3d States Research Articles

Comparative analysis of electronic, mechanical, vibrational, thermodynamical and thermoelectric properties of Li based quaternary Heusler’s LiNbRhAl, LiNbRhGa and LiNbRhIn alloys: A DFT study

We have performed a comprehensive demonstration of electrical, structural, dynamical, mechanical, thermodynamical and transport properties of Li based LiNbRhZ (Z= Al, Ga, and In) quaternary Heusler (QH) alloys utilising Density Functional Theory (DFT). Calculation of electronic properties via generalized gradient approximation (GGA) and Heyd-Scuseria-Ernzerhof (HSE06) hybrid method, disclose the semiconductor nature with indirect band gap. We found the bandgap calculated by HSE06 is close to experimental value as compared to GGA. Density of states (DOS) analysis shows that 4d states of Nb and Rh atoms contributed significantly to the valance and conduction band near Fermi energy. Absence of negative frequency in phonon dispersion curve shows the dynamical stability and the negative value of formation energy suggested the thermodynamic stability of these alloys. To confirm practical reliability, elastic and mechanical properties are calculated. The value of Pugh's ratio and Cauchy pressure ensures the ductile nature of studied alloys. We have calculated all thermoelectric (TE) parameters via Boltzmann Transport theory. The computed melting point of LiNbRhZ (Z = Al, Ga, and In) alloys are found to be 1911.69 ± 300 K, 1790.97 ± 300 K and 1788.42 ± 300 K respectively, which anticipate potential in high temperature thermoelectric devices. The role of lattice vibrations in total thermal conductivity is prominent only up to 300 K in all discussed alloys. The amazing thermoelectric characteristics of LiNbRhZ alloys render these quaternary Heusler alloys as possible thermoelectric materials in high temperatures.

Efficient optical property screening of :Ce (RE = , and Y): Based on activator level position in host band gap

Property screening is essential to identify optical materials with desirable luminescent and thermal properties for specific applications under various conditions. As promising candidates for optical applications across diverse fields, Ce-doped ▪ (RE = ▪, and Y) are studied by utilizing first-principles calculations based on density-functional theory. By employing a simple yet effective approach to analyze the electronic structures, we gain insights into the absorption edge characteristics of ▪, and evaluate the luminescent and thermal properties from the 4f and 5d positions of Ce in the host band gaps. Consequently, it is evident that the absence of luminescence in Ce-doped ▪ is due to the 5d excited states of ▪ being situated in the conduction band of ▪, regardless of its space group. While the 4f and 5d states of ▪ reside within the band gap of other ▪ hosts, potentially leading to luminescence. Excluding certain lanthanides due to potential charge transfer issues with ▪, our focus is directed towards the luminescent properties of Ce-doped ▪, ▪, ▪, and ▪. Based on schematic configuration coordinate diagrams, we conclude more significant thermal quenching resulting from 5d ionization in ▪:Ce and ▪:Ce compared to ▪:Ce and ▪:Ce. Furthermore, we provide valuable insights into the experimental phenomenon that the energy barrier for thermal quenching is smaller than that for thermal ionization in ▪:Ce. Additionally, we assess the photoelectric and thermoelectric performance based on the thermal and optical charge transition levels of Ce in ▪. Based on our study of the luminescent properties and thermal behaviors among Ce-doped ▪, we offer suggestions for screening Ce-activated optical materials, which provide valuable guidance for the design and optimization of specific materials.

Structural properties, thermodynamic stability and reaction pathways for solid-state synthesis of Bi2WO6 polymorphs.

