Valence Electrons Research Articles

Understanding the effect of foreign atoms occupation on the metamagnetic behaviors in MnCoSi-based alloys: taking Pt-doping as an example

Present research on TiNiSi-type MnCoSi-based alloys focuses on finding a suitable doping element to effectively reduce the critical magnetic field (μ 0 H cri) required to induce a metamagnetic transition. This paper provides a guide to achieve this goal through an experimental investigation of Mn1−x Pt x CoSi and MnCo1−x Pt x Si alloys. In Mn1−x Pt x CoSi, as x increases, μ 0 H cri at room temperature decreases, while in MnCo1−x Pt x Si, it increases. This phenomenon can be attributed to the fact that larger Pt atoms prefer Co sites over Mn sites, as predicted by our density-functional theory. Consequently, in Mn1−x Pt x CoSi, larger Co atoms are extruded into the Mn atoms chain, increasing the nearest Mn–Mn distance and resulting in a reduced μ 0 H cri. This finding suggests that transition-metal atoms with more valence electrons preferably occupy the Co site, while those with fewer valence electrons preferably occupy the Mn site. Adhering to this rule, one can easily obtain a low μ 0 H cri and large magnetostrain under a low magnetic field by selecting a suitable foreign element and chemical formula, as demonstrated by the Mn1−x Pt x CoSi alloy.

Band-Edge Electronic Structure on Photo(electro)catalytic Performance of ABO2 (A = Cu, Ag; B = Al, Ga, In): Elucidating the Role of Valence Electron States

Band-Edge Electronic Structure on Photo(electro)catalytic Performance of ABO₂ (A = Cu, Ag; B = Al, Ga, In): Elucidating the Role of Valence Electron States

Pressure-Induced Changes in the Crystal Structure and Electrical Conductivity of GeV4S8.

Lacunar spinels, represented by AM4X8 compounds (A = Ga or Ge; M = V, Mo, Nb, or Ta; X = S or Se), form a unique group of ternary chalcogenide compounds. Among them, GeV4S8 has garnered significant attention due to its distinctive electrical and magnetic properties. While previous research efforts have primarily focused on studying how this material behaves under cooling conditions, pressure is another factor that determines the state and characteristics of solid matter. In this study, we employed a diamond anvil cell in conjunction with high-energy synchrotron X-ray diffraction, Raman spectroscopy, four-point probes, and theoretical computation to thoroughly investigate this material. We found that the structural transformation from cubic to orthorhombic was initiated at 34 GPa and completed at 54 GPa. Through data fitting of volume vs pressure, we determined the bulk moduli to be 105 ± 4 GPa for the cubic phase and 111 ± 12 GPa for the orthorhombic phase. Concurrently, electrical resistance measurements indicated a semiconductor-to-nonmetallic conductor transition at ∼15 GPa. Moreover, we experimentally assessed the band gaps at different pressures to validate the occurrence of the electrical phase transition. We infer that the electrical phase transition correlates with the valence electrons in the V4 cluster rather than the crystal structure transformation. Furthermore, the computational results, electronic density of states, and band structure verified the experimental observation and facilitated the understanding of the mechanism governing the electrical phase transition in GeV4S8.

Machine learning guided prediction of the yield strength and hardness of multi-principal element alloys

Background Multi-Principal Element Alloys (MPEAs) have better properties, such as yield strength, hardness, and corrosion resistance compared to conventional alloys. Compositional optimization is a challenging task to obtain desired properties of MPEAs and machine learning is a potential tool to rapidly accelerate the search and design of new materials. Methods We have implemented different machine learning models to predict the yield strength and Vickers hardness of MPEAs at room temperature and quantify the uncertainty of the predictions. Results Our results suggest that valence electron concentration (VEC) is the key feature dominating the yield strength and hardness of MPEAs. Our predicted yield strength and hardness values for the experimental validation set show < 15 % error for most cases with respect to the experimental values. Conclusions Our machine learning model can serve as a useful tool to screen half a trillion MPEAs and down select promising compositions for useful applications.

