSTRACT FROM AUTHOR]

Simulations of water near extended hydrophobic spherical solutes have revealed the presence of a region of depleted density and accompanying enhanced density fluctuations. The physical origin of both phenomena has remained somewhat obscure. We investigate these effects employing a mesoscopic binding potential analysis, classical density functional theory (DFT) calculations for a simple Lennard-Jones solvent, and Grand Canonical Monte Carlo (GCMC) simulations of a monatomic water (mw) model. We argue that the density depletion and enhanced fluctuations are near-critical phenomena. Specifically, we show that they can be viewed as remnants of the critical drying surface phase transition that occurs at bulk liquid–vapor coexistence in the macroscopic planar limit, i.e., as the solute radius Rs → ∞. Focusing on the radial density profile ρ(r) and a sensitive spatial measure of fluctuations, the local compressibility profile χ(r), our binding potential analysis provides explicit predictions for the manner in which the key features of ρ(r) and χ(r) scale with Rs, the strength of solute–water attraction ɛsf, and the deviation from liquid–vapor coexistence of the chemical potential, δμ. These scaling predictions are confirmed by our DFT calculations and GCMC simulations. As such, our theory provides a firm basis for understanding the physics of hydrophobic solvation. [ABSTRACT FROM AUTHOR]

The pressure-induced geometry and crystal structures, electronic features, hydrogen bonding networks, and vibrational properties of two energetic pentazolate anion salts (N5−)2DABTT2+ and N5−GU+ were investigated by density functional theory (DFT) calculations. The results show that a critical point of phase transition at 9 GPa for (N5−)2DABTT2+ was found, while for N5−GU+ it was not. The distortion of cations DABTT2+ and GU+ was found to be positively correlated with the pressure, thereby affecting the hydrogen bond network distribution in the crystals. The discussions on electronic structures suggest that pressure can lead to an increase of charge overlap and delocalization, thereby improving the ability of electron transition. The topological analysis of the hydrogen bonds reveals that both hydrogen bonding interactions and the degree of contribution of a specific atom to the hydrogen bonding interactions gradually improve as the pressure increases, with a discontinuity at 9 GPa only for (N5−)2DABTT2+. The vibrational properties of both crystals were discussed and the critical point of phase transition at 9 GPa was also found in (N5−)2DABTT2+. Our findings provide a new perspective on the response mechanism of energetic pentazolate anion salts to high pressure, and are expected to provide meaningful guidance for the application of pentazole energetic materials. [ABSTRACT FROM AUTHOR]

Motivated by the recently discovered high-Tc superconductor La3Ni2O7, we comprehensively study this system using density functional theory and random phase approximation calculations. At low pressures, the Amam phase is stable, containing the Y2− mode distortion from the Fmmm phase, while the Fmmm phase is unstable. Because of small differences in enthalpy and a considerable Y2− mode amplitude, the two phases may coexist in the range between 10.6 and 14 GPa, beyond which the Fmmm phase dominates. In addition, the magnetic stripe-type spin order with wavevector (π, 0) was stable at the intermediate region. Pairing is induced in the s±-wave channel due to partial nesting between the M = (π, π) centered pockets and portions of the Fermi surface centered at the X = (π, 0) and Y = (0, π) points. This resembles results for iron-based superconductors but has a fundamental difference with iron pnictides and selenides. Moreover, our present efforts also suggest La3Ni2O7 is qualitatively different from infinite-layer nickelates and cuprate superconductors. Recently superconductivity with Tc of about 80 K was discovered in a bilayer nickelate La3Ni2O7 under high pressure. Here the authors report a density functional theory and random phase approximation study of structural and electronic properties as a function of pressure and discuss the pairing mechanism. [ABSTRACT FROM AUTHOR]