This study presents a density functional theory (DFT) investigation into the structural, electronic, and optical properties, thermodynamic stability, phase competition, and reaction pathways of Bi2WO6, a notable compound within the Aurivillius family of oxides. We examine three distinct polymorphs of Bi2WO6: the low-temperature orthorhombic phase (P1), the intermediate-temperature orthorhombic phase (P2), and the high-temperature monoclinic phase (P3). Electronic structure analysis indicates band gaps of 2.339 eV (P1), 2.312 eV (P2), and 2.128 eV (P3), with the valence band primarily composed of O 2p states and the conduction band of Bi 6p and W 5d states. Optical properties, including the dielectric function and absorption spectra, show distinct behaviors for each phase, particularly P3. Elastic and phonon property analyses confirm the mechanical and dynamical stability of all three phases, with the P1 phase exhibiting the highest bulk modulus and stiffness among the polymorphs. Our effective mass calculations suggest that the P3 phase may have better charge carrier mobility compared to the P1 and P2 phases. Structural optimizations reveal marginal differences in total energy among these phases, suggesting their potential coexistence or easy phase transitions under varying conditions. Detailed Gibbs free energy calculations confirm that the P1 phase is the most stable at low temperatures, in agreement with experimental data in the literature. We also construct a chemical reaction network to explore feasible reaction pathways for the solid-state synthesis of Bi2WO6 from Bi2O3 and WO3 precursors, identifying several low-cost reaction pathways, including both direct and multi-step routes involving intermediates such as Bi14WO24 and Bi2W2O9.

Cementite with substitutional Nb atoms based on first-principles calculations

Abstract Cementite is the strengthening phase in pearlite, and its mechanical properties determine the strength of the pearlite. A crystal model of niobium-substituted Fe3C was constructed. The stability, charge density, and elastic constants of the model were calculated, and the effects of niobium (Nb) substitution on the properties of Fe3C were discussed. FeNb8d 2C is the most stable Nb-substituted structure. The reaction energy of FeNb8d 2C is -0.796 eV, with a magnetic moment of 0.07 μB/f.u. At the Fermi level, the DOS of FeNb8d 2C is 4.24, displaying metallic properties. Nb substitution enhances the symmetry of the DOS in Fe3C. The covalent bonding in FeNb8d 2C, predominantly consisting of σ and π bonds, is formed by the hybridization of Nb 3d states and C 2p states. FeNb8d 2C is mechanical stability. It exhibits the strongest resistance to linear compression along the [100] direction and the highest shear deformation resistance along the (010) plane. Nb substitution increases the toughness of Fe3C, enhances isotropy on the (010) plane, and improves the resistance to linear compression along the [010] direction.

Optimizing Cu 3d Bands with Nanotubular SnO2 to Boost Their Catalytic Transfer Hydrogenation Activity.

Catalytic transfer hydrogenation (CTH) using Cu nanocatalysts offers significant advantages over direct high-pressure hydrogenation. However, the active hydrogen (H*) in this process exhibits poor adsorption and tends to release H2 readily due to the fully occupied 3d states of Cu. To address this issue, a tubular SnO2 support with electron-accepting ability was selected to host Cu nanoparticles, aiming to optimize the Cu 3d bands. The Cu/SnO2 nanohybrids were prepared through an electrospinning technique, followed by hydrothermal synthesis. As evidenced by X-ray photoelectron spectroscopy (XPS) binding energy shifts and density functional theory (DFT) simulations, some electrons from Cu transferred to SnO2 in the Cu/SnO2 nanohybrids due to their different work functions. Such electron transfer enables the Cu 3d orbitals to lose electrons and alters its valence configuration from 3d10 to 3d10-x, which enhances the adsorption of active H* atoms and thereby inhibits undesirable H2 release. The 15 wt % Cu/SnO2 exhibits improved catalytic hydrogenation of 4-nitrophenol with NaBH4, with an optimal normalized rate constant of 56.98 mg-1 min-1 and a turnover frequency of 4.82 min-1, surpassing most reported catalysts. The enhanced activity is attributed to the optimized electronic states, improved hydrogen adsorption, and the tubular structure of the support. This work might shed light on developing more non-noble metal nanocatalysts for CTH by tuning their d bands with appropriate oxide supports.

Asymmetric coordinative modulation boosting the activity and thermal stability of Pt1/CeO2 for CO oxidation under harsh condition

Supported metal catalysts play a central role in chemical industry, but it remains a formidable challenge to realize high activity while maintaining atomically dispersed status to maximize atom efficiency under harsh conditions. Herein, a novel strategy of constructing asymmetric coordination Pt active sites via coating N-doped carbon onto ceria supported Pt single atoms (Pt1/CeO2@CN) is proposed, through which superior low-temperature CO oxidation activity and satisfactory thermal stability are achieved simultaneously. Experimental and density functional theory results confirm that the unique asymmetric N2-Pt1-O2 coordinative configuration on Pt1/CeO2@CN tailors the electronic properties of Pt 5d states, increasing the turnover frequency by 15 times for CO oxidation at 120 °C compared with that on Pt1/CeO2 and sustaining the atomically dispersed status of Pt at as high as 400 °C under H2-containing atmosphere. This strategy suggests a promising avenue to fabricating highly active and stable supported metal catalysts for practical applications under harsh conditions.