Where are the Excess Electrons in Subvalent Compounds? The Case of Ag7Pt2O7.

Subvalent compounds raise the question of where those valence electrons not belonging to chemical bonds are. In the limiting case of Ag7Pt2O7, there is just one-electron excess in the chemical formula requiring the presence of Ag atoms with oxidation states below +1, assuming conventional Pt4+ and O2- ions. Such a situation challenges the understanding of the semiconducting and diamagnetic behavior observed in this oxide. Previous explanations that localize pairwise the electron excess in tetrahedral Ag4 interstices do not suffice in this case, since there are six silver tetrahedral voids and only an excess of nine electrons in the unit cell. Here, we provide an alternative explanation for the subvalent nature of this compound by combining interatomic distances, electron density-based descriptors, and orbital energetic analysis criteria. As a result, Ag atoms that do not participate in their valence electron are revealed. We identify excess electrons located in isolated subvalent silver clusters with electron-deficient multicenter bonds resembling pieces of metallic bonding in fcc-Ag and Ag7Pt2 alloy. Our analysis of the electronic band structure also supports the multicenter bonding picture. This combined approach from the real and reciprocal spaces reconciles existing discrepancies and is key to understanding the new chemistry of silver subvalent compounds.

Structural and mechanical properties of magnetron sputtered (NbxMo1-x)C thin films

While transition metal carbides (TMCs) exhibit favourable mechanical properties, alloying according to the valence electron concentration (VEC) has the potential to further enhance the properties of these hard but inherently brittle materials. This study investigates the influence of alloying on the microstructure and mechanical properties of (NbxMo1-x)C carbide films, including binary references and ternary compositions with varying metal ratios (x between 0.35 and 0.53). Furthermore, the influence of various substrate materials is studied by comparing films deposited on Al2O3, MgO and SiO2. All films exhibit a NaCl-type carbide structure and X-ray photoelectron spectroscopy revealed the presence of small amounts of an additional amorphous carbon (a-C) phase. Hardness values around 20 ± 2 GPa were obtained for the films on Al2O3 and MgO, whereas a reduced hardness of 11 ± 1 GPa was observed for the films on SiO2 which is attributed to larger crystallite size and more polycrystalline structure. Overall no clear trend as a function of composition can be noted, indicating that microstructure effects dominate the mechanical properties in this study overshadowing the effect of varying the metal content.

Structure Sensitivity of Metal Catalysts Revealed by Interpretable Machine Learning and First-Principles Calculations.

The nature of the active sites and their structure sensitivity are the keys to rational design of efficient catalysts but have been debated for almost one century in heterogeneous catalysis. Though the Brønsted-Evans-Polanyi (BEP) relationship along with linear scaling relation has long been used to study the reactivity, explicit geometry, and composition properties are absent in this relationship, a fact that prevents its exploration in structure sensitivity of supported catalysts. In this work, based on interpretable multitask symbolic regression and a comprehensive first-principles data set, we discovered a structure descriptor, the topological under-coordinated number mediated by number of valence electrons and the lattice constant, to successfully address the structure sensitivity of metal catalysts. The database used for training, testing, and transferability investigation includes bond-breaking barriers of 20 distinct chemical bonds over 10 transition metals, two metal crystallographic phases, and 17 different facets. The resulting 2D descriptor composing the structure term and the reaction energy term shows great accuracy to predict the reaction barriers and generalizability over the data set with diverse chemical bonds in symmetry, bond order, and steric hindrance. The theory is physical and concise, providing a constructive strategy not only to understand the structure sensitivity but also to decipher the entangled geometric and electronic effects of metal catalysts. The insights revealed are valuable for the rational design of the site-specific metal catalysts.

Strong electron-phonon coupling and high lattice thermal conductivity in half-Heusler thermoelectric materials.