The ferroelectric phase transition of the perovskite barium titanate as well as its technical importance regarding the switching of respective polar properties is well known and has been thoroughly studied, both experimentally and on theoretical grounds. While details about the phase diagram as well as transition temperatures are experimentally well known, the theoretical approaches still face difficulties in contributing a detailed description of these phase transitions. Within this work, a new methodological approach is introduced to revisit the ferroelectric phase transition with first-principles methods. With the chosen ab initio molecular dynamics (AIMD) method in combination with the applied NpT ensemble, we are able to join the accuracy of density functional theory (DFT) with ambient conditions, realized using a thermostat and barostat in an MD simulation. The derived phase diagram confirms recent corrections in the theoretical models and reproduces the phase boundary pressure dependence of TC. In conclusion of the statistical atomistic dynamics, the nature of the transition can be described in a more detailed way. In addition, this work paves the way towards locally patterned piezoelectrica by means of acoustic standing waves as well as piezoelectrically induced acoustic resonators. [ABSTRACT FROM AUTHOR]

The magnetostructural phase transition in Fe50Al50 alloy with chemically ordered paramagnetic B2 and disordered ferromagnetic (FM) A2 phase has applications in spintronics such as phase-change magnetic memory and magnonic devices. We first conducted a systematic first-principles density functional theory study of the A2 and B2 phases in the Fe50Al50 alloy. A theoretical understanding of this equiatomic alloy's electronic and spin-dynamical properties leads us to the experimental exploration of the FM A2 phase. Therefore, we deposit the 50 nm Fe50Al50 alloy thin film using sputtering and investigate the influence of the Ta buffer layer on its structural and magnetic properties. Our results reveal that the film with a buffer layer exhibits the A2 phase with appreciably higher saturation magnetization (848 emu/cc) than the film without a buffer layer (576 emu/cc). However, the surface roughness and Gilbert damping (α) slightly increase with the presence of the buffer layer from 0.41 to 0.56 nm and 4.35 × 10−3–4.94 × 10−3, respectively. The enhancement in α is due to extrinsic contributions induced by surface inhomogeneities. [ABSTRACT FROM AUTHOR]

Titanium dioxide has been extensively studied in the rutile or anatase phase, while its high-pressure phases are less well-understood, despite that many are thought to have interesting optical, mechanical, and electrochemical properties. First-principles methods, such as density functional theory (DFT), are often used to compute the enthalpies of TiO2 phases at 0 K, but they are expensive and, thus, impractical for long time scale and large system-size simulations at finite temperatures. On the other hand, cheap empirical potentials fail to capture the relative stabilities of various polymorphs. To model the thermodynamic behaviors of ambient and high-pressure phases of TiO2, we design an empirical model as a baseline and then train a machine learning potential based on the difference between the DFT data and the empirical model. This so-called Δ-learning potential contains long-range electrostatic interactions and predicts the 0 K enthalpies of stable TiO2 phases that are in good agreement with DFT. We construct a pressure–temperature phase diagram of TiO2 in the range 0 < P < 70 GPa and 100 < T < 1500 K. We then simulate dynamic phase transition processes by compressing anatase at different temperatures. At 300 K, we predominantly observe an anatase-to-baddeleyite transformation at about 20 GPa via a martensitic two-step mechanism with a highly ordered and collective atomic motion. At 2000 K, anatase can transform into cotunnite around 45–55 GPa in a thermally activated and probabilistic manner, accompanied by diffusive movement of oxygen atoms. The pressures computed for these transitions show good agreement with experiments. Our results shed light on how to synthesize and stabilize high-pressure TiO2 phases, and our method is generally applicable to other functional materials with multiple polymorphs. [ABSTRACT FROM AUTHOR]