Evolution of metallicity, enhancement of [formula omitted] and magnetic anisotropy energy in Y[formula omitted]NiIrO[formula omitted]: Hydrostatic ([111]) strain influence

Ir-based double perovskite oxides (DPO) provide a distinct electronic and magnetic behavior due to entanglement among lattice distortion, strong electron correlation, and spin–orbit coupling (SOC). In this work, we investigated the hydrostatic ([111]) strain impact on the physical properties of the Y2NiIrO6 DPO using ab-initio calculations. Unstrained motif displayed the ferrimagnetic (FiM) spin state owing to strong antiferromagnetic (AFM) interactions between Ni↑ and Ir↓ ions, further confirmed by the computed partial spin magnetic moments and 3D spin-magnetization density iso-surfaces plots. A Mott-insulating state is established with an energy band gap (Eg) of 0.43 eV due to the existence of a rare Ir+4 state having Jeff.=12 and a Curie temperature (TC) of 198 K using the Heisenberg Hamiltonian model, which is up to the experimental observations. The easy magnetic axis is the [010] (b-axis) having a giant magnetic anisotropy energy (MAE) constant of 1.7 × 108 erg/cm3. Moreover, it is predicted that strain holds the FiM spin order as a magnetic ground state for the considered range of ±8%. Notably, an electronic transition from Mott-insulating to a metallic state is established at a critical compressive strain of −8%, where the admixture of Ir 5d states appears at/around the Fermi level. On the other hand, Eg solely increases with the increase of tensile strain amplitude. Due to strong and weak hybridization, the spin/orbital magnetic moment value is reduced and enhanced as a function of compressive and tensile strains, respectively. Along with this, it is found that MAE/TC increases to 25%/18% and 15%/10% at −8% compressive and ＋8% tensile strains due to larger structural distortion than that of the unstrained one, which enhances the system potential for magnetic memory devices.

Theoretical investigations of optoelectronic properties, photocatalytic performance as a water splitting photocatalyst and band gap engineering with transition metals (TM = Fe and Co) of K3VO4, Na3VO4 and Zn3V2O8: a first-principles study.

First-principles density functional investigations of the structural, electronic, optical and thermodynamic properties of K3VO4, Na3VO4 and Zn3V2O8 were performed using generalized gradient approximation (GGA) via ultrasoft pseudopotential and density functional theory (DFT). Their electronic structure was analyzed with a focus on the nature of electronic states near band edges. The electronic band structure revealed that between 6% Fe and 6% Co, 6% Co significantly tuned the band gap with the emergence of new states at the gamma point. Notable variations were highlighted in the electronic properties of Na3V(1-x)Fe x O4, Na3V(1-x)Co x O4, K3V(1-x)Fe x O4, K3V(1-x)Co x O4, Zn3(1-x)V2(1-x)Co x O8 and Zn3(1-x)V2(1-x)Fe x O8 (where x = 0.06) due to the different natures of the unoccupied 3d states of Fe and Co. Density of states analysis as well as α (spin up) and β (spin down) magnetic moments showed that cobalt can reduce the band gap by positioning the valence band higher than O 2p orbitals and the conduction band lower than V 3d orbitals. Mulliken charge distribution revealed the presence of the 6s2 lone pair on Zn, greater population and short bond length in V-O bonds. Hence, the hardness and covalent character develops owing to the V-O bond. Elastic properties, including bulk modulus, shear modulus, Pugh ratio and Poisson ratio, were computed and showed Zn3V2O8 to be mechanically more stable than Na3VO4 and K3VO4. Optimal values of optical properties, such as absorption, reflectivity, dielectric function, refractive index and loss functions, demonstrated Zn3V2O8 as an efficient photocatalytic compound. The optimum trend within finite temperature ranges utilizing quasi-harmonic technique is illustrated by calculating thermodynamic parameters. Theoretical investigations presented here will open up a new line of exploration of the photocatalytic characteristics of orthovanadates.

