Valence Electrons Research Articles

Two-stage feature selection for machine learning-aided DFT-based surface reactivity study on single-atom alloys

This paper presents a feature-centric strategy for predicting adsorption energies of key CO2 reduction reaction (CO2RR) adsorbates, CO and H species, utilizing density functional theory-based calculations for eight adsorption sites and considering alloying effects of nine transition metals at single-atom concentrations. Here, we explore a class of materials consisting of a majority host metal where individual atoms of a different element are dispersed called single-atom alloys (SAA). A total of eight feature selection methods are assessed within Gradient Boosting Regression and Linear Regression models. This study proposes a practical and effective two-stage approach that narrows down the initial 86 features to subsets of 10 and 7 for CO and H adsorption energy predictions, respectively, with the arithmetic mean of valence electrons (VE-am) feature consistently emerging as highly influential, validated through permutation and Shapley additive explanations-based feature importance analyses. The models exhibit robust performance on unseen data, indicating their generalization capability. The findings emphasize VE-am as a potential key machine learning feature for CO2RR on SAA surfaces and underline the effectiveness of the feature-centric approach in understanding feature impacts in machine learning models for CO2RR on SAA systems. Additionally, while other features based on structural, electronic and elemental properties may not individually impact the model significantly, their collective contribution plays a vital role in achieving more accurate adsorption energy predictions.

Exploring the electronic, optical, and thermoelectric features of BaXCu3Se4 (X = In, Tl) quaternary chalcogenides: first-principles study

The tunable optical characteristics and superior thermal stability of Indium and Thallium-based quaternary chalcogenides are significant. We studied the intricate relationship between the optoelectronic, and thermoelectric features of notable BaXCu3Se4 (X = In, Tl) quaternary chalcogenides. Both the maximum of the valence band and conduction band coincide at the Γ-point, confirming these materials as direct band gap materials. By substituting Indium for thallium, the calculated band gap decreases from 0.71 eV to 0.53 eV. These anions have a considerable impact and contribute to a decrease in the energy gap via valence electrons. Partially filled d orbitals of copper play an important role in electronic states at the Fermi level. The components of the complex dielectric function, as well as other important optical parameters, are examined and analyzed for the potential usage in optoelectronic devices. The ε 1(ω) becomes negative at 6.32 eV, suggesting that the medium is reflecting all of the incident light. Thallium affects the absorption spectrum because it changes the density of states and electronic transitions. The absorption spectra indicated that the material absorbs in the visible and near-ultraviolet parts of the spectrum, which is fascinating and might have applications in optoelectronics. The investigated materials are appropriate to be used for thermoelectric devices confirmed by their significant and notable thermoelectric properties. Because the Seebeck coefficient is negative, most charge carriers, typically electrons, flow from the higher temperature area to the lower temperature region. At both low and high temperatures, thallium is accountable for BaTlCu3Se4’s higher thermal conductivity than BaInCu3Se4 material.

Mapping of lithium ion concentrations in 3D structures through development of in situ correlative imaging of X-ray Compton scattering-computed tomography.

Understanding the correlation between chemical and microstructural properties is critical for unraveling the fundamental relationship between materials chemistry and physical structures that can benefit materials science and engineering. Here, we demonstrate novel in situ correlative imaging of the X-ray Compton scattering computed tomography (XCS-CT) technique for studying this fundamental relationship. XCS-CT can image light elements that do not usually exhibit strong signals using other X-ray characterization techniques. This paper describes the XCS-CT setup and data analysis method for calculating the valence electron momentum density and lithium-ion concentration, and provides two examples of spatially and temporally resolved chemical properties inside batteries in 3D. XCS-CT was applied to study two types of rechargeable lithium batteries in standard coin cell casings: (1) a lithium-ion battery containing a cathode of bespoke microstructure and liquid electrolyte, and (2) a solid-state battery containing a solid-polymer electrolyte. The XCS-CT technique is beneficial to a wide variety of materials and systems to map chemical composition changes in 3D structures.

Prediction of the Cohesion Energy, Shear Modulus and Hardness of Single-Phase Metals and High-Entropy Alloys.