Traditional half-Heusler thermoelectric materials, identified as 18-electron compounds, are characterized by the high power factor and the high lattice thermal conductivity. Interestingly, the emerging 19-electron half-Heusler compounds were also found to be promising thermoelectric materials, but with a 5-10 times lower lattice thermal conductivity. Since the two kinds of compounds have similar chemical and physical structures, such as TiCoSb and VCoSb, the large difference in lattice thermal conductivity is a puzzling question. Here, we present a theoretical study to clarify the lattice thermal transport in half-Heusler thermoelectric materials. Based on electronic band structure analysis, we show that the two transition-metal elements in half-Heusler compounds form the strong and direct d-d interaction that is responsible for the high lattice thermal conductivity of 18-electron compounds. In 19-electron half-Heusler compounds, however, the extra valence electron enters the d-d antibonding states, which significantly weakens the atomic bond strength, leading to a large decrease in the cohesive energy. The resulting softened acoustic phonons enhance the phonon-phonon scattering, and thus reduce the lattice thermal conductivity significantly. By constructing an artificial 18-e compound V0.5Sc0.5CoSb, it is proved that the one less electron relative to VCoSb increases the lattice thermal conductivity significantly.

DFT computation of the electron spectra of thiophene

Recent experiments on thiophene using synchrotron radiation stimulated the present density functional theory (DFT) study of the various types of electron spectroscopy using the DFT methods we developed in our laboratory. Besides the confirmation of the experimental interpretations, the results of the present study strongly validate the methods used for the various types of electron spectroscopy. The average absolute deviations of the DFT results for thiophene from experiment are 0.22 eV for vertical ionization energies of outer valence electrons, 0.10 eV for carbon 1s binding energies, 0.19 eV for vertical valence excitation energies, and 0.18 eV for X-ray absorption energies. The calculated carbon 1s binding energies for bithiophene and terthiophene are also within 0.1 eV of synchrotron experiment. We conclude that workers using synchrotron radiation to study electronic processes of gas-phase molecules can apply our DFT methods for the interpretation of their experimental results.

The structural, stability, electronic, optical and thermodynamic properties of MoX2 (X= S, Se, and Te) under hydrostatic pressures: a plasmon approach and first-principle study.

The new equations have been developed for the structural and electronic properties using the plasmon calculations for the first time for 2-D MoX2 structures. Literature shows still an extensive study is required on the stability and optical properties of MoX2 under different hydrostatic pressures and thermal properties under different temperatures using the first principles, for electronic industrial applications. The stability is analyzed using binding energy and phonon calculations. The phase transition of metallization of MoX2 is discussed using band structure calculations under different hydrostatic pressures. The calculated work function shows the photoemission starts from the threshold frequency of 4.189×104 cm-1, 3.184×104 cm-1, and 3.651×104 cm-1, respectively, for MoS2, MoSe2, and MoTe2 materials. The optical properties such as refractive index n(0), and static dielectric permittivity ε(0) for three successive materials are calculated under different hydrostatic pressures, applicable for optoelectronic applications. The calculated theoretical and computational values agree well with each other and also agree with reported and experimental values. Some of the values are calculated for the first time. The theoretical equations are derived using the molecular weight, effective valence electrons, and density of molecule of MoX2 structures. The simulation work is performed using GGA-PBE approximation in the CASTEP simulation package with DFT+D semi-empirical dispersion correction. An ultra-soft pseudopotential representation calculates the electronic and optical properties with a finite basis set kinetic energy cut-off of 381.0 eV. Each geometry has been optimized using Broyden, Fletcher, Goldfarb, and Shanno's (BFGS) algorithm for 100 iterations with a fixed basis quality variable cell method and finite electronic minimization parameters. The phonon calculations were performed using TDFT with a kinetic energy cut of 460 eV in a norm-conserving linear response method. The interpolation with a finite dispersion quality and q-vector grid spacing is performed.