In this paper, we study the structural, electronic, magnetic, mechanical and thermal properties of Half-Heusler CoMnTe, using the Density Functional Theory (DFT) with the generalized gradient approximation (GGA) and GGA plus the Hubbard correction (GGA+U). The obtained results show that the compound is more stable in γ -phase with the ferromagnetic (FM) configuration than the nonmagnetic (NM) among the three considered structural phases (α , β , γ) and there is no possibility of martensitic transformation under the tetragonal distortion effect. The computed total magnetic moments of CoMnTe by GGA and GGA+U are 4 μ B and 4.13 μ B , respectively, arising mainly from the Mn atom. The obtained density of states and band structures using GGA approach show that the compound exhibits half-metallic behavior with 100% spin polarization. As the Hubbard is added, the electronic properties show an electronic phase transition from half-metallic to metallic behavior with 68% spin polarization. While the thermal and mechanical properties show very small changes, confirming that the studied compound is mechanically and dynamically stable. • CoMnTe compound is more stable in γ -phase with the ferromagnetic configuration. • CoMnTe compound is mechanically and thermodynamically stable. • CoMnTe compound is promising for spintronic applications. [ABSTRACT FROM AUTHOR]

For efficient usage of the rare earth-based materials in electronic or optoelectronic applications, their 4f electrons' behaviors must be understood properly. We have presented Hubbard U corrected density functional theory (DFT+U) study of structural, magnetic, electronic, and optical properties of a rare-earth chalcogenide system, Ca(La1−xCex)2S4 (0 ≤ x ≤ 1). A unique site selection technique based on local magnetic moment arrangements was applied to build the atomic arrangements for a Ce doped Ca(La1−xCex)2S4 solid solution. The incorporation of f-electrons by Ce doping modifies the properties of the parent compound, CaLa2S4. In conjunction with the DFT + U method, we applied spin–orbit coupling to determine the magnetic ground state. The inclusion of 25% Ce transforms the non-magnetic parent compound to an antiferromagnetic (AFM) compound, and AFM magnetic ordering remains unaltered throughout the whole solid solution series. In addition, these compounds also undergo insulator to semiconducting to metallic phase transitions as Ce concentration increases. While CaLa2S4 is an insulator, Ca(La1−xCex)2S4 with x = 0.25 and 0.50 are n-type semiconductors, and on the other hand, compounds with x = 0.75 and 1.0 are found to have metallic band structures. The Ce atoms in these materials were found to be in a mixed valence state, Ce3+/4+. We explained these phase transitions from the calculated electronic structures. In addition, we have presented an explanation for the experimentally observed red-orange colors of Ca(La0.25 Ce0.75)2S4 and CaCe2S4 compounds. [ABSTRACT FROM AUTHOR]

Hydrogen hydrates are among the basic constituents of our solar system's outer planets, some of their moons, as well Neptune-like exo-planets. The details of their high-pressure phases and their thermodynamic conditions of formation and stability are fundamental information for establishing the presence of hydrogen hydrates in the interior of those celestial bodies, for example, against the presence of the pure components (water ice and molecular hydrogen). Here, we report a synthesis path and experimental observation, by X-ray diffraction and Raman spectroscopy measurements, of the most H2-dense phase of hydrogen hydrate so far reported, namely the compound 3 (or C3). The detailed characterisation of this hydrogen-filled ice, based on the crystal structure of cubic ice I (ice Ic), is performed by comparing the experimental observations with first-principles calculations based on density functional theory and the stochastic self-consistent harmonic approximation. We observe that the extreme (up to 90 GPa and likely beyond) pressure stability of this hydrate phase is due to the close-packed geometry of the hydrogen molecules caged in the ice Ic skeleton. [ABSTRACT FROM AUTHOR]

The effect of pressure on the structural and electronic properties of the two binary compounds SrSe and SrTe have been investigated in the present work. The calculations were carried out with the CASTEP code which uses the density functional theory (DFT) based on the pseudopotential and plane-wave (PW-PP) method. To evaluate the exchange and correlation energy, we used the generalized gradient approximation (GGA-PBE). Our results show that, at ambient conditions, the two studied compounds are more stable in the NaCl-type phase (B1). Under the effect of pressure, the two materials undergo a phase transition to crystallize in the CsCl-like structure (B2) at pressures equal to 16 GPa and 12.33 GPa for the compounds SrSe and SrTe, respectively. Analysis of the band structure under conditions of pressure and zero temperature shows that the two compounds are indirect semiconductors. Under the effect of pressure, our calculations predict that the two compounds change electronically to become metals at pressures greater than 46 GPa for SrSe and 13 GPa for SrTe. To give a quantitative description of the variation of the structural and electronic parameters of the two structures with the pressure, we extracted the parametric functions of these parameters as a function of the pressure. Our results are presented and compared with a different experimental and theoretical works. [ABSTRACT FROM AUTHOR]