In order to facilitate the prediction of some physical properties, we propose several simple formulas based on two parameters only, the metallic valence and metallic atomic radii. Knowing the composition, for single-phase alloys, the average parameters can be calculated by the rule of mixture. The input parameters can be obtained from tabulated databases. Adopting from the literature the results of Coulomb crystal model for metals and single-phase high-entropy alloys, we have derived formulas for the shear modulus (G) and the cohesion energy (Ecoh). Based on these parameters separately, we set up two formulas to estimate the hardness in the case of pure metals. For single-phase (solid-solution) HEAs, by simplifying the Maresca and Curtin model, we obtained a formula for estimating the hardness, which takes into account the atomic misfit in addition to G. The maximal hardness for single-phase HEA is approximately 600 kg/mm2 and is obtained for a composition with a valence electron concentration of approximately 6 ÷ 7.

Alternative CNDOL Fockians for fast and accurate description of molecular exciton properties.

CNDOL is an a priori, approximate Fockian for molecular wave functions. In this study, we employ several modes of singly excited configuration interaction (CIS) to model molecular excitation properties by using four combinations of the one electron operator terms. Those options are compared to the experimental and theoretical data for a carefully selected set of molecules. The resulting excitons are represented by CIS wave functions that encompass all valence electrons in the system for each excited state energy. The Coulomb-exchange term associated to the calculated excitation energies is rationalized to evaluate theoretical exciton binding energies. This property is shown to be useful for discriminating the charge donation ability of molecular and supermolecular systems. Multielectronic 3D maps of exciton formal charges are showcased, demonstrating the applicability of these approximate wave functions for modeling properties of large molecules and clusters at nanoscales. This modeling proves useful in designing molecular photovoltaic devices. Our methodology holds potential applications in systematic evaluations of such systems and the development of fundamental artificial intelligence databases for predicting related properties.

Intermediate Intercluster Bond Orders. Electronic Communication in Au38(SR)24 Superatomic Molecules.

Ligand-protected gold clusters remain potential building blocks for envisaged molecular materials. The archetypal Au38(SR)24 cluster can be viewed as a robust template for the fusion of two Au25(SR)18- cluster units, retaining a bi-icosahedral Au23 core. Via electrochemical properties, the overall charge state can be selectively tuned, enabling the access of 14 valence electron (ve) species featuring a single intercluster bond and nearby charge from -1 to +3, achieving related species bearing 15- to 11-ve with variable intercluster bond orders. Here, we explore the characteristics of intermediate intercluster bond orders in order to provide insights into the plausible electron communication between the constituent building blocks, with Au38(SR)24, as a representative template. Our results denote a small structural variation along -1 to +3 charge states, provided by the core-protecting ligand interaction, which is enhanced towards more oxidized species. The remaining unpaired electron from intermediate intercluster bond orders of 1.5 for Au38(SR)241-, 1.5 for Au38(SR)241+, and 2.5 for Au38(SR)243+, holds delocalized characteristics between the building block units, favoring electron communication for conductive and cooperative cluster aggregates. Such features are relevant for the formation of molecular electronic device applications, favoring the rationalization prior to engaging in explorative synthesis of larger ligand-protected cluster aggregates.

Microstructural and chemical dependences of fracture toughness in stainless steel welds at 4.2 K

In this study, 316LN-Mn and 316LN plates were welded by gas tungsten arc welding. The fracture toughness of 316LN-Mn and 316LN steel welds at the cryogenic-temperature of 4.2 K was evaluated by focusing on the effects of microstructures and chemical elements. The results showed that both steel welds did not experience stress-induced martensite transformation during deformation. At 4.2 K, the microbands and dislocation pileup at grain boundaries blocked dislocations, thus reducing the fracture toughness of the welds. The embedded nano-twin clusters inhibited void initiation in the nano-grained matrix and formed a crack-bridging ligament behind the crack, which contributed to higher fracture toughness. It was found that microstructures with dominant equiaxed grains at an average grain size of 30 μm had the highest fracture toughness. While chemically, the reduction of manganese content blurred the outline of the electron density map, reduced the valence electron concentration, resulting in unstable austinites, which informed the significance of manganese content control in design and selection of weld metals.