GRNAde: A Geometric Deep Learning Pipeline for 3D RNA Inverse Design.

Fundamental to the diverse biological functions of RNA are its 3D structure and conformational flexibility, which enable single sequences to adopt a variety of distinct 3D states. Currently, computational RNA design tasks are often posed as inverse problems, where sequences are designed based on adopting a single desired secondary structure without considering 3D geometry and conformational diversity. In this tutorial, we present gRNAde, a geometric RNA design pipeline operating on sets of 3D RNA backbone structures to design sequences that explicitly account for RNA 3D structure and dynamics. gRNAde is a graph neural network that uses an SE (3) equivariant encoder-decoder framework for generating RNA sequences conditioned on backbone structures where the identities of the bases are unknown. We demonstrate the utility of gRNAde for fixed-backbone re-design of existing RNA structures of interest from the PDB, including riboswitches, aptamers, and ribozymes. gRNAde is more accurate in terms of native sequence recovery while being significantly faster compared to existing physics-based tools for 3D RNA inverse design, such as Rosetta.

Electronic structure and thermoelectric properties of epitaxial Sc1−xVxNy thin films grown on MgO(001)

The electronic structure of Sc1−xVxNy epitaxial films with different alloying concentrations of V are investigated with respect to effects on thermoelectric properties. Band structure calculations on Sc0.75V0.25N indicate that V 3d states lie in the band gap of the parent ScN compound in the vicinity of the Fermi level. Thus, theoretically, the presence of light (dispersive) bands at the Γ point with band multiplicity is expected to lead to lower electrical resistivity, while flat (heavy) bands at X−W−K symmetry points are associated with higher Seebeck coefficients than that of ScN. Hence, to probe the thermoelectric properties experimentally, epitaxial Sc1−xVxNy thin film samples were deposited on MgO(001) substrates. All the samples showed N substoichiometry and pseudocubic crystal structure. The N-vacancy-induced states were visible in the Sc 2p x-ray absorption spectroscopy spectra. The reference ScN and Sc1−xVxNy samples up to x=0.12 were n type, exhibiting carrier concentration of 1021 cm−3, typical for degenerate semiconductors. For the highest V alloying of x=0.15, holes became the majority charge carriers, as indicated by the positive Seebeck coefficient. The underlying electronic structure and bonding mechanisms in Sc1−xVxNy influence the electrical resistivity, Seebeck coefficient, and Hall effect. Thus, this paper contributes to the fundamental understanding to correlate defects and thermoelectric properties to the electronic structure in the Sc-N system with V alloying. Published by the American Physical Society 2024

Optical and electrochemical performance of NdVO4 nanorods

Tetragonal structured NdVO4 nanorods were prepared by simple solvothermal route with various temperatures. An optimized NdVO4 sample was characterized through XRD, FTIR, RAMAN, UV–Visible, PL, SEM, HRTEM and EDX techniques. Electrochemical performance of NdVO4 electrodes were analyzed by CV instrument. From XRD, it is confirmed that the prepared samples were perfectly formed in pure tetragonal structured NdVO4 nanoparticles with the crystallite size of 41.08 nm, 44.92 nm and 54.78 nm for the temperature of 240 °C, 260 °C, and 280 °C, respectively. In UV–Visible absorption spectrum, there are seven peaks raised for NdVO4. The XPS shows the three elements that are Nd, V and O with the corresponding electronic configuration states of 3d, 2p and 1 s, respectively. SEM and HRTEM picture the nanorod morphology of the prepared NdVO4 nanoparticles. The electrochemical performance of the NV1 electrode showed pseudocapacitive existence with the current density of 10 mV/s (362F/g) and it will act as well behaved electrode in supercapacitors.