High pressure is an effective means to optimize the thermoelectric (TE) performance by sharply improving the electrical properties of materials. Studying the carrier effective mass (m*) is a feasible way to uncover the basic reason for superior electrical properties under high pressure. However, it is still difficult to obtain the m* under pressure in experiments. Thus, in this work, the m* of α-PbSe (F m 3 ̄ m) under high pressure is calculated by band dispersion relation based on the density functional theory. It is found that the high pressure decreases m* of α-PbSe, which is the cause for excellent electrical properties. Moreover, the isotropy of m* enhances with the increase in the pressure, which means the high pressure further optimizes the isotropy of the carrier migration in the structure. It is reveled that the higher the pressure, the more beneficial to improve the electrical properties of α-PbSe, thus optimizing the TE performance before the phase transition pressure (4.5 GPa). This work is of great significance for exploring the mechanism of in situ high-pressure TE properties in the future, as well as the prediction and selection of high-performance TE materials under high pressure. [ABSTRACT FROM AUTHOR]

We focus on the ferric end-member of phase H: ε-FeOOH using density functional theory at the PBEsol+U level. At 300 K, we find that ε-FeOOH undergoes a hydrogen bond symmetrization at 37 GPa and a sharp high-spin to low-spin transition at 45 GPa. We find excellent agreement with experimental measurements of the equation of state, lattice parameters, atomic positions, vibrational frequencies, and optical properties as related to the band gap, which we find to be finite and small, decreasing with pressure. The hydrogen bond symmetrization transition is neither first- nor second-order, with no discontinuity in volume or any of the elastic moduli. Computed IR and Raman frequencies and intensities show that vibrational spectroscopy may provide the best opportunity for locating the hydrogen bond symmetrization transition experimentally. We find that ε-FeOOH is highly anisotropic in both longitudinal- and shear-wave velocities at all pressures, with the shear wave velocity varying with propagation and polarization direction by as much as 24% at zero pressure and 43% at 46 GPa. The shear and bulk elastic moduli increase by 18% across the high-spin to low-spin transition. [ABSTRACT FROM AUTHOR]

The aim of this study is to comprehensively examine the structural composition and properties of the AgAlS2 crystal during its high-pressure phase. This analysis delves into the second coordination environment of the crystal structure and elucidates the distinct transformations it undergoes during the phase transition. The band energy structure was calculated, and the origin of electronic levels was clarified. It is shown that the crystal becomes non-stratified during the phase transition. This study also determined the values of the crystal's carrier effective masses, underscoring its spatial anisotropy. It was found that the calculated optical functions are similar to the crystal in the chalcopyrite structure, and their differences are shown. Further, this study involved the calculation of the crystal's phonon spectrum, revealing the spectrum's transformation during the phase transition. The vibrational frequencies were also obtained, with a symmetrical classification of vibrational modes. Finally, this study derived the infrared and Raman spectra of the AgAlS2 crystal, thereby providing a comprehensive picture of the crystal during its high-pressure phase. [ABSTRACT FROM AUTHOR]

Reflection asymmetric, octupole shapes in nuclei are a prominent aspect of nuclear structure, and have been recurrently studied over the decades. Recent experiments using radioactive-ion beams have provided evidence for stable octupole shapes. A variety of nuclear models have been employed for the related theoretical analyses. We review recent studies on the nuclear octupole shapes and collective excitations within the interacting boson model. A special focus is placed on the microscopic formulation of this model by using the mean-field method that is based on the framework of nuclear density functional theory. As an illustrative example, a stable octupole deformation, and a shape phase transition as a function of nucleon number that involves both quadrupole and octupole degrees of freedom are shown to occur in light actinides. Systematic spectroscopic studies indicate enhancement of the octupole collectivity in a wide mass region. Couplings between the octupole and additional degrees of freedom are incorporated in a microscopic manner in the boson system, and shown to play a crucial role in the description of the related intriguing nuclear structure phenomena such as the shape coexistence. [ABSTRACT FROM AUTHOR]