A Fundamental Perspective on the Selective Zn Substitution in Ternary Ordered Intermetallic Compound Cu 6 Zn 2 Sb 2

The unique site substitution of Zn in the structure of tetragonal and atomically ordered Cu6Zn2Sb2, followed by the formation of Cu5Zn3Sb2, has been addressed from fundamental perspectives. First principles energy calculations, and semi‐empirical electronic structure calculations using the density of states, crystal orbital Hamilton population, crystal orbital bond index and Mulliken population analysis were performed to understand the observed substitution pattern and the narrow homogeneity range of the titled compound. Mulliken and Löwdin’s approach of charge analysis explain the experimentally proved ‘coloring’ based on the ‘site energy’ argument. The concept of ‘topological charge stabilization’ has been introduced in this context. Valence electron concentration and the optimization of the partially covalent Cu‐Sb interactions also play pivotal role in the electronic stability of the titled phase.

Fine tuning of Mn/Co vacancies for optimized magnetocaloric performance in MnCoGe alloys

As a family of rare-earth-free magnetocaloric materials, MnCoGe-based alloys have attracted intensive attention in the past few years. In this study, we reveal that the deficiency of either Mn or Co reduces the valence electron concentration of MnCoGe system, thereby lowering its structural transition temperature (Ttr) by pushing the Fermi level away from the Mn-Mn antibonding electronic states. Moreover, Ttr is found to be more sensitive to Co content. By systematically analyzing the temperature dependent magnetization plots and Arrott curves, it is proven that the magnetic and structural degrees of freedom are coupled in the Mn0.97CoGe alloy, resulting in a first-order magnetic phase transition. On the contrary, MnCo0.98Ge undergoes a second-order transition. As a consequence, the Mn0.97CoGe alloy exhibits a maximum magnetic entropy change of 27.6 J kg−1K−1 under a magnetic field of 5 T near the Curie temperature of 265 K, corresponding to a refrigeration capacity of 212.1 J kg−1. Our work demonstrates Mn/Co vacant MnCoGe alloy as a robust candidate of magnetic substance for room temperature magnetocaloric refrigeration.

Applicability of the Zintl Concept to Understanding the Crystal Chemistry of Lithium-Rich Germanides and Stannides.

With this contribution, we take a new, critical look at the structures of the binary phases Li5Ge2 and Li5Sn2. Both are isostructural (centrosymmetric space group R3̅m, no. 166), and in their structures, all germanium (tin) atoms are dimerized. Application of the valence rules will require the allocation of six additional valence electrons per [Ge2] or [Sn2] unit considering single covalent bonds, akin to those in the dihalogen molecules. Alternatively, four additional valence electrons per [Ge2] or [Sn2] anion will be needed if homoatomic double bonds exist, in an analogy with dioxygen. Therefore, five lithium atoms in one formula unit cannot provide the exact number of electrons, leaving open questions as to what is the nature of the chemical bonding within these moieties. Additionally, by means of single-crystal X-ray diffraction, synchrotron powder X-ray diffraction, and neutron powder diffraction, we established that the Li and Sn atoms in Li5Sn2 are partially disordered, i.e., the actual chemical formula of this compound is Li5-xSn2+x (0 < x < 0.1). The convoluted atomic bonding in the case where tin atoms partially displace lithium atoms results in the formation of larger covalently bonded fragments. Our first-principle calculations suggest that such disorder leads to electron doping. Contrary to that, both experimental and computational findings indicate that in the Li5Ge2 structure, the [Ge2] dimers are slightly oxidized, i.e., hole-doped, as a result of approximately 30% vacancies on a Li site, i.e., the actual chemical formula of this compound is Li5-xGe2 (x ≈ 0.3).