Antiferromagnetism, superparamagnetism, and effects of A-cation distribution on magnetic properties of LaSrFeO[formula omitted] Ruddlesden–Popper oxides: A combined experimental and theoretical study

Understanding the structure–property relationship in complex magnetic oxides is crucial for developing new magnetic materials. In this study, we synthesized LaSrFeO4 Ruddlesden–Popper oxide using spray-pyrolysis method. Electron paramagnetic resonance (EPR) spectra revealed two Fe-associated lines: a broad line indicating short-range antiferromagnetic (AFM) order, disappearing at 375 K, and a narrow line due to the presence of superparamagnetic phase. The magnetic moment from the linewidth’s temperature dependence was 10−19 A m2. EPR intensity analysis estimated the superparamagnetic to antiferromagnetic phase ratio at 0.014 ± 0.004.Using density-functional theory with Agapito–Curtarolo–Buongiorno–Nardelli (ACBN0) pseudo-hybrid functional including self-consistent Hubbard U correction, we calculated the density of states for two La/Sr atomic distributions. The calculations show that a uniform La/Sr distribution perturbs 2D symmetry of Fe 3d states, which can contribute to the observed lowering of the critical exponent β for the magnetically ordered phase. These results highlight the impact of La/Sr distribution on LaSrFeO4’s properties.

The Effect of Silicon Substitution by Boron for the α-Nb5Si3: INSIGHTS into the Constitutive Properties of Nb5Si2B Through Theory and Experimental Approach

We investigated the structural, elastic, vibrational, and electronic properties of the Nb5Si2B compound combining density functional theory (DFT) calculations and experimental methods. We compared our results with the parent compound Nb5Si3 with two non-equivalent Si sites namely Si(4a) and Si(8h). The analysis of elastic constants and phonon spectra indicate that Nb5Si2B is respectively mechanically and dynamically stable. Based on the phonon calculation we evaluate the theoretical constant volume lattice specific heat (CV) for different site occupancies and compare it with experimental specific heat (Cp) measurements. We found an excellent agreement between theoretical and experimental results for Nb5Si2B with the B at the Si(8h) site, which agrees with the calculated formation energy. In addition, we also performed DFT calculations aimed at showing and comparing the total DOS near the Fermi level (EF) for Nb5Si3 and Nb5Si2B. The XPS valence band (VB) of the Nb5Si2B is largely dominated by two characteristic peaks at − 8.7 and − 1.7 eV, respectively. Based on DFT calculations, it follows that the main sharp peak at − 1.7 eV comes as a contribution from Nb 4d states, while the smaller and broader one located at − 8.7 eV results mainly from Si 3s states weakly hybridized with Nb 4d states. In this connection, the majority contribution in the binding energy range from − 12 eV to the EF comes from Nb 4d states, while the contribution from Si and B atoms is very small in this region. The core levels of Nb 3d, Si 2s, 2p, and B 1s were also identified using the XPS technique.

Unravelling NbSe2 single crystal: First principle insights, optical properties, synthesis and X-ray diffraction profile investigation

Niobium diselenide (NbSe2) crystals were synthesized using the chemical vapour transport (CVT) technique. The Generalized Gradient Approximation (GGA) method was used in the density functional theory (DFT) calculation. The Perdew-Burke-Ernzerhof (PBE) pseudopotential function was utilised to emphasise the structural, electrical, elastic constant, and optical characteristics of the hexagonal P63/mmc symmetric structured crystalline phase of NbSe2. Additionally, the density of states (DOS) revealed that, at the Fermi level, the Nb (3d) states predominated, with the Se (5p) and Se (5 s) states playing a minor role. The band structure confirmed the crystal's metallic nature, which showed an overlap between the conduction and valence bands. Scherrer method, Williamson-Hall (WH) analysis, Size-Strain Plot (SSP), and Halder-Wagner (HW) plot utilising uniform deformation model (UDM), uniform stress deformation model (USDM), and uniform deformation energy density model (UDEDM) models were used to investigate the X-ray diffraction (XRD) profile. The crystallite size, inherent stress, strain, and energy density parameters were estimated. Optical microscopy and transmission electron microscopy (TEM) had both been used to study surface morphology.