Structural stabilities and natural half-metallic properties of OsXCoSi (X=Ti, Zr, Hf) quaternary Heusler alloys series first-principles calculations

Based on first-principles calculation of density functional theory, this study investigates the structural stability, magnetic properties, and electronic properties of the three different phases (i.e. type 1, type 2, and type 3) of OsXCoSi (X=Ti, Zr, Hf) in a new quaternary Heusler alloy series. The corresponding equilibrium lattice constants of each type are optimized, and the change of formation enthalpy and elastic constant phonon spectrum show that the OsXCoSi (X=Ti, Zr, Hf) alloy is thermodynamically, dynamically and mechanically stable. Furthermore, the bonding features of each phase are discussed. It is found that all type 1 structures of OsXCoSi (X=Ti, Zr, Hf) exhibit natural half-metallicity (HM) in equilibrium lattice constant, and their equilibrium lattice constants in the ground state were determined to be 5.909 Å for OsTiCoSi, 6.155 Å for OsZrCoSi, and 6.100 Å for OsHfCoSi. Meanwhile, by testing the alloy under different pressures, the range of the integer magnetic moment non-equilibrium lattice constants for the three alloys OsTiCoSi, OsZrCoSi, and OsHfCoSi are 5.710 Å ∼ 6.329 Å, 5.696 Å ∼ 6.1557 Å and 5.716 Å ∼6.1009 Å, respectively, which is wide and is more close to the practical application for spin-polarized materials. In addition, its magnetic moment is consistent with the values given by the Slater–Pauling rule. Furthermore, the forming of the HM gap is examined by analysing the total and partial density of states, energy bands of alloy’s electronic property, with respect to the calculated results. What’s more, special attention is paid to the differences of the properties for series Heusler alloys. It is found that the electronics properties distinction is mainly based on valence electron changes. However, the lattice constants are susceptible to size of a nucleus.

Effect of Yb doping on the optical and photoelectric properties of CsPbCl3 single crystals

In this paper, the effect of Yb dopant on the optical and photoelectric properties of CsPbCl3:Yb single crystals is studied. The position of the main energy level of Yb3+ ions relative to the energy band was determined. Two channels of electron photoionization into the conduction band were identified, which are photo-stimulated transition of valence electrons and transition of electrons from the ground level of the Yb3+ dopant ion. The crucial role of Yb2+ ions in the manifestation of the quantum cutting effect was shown. A non-uniform distribution of ytterbium dopant in the CsPbCl3:Yb single crystal volume was revealed. It was established that the absolute quantum yield of ytterbium luminescence in CsPbCl3:Yb single crystals in the near-infrared region at Yb concentration of 2 mol.% reaches 86%.

Unveiling the influence of various vacancies on the mechanical properties of Ti4C3O2

Two-dimensional transition metal carbides and nitrides are widely recognized as potential candidates in areas such as energy storage and sensors. In this work, we explore the mechanical properties of Ti4C3O2 with a perfect cell and Ti4C3O2 with vacancies by a first-principles method. The results indicate that the loss of C in the outer layer of Ti4C3O2 is more likely to occur due to the smaller formation energy of C-vacancy. By considering the charge contribution, the removed atom changes the valence electron equilibrium concentration between that atom and its neighbors, which in turn affects the mechanical properties of Ti4C3O2. The presence of various vacancies affects the strength of the TiO bonds on the shear plane, causing Ti4C3O2 to exhibit changes towards brittleness or ductility. It is noteworthy that the elastic modulus, critical strain, and ideal stress of Ti4C3O2 with different concentrations of O vacancies are all approximately the same, suggesting that the variations in their mechanical properties are not significant for Ti4C3O2 with different degrees of O functional group coverage. Our findings offer valuable insights for the development of MXene-based nanoflexible energy storage devices, gas-sensitive sensors, high-temperature environmental applications, and beyond.

Preparation and characterization studies of silica tin nanospheres from Indian rice husk for suitable photocatalytic applications