Tuning the electronic and magnetic properties of MoS2 bilayers by transition-metal intercalation

Two dimensional (2D) transition-metal dichalcogenides and their heterostructures are important materials for future electronic device applications. By using the first-principles calculations we investigate how the electronic and magnetic properties of MoS2 bilayer is modified by intercalating transition-metals (Fe, Co, and Ni). The Fe-intercalated MoS2 bilayer is found to be a non-magnetic gapless semimetal. Without spin–orbit coupling it has the Dirac state on the Γ-M line in k space at the Fermi level. The spin degeneracy of the Dirac state is removed by spin–orbit coupling. However, the bottom of the conduction band and the top of the valence band are in contact with the Fermi level making the system a non-magnetic gapless semimetal. The Dirac state is also observed in both Co-intercalated and Ni-intercalated MoS2 bilayers because they have the same symmetry in crystal structure as the Fe-intercalated MoS2 bilayer. The difference in the number of valence electrons in the intercalated atoms drastically change the electronic and magnetic properties. The Co-intercalated MoS2 bilayer is a ferromagnetic metal, where the Dirac state is below the Fermi level. The Ni-intercalated MoS2 bilayer is a non-magnetic narrow gap semiconductor, where the Dirac state is far below the Fermi level. The results revealed the electronic and magnetic properties of the transition-metal-intercalated MoS2 bilayers and will be useful for designing a variety of 2D materials.

The eutectic compositional space in Al-Cr-Fe-Ni system utilizing the high-throughput Calphad approach

Eutectic High-Entropy Alloys (EHEAs) show promise in balancing strength and ductility, along with excellent castability and chemical homogeneity. However, identifying optimal eutectic compositions in HEAs has been a persistent challenge, primarily due to the lacuna of multi-component phase diagrams. In addressing this challenge, the eutectic compositional space in Al-Cr-Fe-Ni is unveiled for the first time as a case study using the High-Throughput Calphad methodology. By integrating the Python application programming interface into Thermo-Calc, Latin Hypercube Sampling generated 100,000 compositions examined for eutectic formation based on thermodynamic parameters. This analysis revealed the significant correlation of valence electron concentration, enthalpy of mixing, and atomic size difference on eutectic formation. Moreover, the predictions of the proposed design methodology were compared with reported eutectic compositions, exhibiting excellent agreement. Further, a novel dual-phase eutectic alloy featuring an FCC+B2 combination predicted through the proposed design framework has been successfully cast, providing further experimental validation.

Unraveling the role of the BCC-B2 transition and V occupancies in the contradictory magnetism-ductility relationship of FeCoV alloys

The contradictory relationship between magnetism and ductility restricts further applications of FeCoV alloys in high-performance electrical machines. The role of the BCC-B2 transition, accompanied by vanadium (V) site occupancies, in magnetic moments and ductility has been explored using first-principles calculations. The variations in magnetism and ductility of FeCoV alloys are attributed to the coupling of the BCC-B2 transition and V occupancies. When V replaces Fe atoms in the equiatomic B2-FeCo alloy, the superior magnetism observed in B2-Fe50-cCo50Vc alloys is a consequence of the enhanced local magnetic moment of Fe and the ferrimagnetic-ferromagnetic transition in the magnetic state. Moreover, due to the preferential V occupancy in the B2 phase, the B2-Fe46Co50V4 alloy exhibits comparable ductility to the BCC-Fe50Co46V4 alloy. The results indicate that the increased brittleness in the B2 phase arises from the raised Peierls stress and the enhanced covalent component in interatomic bonding, which is caused by the strong hybridization between Fe and Co atoms. Pearson correlation analysis illustrates that valence electron concentration (VEC) and V content are significant factors in the contradictory relationship between magnetization and ductility. The theoretical results demonstrate that tuning the V content and atomic occupancies is helpful to achieve a trade-off between magnetization and ductility in B2-FeCoV alloys.

Engineering the Activity and Stability of the Zn-MOF Catalyst via the Interaction of Doped Zr with Zn.