Multifaceted control of focal points along an arbitrary 3D curved trajectory

Metalenses can integrate the functionalities of multiple optical components thanks to the unprecedented capability of optical metasurfaces in light control. With the rapid development of optical metasurfaces, metalenses continue to evolve. Polarization and color play a very important role in understanding optics and serve as valuable tools for gaining insights into our world. Benefiting from the design flexibility of metasurfaces, we propose and experimentally demonstrate a super metalens that can realize multifaceted control of focal points along any 3D curved trajectory. The wavelengths and polarization states of all focal points are engineered in a desirable manner. The super metalens can simultaneously realize customized 3D positioning, polarization states, and wavelengths of focal points, which are experimentally demonstrated with incident wavelengths ranging from 501 to 700 nm. We further showcase the application of the developed super metalenses in 3D optical distance measurement. The compact nature of metasurfaces and unique properties of the proposed super metalenses hold promise to dramatically miniaturize and simplify the optical architecture for applications in optical metrology, imaging, detection, and security.

DFT study of electron energy loss spectra of sulfur in Janus MoSSe, MoSTe, WSSe and WSTe monolayers

In this paper, electronic properties of Janus MSX monolayers (M = Mo, W; X = Se or Te), including electron energy loss near edge structures (ELNES), as K and [Formula: see text] edge spectra of sulfur atoms, have been calculated using a full potential scheme and the augmented plane wave plus local orbitals (APW+lo) method in the framework of density functional theory (DFT). Due to the lack of experimental results, the obtained spectra are compared with the corresponding spectra of MoS2 monolayer. The band structure analysis shows that the Janus MoSSe and WSSe monolayers are direct bandgap semiconductors, while the Janus MoSTe and WSTe monolayers are indirect bandgap semiconductors. In comparison to MoS2, the main structures of the K edge ELNES spectra of sulfur atoms in Janus monolayers occur at lower energies, due to the structural change and increased bond length. The relative reduction of intensities in the main structures predicts the possibility of reducing the number of partial densities of states (PDOS), that is, the reduced p PDOS for the K edge in the corresponding energy range. In the K ([Formula: see text] edge ELNES spectra of the sulfur atom in Janus monolayers and MoS2 monolayers, the main structures are due to the electron transfer to the unoccupied [Formula: see text] and [Formula: see text] states (3s of sulfur and 4d states of the transition metal).

Spin reorientation transition in epitaxial nanometric Nd-Fe-B films with large perpendicular magnetic anisotropy

The development of hard magnetic properties in Nd-Fe-B films with a thickness below 50 nm has been studied and the effect of the film strain on the spin reorientation transition temperature (TSR) evaluated. A value of 35 nm is found to be the lowest thickness threshold in the films grown by DC magnetron sputtering in this study for the full development of both the Nd2Fe14B phase and the broadening of the out of plane hysteresis loops. Large perpendicular magnetic anisotropy is obtained for a film of 40 nm in thickness, reaching a coercivity of 8 kOe at room temperature. A detailed characterization of epitaxial Nd2Fe14B with high magnetic anisotropy has been done by Grazing Incidence X-Ray Diffraction, lattice parameters are reported and the chemical states of Nd 3d and Fe 2p have been stablished by the analysis of Hard X-Rays Photoelectron Spectroscopy spectra. The spin reorientation transition temperature has been estimated to a value of 124.2 K. Based on experimental results, lattice strain is attributed as the main reason for the deviation of TSR by comparison with the theoretical value (135 K). No evidence of tetragonal distortion in the lattice is found below TSR in this system in contrast to previous considerations.

Hole statistics of equilibrium 2D and 3D hard-sphere crystals.

The probability of finding a spherical "hole" of a given radius r contains crucial structural information about many-body systems. Such hole statistics, including the void conditional nearest-neighbor probability functions GV(r), have been well studied for hard-sphere fluids in d-dimensional Euclidean space Rd. However, little is known about these functions for hard-sphere crystals for values of r beyond the hard-sphere diameter, as large holes are extremely rare in crystal phases. To overcome these computational challenges, we introduce a biased-sampling scheme that accurately determines hole statistics for equilibrium hard spheres on ranges of r that far extend those that could be previously explored. We discover that GV(r) in crystal and hexatic states exhibits oscillations whose amplitudes increase rapidly with the packing fraction, which stands in contrast to GV(r) that monotonically increases with r for fluid states. The oscillations in GV(r) for 2D crystals are strongly correlated with the local orientational order metric in the vicinity of the holes, and variations in GV(r) for 3D states indicate a transition between tetrahedral and octahedral holes, demonstrating the power of GV(r) as a probe of local coordination geometry. To further study the statistics of interparticle spacing in hard-sphere systems, we compute the local packing fraction distribution f(ϕl) of Delaunay cells and find that, for d ≤ 3, the excess kurtosis of f(ϕl) switches signat a certain transitional global packing fraction. Our accurate methods to access hole statistics in hard-sphere crystals at the challenging intermediate length scales reported here can be applied to understand the important problem of solvation and hydrophobicity in water at such length scales.