Layered perovskite-like oxide K 2 La 2 Ti 3 O 10 is a perspective photocatalyst for water decomposition at ultraviolet irradiation. In this paper using a density functional theory approach electronic structures of dehydrated and hydrated phases of K 2 La 2 Ti 3 O 10 have been studied. It has been found that the water induced phase transformation from I 4/ mmm to P 4/ mmm occurs in two steps: at first an increase in the distance between the perovskite layers prevails, and then a shift of the perovskite layers occurs with a moderate changing of the interlayer distance. The band structure calculations have done using the modified Becke-Johnson potential show that K 2 La 2 Ti 3 O 10 exhibits indirect band gap of about 3.1 eV. The valence band maximum potential is located at 2.23 eV (vs. normal hydrogen electrode), while the conduction band minimum potential is at −0.91 eV. This confirms the ability of K 2 La 2 Ti 3 O 10 both for photocatalytic oxidation of water (producing dioxygen and protons) or pollutants, and to reduce H+ to H 2. However, the transfer rate, estimated implicitly from the ratio of the effective masses of the photogenerated holes and electrons in valence and conduction bands, limits photocatalytic performance of K 2 La 2 Ti 3 O 10 due to separation ineffectiveness of charge carriers. • Both LDA and GGA fail to describe correctly band gap in K 2 La 2 Ti 3 O 10. • The mBJ potential gives an indirect band gap of about 3.1 eV in K 2 La 2 Ti 3 O 10. • Photocatalytic properties of K 2 La 2 Ti 3 O 10 for water splitting was studied. • Low transfer rate limits photocatalytic performance of pristine K 2 La 2 Ti 3 O 10. • Water induced phase transition in K 2 La 2 Ti 3 O 10 from I 4/ mmm to P 4/ mmm was modelled. [ABSTRACT FROM AUTHOR]

Defects passivation is widely devoted to improving the performance of formamidinium lead triiodide perovskite solar cells; however, the effect of various defects on the α-phase stability is still unclear. Here, using density functional theory, we first reveal the degradation pathway of the formamidinium lead triiodide perovskite from α to δ phase and investigate the effect of various defects on the energy barrier of phase transition. The simulation results predict that iodine vacancies are most likely to trigger the degradation, since they obviously reduce the energy barrier of α-to-δ phase transition and have the lowest formation energies at the perovskite surface. A water-insoluble lead oxalate compact layer is introduced on the perovskite surface to largely suppress the α-phase collapse through hindering the iodine migration and volatilization. Furthermore, this strategy largely reduces the interfacial nonradiative recombination and boosts the efficiency of the solar cells to 25.39% (certified 24.92%). Unpackaged device can maintain 92% of its initial efficiency after operation at maximum power point under simulated air mass 1.5 G irradiation for 550 h. The effect of various defects on α-phase stability of formamidinium lead triiodide perovskite remains unclear. Here, the authors reveal the degradation pathway, introduce water-insoluble compact layer to suppress α-phase collapse, and achieve maximum efficiency over 25% for perovskite solar cells. [ABSTRACT FROM AUTHOR]