The synthesis of nanomaterials is advancing due to its ease of synthesis, inexpensiveness, nontoxicity, and renewability. In the present study, an eco-friendly sol–gel method was employed to prepare silica tin nanospheres from Indian rice husks. We used cetyl trimethyl ammonium bromide as the surfactant at room temperature. The calcination of the material at 1000 °C for 24 h gave nanospheres with a diameter of 3–5 nm. The PXRD pattern confirms that the material is amorphous. The FTIR analysis confirms the existing functional groups of silica tin nanospheres. The optical properties of silica tin nanospheres were evaluated using UV visible spectra analysis. The Energy gap of the silica tin nanospheres was calculated and it was found to be 4.85 eV. Using DLS measurements the size of the silica tin nanospheres was estimated to lie between 43 nm and 226 nm and the particles having 108 nm diameter showed an intensity of 90 %. The dielectric properties of silica tin nanospheres were studied in the range of 1 Hz to 1000 MHz at various temperatures. The dielectric properties of silica tin nanospheres were found to be significantly enhanced, especially in the low-frequency range due to quantum confinement. Besides, the values such as valency electron plasma energy, Penn energy, fermi energy, and electric polarizability were evaluated. From the SEM analysis it is observed that the material is nanosized sphere. Also, the SEM analysis reveals that the spherical structure has a high porous and uniformly agglomerated structure. The study of the elemental composition of silica tin nanospheres through XRF analysis has been carried out.

Predication of novel ordered phases in Cu-Be system at ambient pressure: Crystal structures, electronic properties, and alloying mechanism

The crystal and electronic structures determine a material's physical and chemical properties. Here, using particle swarm optimization method and first-principles calculations, we conducted a systematically investigation on the ordered CuxBey alloys. Two novel stable phase CuxBey (P4¯m2-CuBe3, and I4/mmm-Cu2Be) and five novel metastable CuxBey alloys (F4¯3m-CuBe5, P21/m-CuBe4, P4/mmm-Cu3Be, P4/mmm-Cu4Be, and P4/mmm-Cu5Be) are predicted for the first time. The primary alloying form of ordered CuxBey alloys is the substitution of certain Be atoms for some Cu atoms in the FCC-Cu according to structural study. Obvious valence electrons transfer from Be to Cu and the lattice interstice are observed, which leads to Cu exhibiting negative valence state in CuxBey and Be-rich CuxBey acting as zero-dimensional electrides. Furthermore, these Be-rich CuxBey have good structural stability due to the interaction between interstitial anionic electrons and Be atoms in the Be-tetrahedron. These findings help us in the design and application of CuxBey alloys and related materials.

Alkaline Earth Metal Superatom of W@Si16: Characterization of Group 6 Metal Encapsulating Si16 Cage on Organic Substrates.

Transition metal atom (M)-encapsulating silicon cage nanoclusters (M@Si16) exhibit a superatomic nature, depending on the central M atom owing to the number of valence electrons and charge state on organic substrates. Since M@Si16 superatom featuring group 4 and 5 transition metal atoms exhibit rare-gas-like and alkali-like characteristics, respectively, group 6 transition metal atoms are expected to show alkaline earth-like behavior. In this study, M@Si16, comprising a central atom from group 6 (MVI = Cr, Mo, and W) were deposited on C60 substrates, and their electronic and chemical stabilities were investigated in terms of their charge state and chemical reactivity against oxygen exposures. In comparison to alkali-like Ta@Si16, the extent of charge transfer to the C60 substrate is approximately doubled, while the oxidative reactivity is subdued for MVI@Si16 on C60, especially for W@Si16. The results show that a divalent state of MVI@Si162+ appears on the C60 substrate, which is consistently calculated to be a symmetrical cage structure of W@Si162+ in C3v, revealing insights into the "periodic law" of M@Si16 superatoms pertaining to the characteristics of alkaline earth metals.

Effective Descriptor for Screening Single-Molecule Conductance Switches.

The adsorption-type molecular switch exhibits bistable states with an equivalently long lifetime at the organic/inorganic interface, promising reliable switching behavior and superior assembly ability in the electronic circuits at the molecular scale. However, the number of reported adsorption-type molecular switches is currently less than 10, and exploring these molecular switches poses a formidable challenge due to the intricate interplay occurring at the interface. To address this challenge, we have developed a model enabling the identification of diverse molecular switches on metal surfaces based on easily accessible physical characteristics. These characteristics primarily include the metal valency electron concentration, the work function of metal surfaces, and the electronegativity difference of molecules. Using this model, we identified 56 new molecular switches. Employing the gradient descent algorithm and statistical linear discriminant analysis, we constructed an explicit descriptor that establishes a relationship between the interfacial structure and chemical environment and the stability of molecular switches. The model's accuracy was validated through density functional theory calculations, achieving a 90% accuracy for aromatic molecular switches. The conductive switching behaviors were further confirmed by nonequilibrium Green's function transport calculations.