Towards the Direct Observation of the Local and Electronic Structure Evolution of Positive Electrode Materials by X-Ray Raman Scattering and X-Ray Emission Spectroscopy

The emergence of high brilliance synchrotron sources and the availability of more sophisticated infrastructure have opened doors to advanced material characterization, enabling a profound understanding of new processes and mechanisms in battery research. [1] More specifically, non-resonant Valence-to-Core X-ray emission spectroscopy (VtC-XES) and X-ray Raman scattering (XRS) stand out as complementary photon-in/photon-out X-ray spectroscopy techniques. The unique capability of these techniques lies in their access to soft X-ray edges using a hard X-ray beam, providing comprehensive information on both the electronic and local structure, due to their high sensitivity to local structure and coordination. [2,3] In this work, two different families of materials with different compositions and crystal structures are studied using XES and XRS.The first part focuses on the detailed study of oxygen substituted KVPO4F1-xOx (x = 0, 0.25, 0.5, 0.75, 1) using VtC-XES combined to ab initio theoretical calculations. The average working potential increases with increasing oxygen content by activating the V3+/4+ and V4+/5+ redox couples and replacing the VO4F2 “ionic” entity by {V=O}O5 unit with a highly “covalent” vanadyl-type bond. [4] VtC-XES allowed us to distinguish the contributions of V-F, V-O, and V=O bonds in the local environment, with spectral intensities highly correlated to the F- and O2- anionic composition. In addition, specific spectral evolutions are also associated to the presence of highly covalent V=O bonds, strongly influencing the electrode potential of the material.The second part examines a series of electrochemically obtained LiyNiO2 layered oxide materials (y = 0.02, 0.25, 0.33, 0.5, 0.67, 1.0). VtC-XES performed at the Ni Kβ emission line allows to study the local structure evolution around Ni, whereas XRS spectroscopy at both the O K-edge and Ni L2,3 L-edges gives an access to the interplay between Ni and O through the hybridized states of Ni 3d and O 2p. [5] This work provides a survey on both the cationic and anionic redox processes, and unveils the underlying phenomena related to the charge compensation processes occurring in this material.Overall, we present a comprehensive study of different polyanionic and layered oxide positive electrode materials using complementary X-ray spectroscopic techniques to probe both the local and electronic structures. We demonstrate that they allow to follow and understand evolution and changes in the local structure in materials of intricate crystal structures.AcknowledgementThis work was supported by the DESTINY Marie Skłodowska-Curie Actions COFUND PhD Programme (Grant Agreement #945357) co-funded by the European Union's Horizon2020 research and innovation program and the Synchrotron SOLEIL. ANR is also acknowledged for funding the RS2E network through the STORE-EX Labex Project ANR-10-LABX-76-01. Alistore-ERI network is also acknowledged.

Band Structure Engineering in 2D Metal-Organic Frameworks.

The design of 2D metal-organic frameworks (2D MOFs) takes advantage of the combination of the diverse electronic properties of simple organic ligands with different transition metal (TM) centers. The strong directional nature of the coordinative bonds is the basis for the structural stability and the periodic arrangement of the TM cores in these architectures. Here, direct and clear evidence that 2D MOFs exhibit intriguing energy-dispersive electronic bands with a hybrid character and distinct magnetic properties in the metal cores, resulting from the interactions between the TM electronic levels and the organic ligand π-molecular orbitals, is reported. Importantly, a method to effectively tune both the electronic structure of 2D MOFs and the magnetic properties of the metal cores by exploiting the electronic structure of distinct TMs is presented. Consequently, the ionization potential characteristic of selected TMs, particularly the relative energy position and symmetry of the 3d states, can be used to strategically engineer bands within specific metal-organic frameworks. These findings not only provide a rationale for band structure engineering in 2D MOFs but also offer promising opportunities for advanced material design.