The effect of Ru substitution on the structure and electrochemical properties of P2‐type Na0.67CoO2 is investigated. The first‐discharge capacities of Na0.67CoO2 and Na0.6 [Co0.78Ru0.22]O2 materials are 128 and 163 mAh g−1 (23.5 mA g−1), respectively. Furthermore, the rate capability is improved due to the electro‐conducting nature of Ru doping. Operando X‐ray diffraction analysis reveals that the Na0.67CoO2 does not undergo a phase transition; however, multiple Na+/vacancy ordered superstructures within the P2 phase appear during Na+ extraction/insertion. In contrast, the Na0.6[Co0.78Ru0.22]O2 material undergoes a P2–OP4 phase transition during desodiation, with no formation of Na+/vacancy ordering within the P2 phase. The increased discharge capacity of Na0.6[Co0.78Ru0.22]O2 is most likely associated with additional cationic Ru4+/Ru5+ redox and increased anionic O2−/(O2n−) redox participation. Combined experimental (galvanostatic cycling, X‐ray absorption spectroscopy, differential electrochemical mass spectrometry) and theoretical (density functional theory calculations) studies confirm that Ru substitution provokes the oxygen‐redox reaction and that partial O2 release from the oxide lattice is the origin of the reaction. The findings provide new insight for improving the electrode performance of cathode materials via 4d Ru substitution and motivate the development of a new strategy for the design of high‐capacity cathode materials for sodium‐ion batteries. [ABSTRACT FROM AUTHOR]

Since its inception, chemistry has been predominated by the use of temperature to generate or change materials, but applications of pressure of more than a few tens of atmospheres for such purposes have been rarely observed. However, pressure is a very effective thermodynamic variable that is increasingly used to generate new materials or alter the properties of existing ones. As computational approaches designed to simulate the solid state are normally tuned using structural data at ambient pressure, applying them to high-pressure issues is a highly challenging test of their validity from a computational standpoint. However, the use of quantum chemical calculations, typically at the level of density functional theory (DFT), has repeatedly been shown to be a great tool that can be used to both predict properties that can be later confirmed by experimenters and to explain, at the molecular level, the observations of high-pressure experiments. This article's main goal is to compile, analyze, and synthesize the findings of works addressing the use of DFT in the context of molecular crystals subjected to high-pressure conditions in order to give a general overview of the possibilities offered by these state-of-the-art calculations. [ABSTRACT FROM AUTHOR]

The magnetic ground state of LaCrO3 and CaCrO3 solid solution is investigated in the complete miscibility range, that is, La1−xCaxCrO3 with 0 ≤ x ≤ 1 at the interval of x = 0.25 to understand the magnetic phase transitions with Ca substitutional fraction. The density functional theory is used with Hubbard correction to account for Cr exchange interaction. The isostructural La1−xCaxCrO3 system exhibits antiferromagnetic insulator (x = 0, i.e., LaCrO3) to ferromagnetic half metal (x = 0.75, i.e., La0.25Ca0.75CrO3) to antiferromagnetic metal (x = 1, i.e., CaCrO3) phase transitions. Cr changes its oxidation state from 3+ in LaCrO3 to 4+ in CaCrO3, showing both 3+ and 4+ states for intermediate compositions. Interestingly, the introduction of Hubbard energy U and Ca substitution affects Cr–O hybridization, as observed in partial density of states. A compositional phase diagram with magnetic ground state is presented for the La1−xCaxCrO3 (0 ≤ x ≤ 1) system. [ABSTRACT FROM AUTHOR]

Ga2O3 is a wide-band gap semiconductor of emergent importance for applications in electronics and optoelectronics. However, vital information of the properties of complex coexisting Ga2O3 polymorphs and low-symmetry disordered structures is missing. We develop two types of machine-learning Gaussian approximation potentials (ML-GAPs) for Ga2O3 with high accuracy for β/κ/α/δ/γ polymorphs and generality for disordered stoichiometric structures. We release two versions of interatomic potentials in parallel, namely soapGAP and tabGAP, for high accuracy and exceeding speedup, respectively. Both potentials can reproduce the structural properties of all the five polymorphs in an exceptional agreement with ab initio results, meanwhile boost the computational efficiency with 5 × 102 and 2 × 105 computing speed increases compared to density functional theory, respectively. Moreover, the Ga2O3 liquid-solid phase transition proceeds in three different stages. This experimentally unrevealed complex dynamics can be understood in terms of distinctly different mobilities of O and Ga sublattices in the interfacial layer. [ABSTRACT FROM AUTHOR]