XB2Bi2 (X = Si, Ge, Sn, Pb): Penta-Atomic Planar Tetracoordinate Si/Ge/Sn/Pb Clusters with 20 Valence Electrons.

Planar tetracoordinate silicon, germanium, tin, and lead (ptSi/Ge/Sn/Pb) species are scarce and exotic. Here, we report a series of penta-atomic ptSi/Ge/Sn/Pb XB2Bi2 (X = Si, Ge, Sn, Pb) clusters with 20 valence electrons (VEs). Ternary XB2Bi2 (X = Si, Ge, Sn, Pb) clusters possess beautiful fan-shaped structures, with a Bi-B-B-Bi chain surrounding the central X core. The unbiased density functional theory (DFT) searches and high-level CCSD(T) calculations reveal that these ptSi/Ge/Sn/Pb species are the global minima on their potential energy surfaces. Born-Oppenheimer molecular dynamics (BOMD) simulations indicate that XB2Bi2 (X = Si, Ge, Sn, Pb) clusters are robust. Bonding analyses indicate that 20 VEs are perfect for the ptX XB2Bi2 (X = Si, Ge, Sn, Pb): two lone pairs of Bi atoms; one 5c-2e π, and three σ bonds (two Bi-X 2c-2e and one B-X-B 3c-2e bonds) between the ligands and X atom; three 2c-2e σ bonds and one delocalized 4c-2e π bond between the ligands. The ptSi/Ge/Sn/Pb XB2Bi2 (X = Si, Ge, Sn, Pb) clusters possess 2π/2σ double aromaticity, according to the (4n + 2) Hückel rule.

Phase Transformation Temperature Prediction in Steels via Machine Learning.

The phase transformation temperature plays an important role in the design, production and heat treatment process of steels. In the present work, an improved version of the gradient-boosting method LightGBM has been utilized to study the influencing factors of the four phase transformation temperatures, namely Ac1, Ac3, the martensite transformation start (MS) temperature and the bainitic transformation start (BS) temperature. The effects of the alloying element were discussed in detail by comparing their influencing mechanisms on different phase transformation temperatures. The training accuracy was significantly improved by further introducing appropriate features related to atomic parameters. The melting temperature and coefficient of linear thermal expansion of the pure metals corresponding to the alloying elements, atomic Waber-Cromer pseudopotential radii and valence electron number were the top four among the eighteen atomic parameters used to improve the trained model performance. The training and prediction processes were analyzed using a partial dependence plot (PDP) and Shapley additive explanation (SHAP) methods to reveal the relationships between the features and phase transformation temperature.

Constructing diatomic catalyst on MoSSe achieves ultra-low overpotential for nitrogen fixation: First-principles study

The collaborative interaction between the diatoms can enhance the catalytic activity of the transition metal atoms and facilitate the activation of nitrogen molecules. Based on first-principles calculations, the feasibility of electrocatalytic nitrogen fixation as a diatomic catalyst (TMs@MoSSe) formed by transition metal dimers (V, Cr, Mn, Fe, Co, and Ni) dispersed on a two-dimensional Janus MoSSe monolayer is systematically investigated. The results demonstrate that the MoSSe monolayer loaded with double chromium atoms (Cr2@MoSSe) exhibits exceptional catalytic activity, showcasing an extremely low overpotential of 0.17 V in the enzymatic mechanism. Specifically, this research elucidates the nitrogen reduction capacity of the electrocatalyst by considering its magnetic moment and work function, concluding that a lower work function corresponds to enhanced catalytic activity. Additionally, the product of the magnetic moment and valence electron numbers of the transition metal atoms (μTM⋅dTM) exhibits an inverted volcano relationship with the overpotential. These findings can provide valuable insights for the design of diatomic catalysts.