Metallic atoms within metal-organic framework (MOF) materials exhibit a distinctive and adaptable coordination structure. The three-dimensional (3D) pore configuration of MOFs enables the complete exposure of metal active sites, rendering them prevalent in various catalytic reactions. In this study, zinc (Zn) atoms within Zn-based MOF materials, characterized by an abundance of valence electrons, are utilized for the transesterification of dimethyl carbonate (DMC). Additionally, the introduction of zirconium (Zr) effectively addresses the susceptibility of the MOFs' crystal structure to dissolution in organic solvents. The formulated catalyst, Zn-10%Zr-MOF(300), demonstrates remarkable catalytic performance with 91.5% DMC selectivity, 61.9% propylene carbonate (PC) conversion, and 56.6% DMC yield. Impressively, the catalyst maintains its high performance over five cycles. Results indicate that Zr interacts with Zn, forming new coordination bonds and enhancing the catalyst crystal structure stability. Moreover, electron transfer intensifies the alkalinity of the active Zn atoms, enhancing the overall catalyst performance. This research informs the development of transesterification heterogeneous catalysts and broadens the application scope of MOF catalysts.

“Capture-activation-recapture” mechanism-guided design of double-atom catalysts for electrocatalytic nitrogen reduction

Compared with the traditional industrial nitrogen fixation, electrocatalytic methods, especially those utilizing double-atom catalysts containing nonmetals, can give good consideration to the economy and environmental protection. However, the existing “acceptance-donation” mechanism is only applicable to bimetallic catalysts and nonmetallic double-atom catalysts containing boron atoms. Herein, a novel “capture-activation-recapture” mechanism for metal-nonmetal double-atom catalyst is proposed to solve the problem by adjusting the coordination environments of nonmetallic atoms and utilizing the activation effect of metal atoms on nitrogen. Based on this mechanism, the nitrogen reduction reaction (NRR) activity of 48 structures is calculated by density functional theory calculation, and four candidates are selected as outstanding electrocatalytic nitrogen reduction catalysts: Si-Fe@NG (UL = –0.14 V), Si-Co@NG (UL = –0.15 V), Si-Mo@BP1 (UL = 0 V), and Si-Re@BP1 (UL = –0.02 V). The analyses of electronic properties further confirm “capture-activation-recapture” mechanism and suggest that the difference in valence electron distribution between metal and Si atoms triggers the activation of N≡N bonds. In addition, a machine learning approach is utilized to generate an expression and an intrinsic descriptor that considers the coordination environment to predict the limiting potential. This study offers profound insight into the synergistic mechanism of TM and Si for NRR and guidance in the design of novel double-atom nitrogen fixation catalysts.

Phase stability and elastic properties of NbMoTaWMx (M = Al, V, Zr, Tc, Re and Ir) RHEAs: A first-principles assessment

Using the exact muffin-tin orbital (EMTO) method in combination with the coherent potential approximation (CPA), we studied the influence of alloying elements Al, V, Zr, Tc, Re and Ir on the phase stability, lattice parameters, densities, and elastic properties of NbMoTaWMx (M = Al, V, Zr, Tc, Re, Ir; 0 ≤ x ≤ 1.0) refractory high entropy alloys. The predicted lattice parameter and elastic parameters agree with the available experimental and theoretical data. We calculated the atomic size difference (δ), the valence electron concentration (VEC) and the total energy of face-centered cubic (fcc), body-centered cubic (bcc) and hexagonal closed-pack (hcp) structures. We found that all studied NbMoTaWMx (M = Al, V, Zr, Tc, Re, Ir; 0 ≤ x ≤ 1.0) systems have the bcc structure. We showed that Al and Ir decrease the stability of the bcc structure and Zr significantly enlarges the lattice parameter of the host. Vanadium increases the ductility of NbMoTaW system. On the other hand, Al and Ir have strongly non-linear effect on the ductility, with clear minimum at x = 0.55. The present results provide consistent theoretical data and call for experimental investigation for these alloys.

Modulating Valence Electrons and Na Occupancy in Layered Cathodes for High-Performance Na-Ion Batteries.