We have simulated BiCoO 3 films epitaxially grown along (001) direction with density functional theory computations. Leading candidates for the lowest-energy phases have been identified. The tensile strains induce magnetic phase transition in the ground state ( P 4 m m symmetry) from a C-type antiferromagnetic order to a G-type order for the in-plane lattice parameter above 3.922 Å. The G-type antiferromagnetic order will be maintained with larger tensile strains; however, a continuous structural phase transition will occur, combining the ferroelectric and antiferrodistortive modes. In particular, the larger tensile strain allows an isostructural transition, the so-called Cowley's "Type Zero" phase transitions, from C c -(I) to C c -(II), with a slight volume collapse. The orientation of ferroelectric polarization changes from the out-of-plane direction in the P 4 m m to the in-plane direction in the P m c 2 1 state under epitaxial tensile strain; meanwhile, the magnetic ordering temperature T N can be strikingly affected by the variation of misfit strain. [ABSTRACT FROM AUTHOR]

The use of a sacrificial cathode additive that contains a large amount of lithium is one potential solution to compensate for the irreversible capacity loss associated with next‐generation anodes such as silicon. Antifluorite‐type Li6CoO4 has attracted attention as a potential cathode additive owing to its remarkably high theoretical lithium extraction capacity. However, the complex mechanism of lithium extraction as well as the oxygen loss from Li6CoO4 is not well understood. A generalizable computational thermodynamics and experimental framework is presented to understand the lithium‐extraction pathway of Li6CoO4. It is found that one lithium per formula unit can be topotactically extracted from Li6CoO4, followed by an irreversible and nontopotactic phase transformation to Li2CoO3 or LiCoO2 depending on the temperature. The results show that peroxide species may form to charge‐compensate for Li extraction which is undesirable as this can lead to gas release during battery operation. It is suggested that charging Li6CoO4 at an elevated temperature that the electrolyte can withstand, redirects the reaction pathway and prevents the formation of intermediate peroxide species making it an effective and stable sacrificial cathode additive. [ABSTRACT FROM AUTHOR]

Transition metal-free half-metallicity recently has been the subject of intense research activity due to its potential in spintronics application. By employing density functional theory calculations, we revealed that F-intercalated GaN bilayers exhibit robust p orbital-based half-metallicity against the intercalant concentration and biaxial strain of −10% to 10%. The stable half-metallicity results from a spontaneous phase transition through Stoner instability and can be attributed to unusually high density of states near the Fermi level due to the quasi-flat bands. These findings provide a strategy to design transition-metal free half-metallic materials for futuristic spintronic applications. [ABSTRACT FROM AUTHOR]

This study presents a thorough analysis of the electronic structures of the TaPxAs1−x series of compounds, which are of significant interest due to their potential as topological materials. Using a combination of first principles and Wannier‐based tight‐binding methods, this study investigates both the bulk and surface electronic structures of the compounds for varying compositions (x = 0, 0.25, 0.50, 0.75, 1), with a focus on their topological properties. By using chirality analysis, (111) surface electronic structure analysis, and surface Fermi arcs analysis, it is established that the TaPxAs1−x compounds exhibit topologically nontrivial behavior, characterized as Weyl semimetals (WSMs). The effect of spin–orbit coupling (SOC) on the topological properties of the compounds is further studied. In the absence of SOC, the compounds exhibit linearly dispersive fourfold degenerate points in the first Brillouin zone (FBZ) resembling Dirac semimetals. However, the introduction of SOC induces a phase transition to WSM states, with the number and position of Weyl points (WPs) varying depending on the composition of the alloy. For example, TaP has 12 WPs in the FBZ. The findings provide novel insights into the electronic properties of TaPxAs1−x compounds and their potential implications for the development of topological materials for various technological applications. [ABSTRACT FROM AUTHOR]