Na-ion batteries (NIBs) hold promise as a leading option for large-scale energy storage. However, their development faces challenges due to the lack of high-performance cathode materials. P2-type layered oxides are seen as potential cathode materials for NIBs due to higher structure stability, yet their commercialization is hindered by limited capacity and subpar phase transitions during Na extraction and insertion at high voltages. In this study, we introduce a new P2-type cathode material, Na0.76Ni0.23Li0.1Ti0.02Mn0.65O1.998F0.02 (NLTMOF), synthesized with ternary Li/Ti/F substitution. This modification of ternary Li/Ti/F substitution significantly tailors the electronic structures, increasing the number of valence electrons near the Fermi energy level. This facilitates the electronic conductivity and their involvement in charge compensation, thereby enhancing reversible capacity. Additionally, ternary doping synergistically adjusts the Na occupancy at the Na layer for favorable Na extraction without P2-O2 phase transitions even under a high voltage of 4.4 V, boosting cycling stability. As a result, NLTMOF demonstrates a reversible capacity of 110.0 and 132.2 mAh g-1 at 2-4.2 and 2-4.4 V, respectively, and maintains greatly enhanced cycling stability over long cycles. This study sheds light on the design of transition metal oxides for advanced cathode materials through the modulation of electronic structure and Na occupancy in cathode materials, thus promoting the development of NIBs.

Field-assisted sintering of high-entropy alloy-reinforced aluminium matrix composites: phase identification and microstructural properties

This study investigates the design, phase identification, and microstructural properties of high-entropy alloy (HEA)-reinforced aluminium (Al) matrix composites. Thermophysical expressions for HEAs were employed during the design phase of the HEA; both theoretical frameworks and experimental analyses were used to anticipate stable phases while a field-assisted sintering technique was employed to consolidate the samples. Calculation of phase diagram (CALPHAD) predictions for the phases present in the HEA align with valence electron concentration (VEC) calculations as both predicted the presence of BCC and FCC phases. The microhardness results reveal a substantial increase in the hardness value of the composites as compared to the pure Al, such that as low as 5 wt% HEA addition resulted in over a 100% improvement, while the densification of the composites was found to decrease with an increase in the wt% of HEA. SEM micrographs and XRD analyses show fair dispersion, bonding, and phase integration in the HEA-reinforced composites.

Sparse-Stochastic Fragmented Exchange for Large-Scale Hybrid Time-Dependent Density Functional Theory Calculations.

We extend our recently developed sparse-stochastic fragmented exchange formalism for ground-state near-gap hybrid DFT to calculate absorption spectra within linear-response time-dependent generalized Kohn-Sham DFT (LR-GKS-TDDFT) for systems consisting of thousands of valence electrons within a grid-based/plane-wave representation. A mixed deterministic/fragmented-stochastic compression of the exchange kernel, here using long-range explicit exchange functionals, provides an efficient method for accurate optical spectra. Both real-time propagation as well as frequency-resolved Casida-equation-type approaches for spectra are presented, and the method is applied to large molecular dyes.

Nature of C-C coupling and strategy of tuning the catalytic activity of Cu-N-C catalysts for electro-reduction of CO2 to ethanol

With high atomic utilization and remarkable catalytic activity, Cu-N-C type catalysts display great potential for electro-catalysis in CO2 reduction. However, the relationship between the active moiety and catalytic activity of generating high-value C2 products is still unclear, and the explicit screening criteria is scarcity. Herein, based on the first-principle simulation, the structure-performance relationship on Cu-N-C type catalysts has been investigated by modulating the CO2 reduction process as the number of Cu atom (Cu1, Cu2, Cu3) and the ligand environment (B, C, N, O, P, S) changed. We find the adsorption strength of intermediate *CO strongly affect the possibility of C-C coupling, which can be determined by Bader charge on Cu atom, mainly depending on the number of loaded atomic Cu on Cu-N-C catalysts. Furthermore, the Bader charge can be refined by adjusting the coordination atom of Cu, thus optimizing catalytic activity for the CO2 to ethanol. The moderate Bader charge value, between +0.35 and +0.45, enables the catalyst to behave as a potentially excellent activity with low limiting potential for generating ethanol. More importantly, an intrinsic descriptor, composed of the radius, electronegativity, and number of valence electrons of coordination atoms (φ=∑χ∑r*∑np), was established to characterize the catalytic activity of Cu-N(X)-C catalysts for producing ethanol. Two excellent catalysts, Cu3-N2O2 (-0.51 V) and Cu3-N3S (-0.64 V), are screened out for the CO2RR to generate ethanol. This work discloses theoretical basis for catalytic selectivity of C2 products on Cu-N-C catalysts and provides a regulating and screening principle for high performance catalysts to ethanol.