In situ thermo‐Raman spectroscopy is used to study a single crystal of natural orthorhombic cubanite from the Talnakh copper‐nickel sulfide deposit (Norilsk, Russia). The dependence of the wavenumbers and FWHMs of vibrational modes on temperature, including the cubanite → isocubanite phase transition region, is studied in the range of 83–873 K. For the first time, the polarized Raman spectra of cubanite have been interpreted according to ab initio calculations performed using the density functional theory (DFT) implemented in the VASP package. In spite of the tetrahedral environment of Cu and Fe atoms in cubanite, the spectra demonstrate the presence of different types of bending (twisting, wagging, scissoring, etc.) and stretching vibrations in two different triatomic (S2‐Cu‐S2) and (Fe‐S1‐Fe) fragments for Ag and B2g symmetries, and in three‐ and four‐atomic (S1‐Cu‐S1) and FeS(2)3 fragments, respectively, for B1g and B3g symmetries. [ABSTRACT FROM AUTHOR]

The confluence of major theoretical, experimental, and observational advances are providing a unique perspective on the equation of state of dense neutron-rich matter—particularly its symmetry energy—and its imprint on the mass-radius relation for neutron stars. In this contribution, we organize these developments in an equation of the state density ladder. Of particular relevance to this discussion are the impact of the various rungs on the equation of state and the identification of possible discrepancies among the various methods. A preliminary analysis identifies possible tension between laboratory measurements and gravitational-wave detections that could indicate the emergence of a phase transition in the stellar core. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) are a class of 2D materials demonstrating promising properties, such as high capacities and cycling stabilities, making them strong candidates to replace graphitic anodes in lithium-ion batteries. However, certain TMDs, for instance, MoS2, undergo a phase transformation from 2H to 1T during intercalation that can affect the mobility of the intercalating ions, the anode voltage, and the reversible capacity. In contrast, select TMDs, for instance, NbS2 and VS2, resist this type of phase transformation during Li-ion intercalation. This manuscript uses density functional theory simulations to investigate the phase transformation of TMD heterostructures during Li-, Na-, and K-ion intercalation. The simulations suggest that while stacking MoS2 layers with NbS2 layers is unable to limit this 2H → 1T transformation in MoS2 during Li-ion intercalation, the interfaces effectively stabilize the 2H phase of MoS2 during Na- and K-ion intercalation. However, stacking MoS2 layers with VS2 is able to suppress the 2H → 1T transformation of MoS2 during the intercalation of Li, Na, and K-ions. The creation of TMD heterostructures by stacking MoS2 with layers of non-transforming TMDs also renders theoretical capacities and electrical conductivities that are higher than that of bulk MoS2. [ABSTRACT FROM AUTHOR]

Electrocatalytic hydrogen evolution reaction (HER) is regarded as a sustainable and green way for H2 generation, which faces a great challenge in designing highly active, stable electrocatalysts to replace the state‐of‐art noble metal‐platinum catalysts. 1T MoS2 is highly promising in this regard, but the synthesis and stability of this is a particularly pressing task. Here, a phase engineering strategy has been proposed to achieve a stable, high‐percentage (88%) 1T MoS2/chlorophyll‐a hetero‐nanostructure, through a photo‐induced donation of anti‐bonding electrons from chlorophyll‐a (CHL‐a) highest occupied molecular orbital to 2H MoS2 lowest unoccupied molecular orbital. The resultant catalyst has abundant binding sites provided by the coordination of magnesium atom in the CHL‐a macro‐cycle, featuring higher binding strength and low Gibbs‐free energy. This metal‐free heterostructure exhibits excellent stability via band renormalization of Mo 4d orbital which creates the pseudogap‐like structure by lifting the degeneracy of projected density of state with 4S in 1T MoS2. It shows extremely low overpotential, toward the acidic HER (68 mV at the current density of 10 mA cm−2), very close to the Pt/C catalyst (53 mV). The high electrochemical‐surface‐area and electrochemical turnover frequency support enhanced active sites along with near zero Gibbs free energy. Such a surface‐reconstruction strategy provides a new avenue toward the production of efficient non‐noble‐metal‐catalysts for the HER with the aim of green‐hydrogen production. [ABSTRACT FROM AUTHOR]