Trends In Electronic Structure Research Articles

Stability and chemical bonding in a series of inverse sandwich actinide boride clusters (An2B8) with δ bonding.

An inverse sandwich structure has been computationally predicted for uranium boride and extended to the series of actinide elements (An) from Th to Cm. The electronic structure and chemical bonding of these novel compounds have been analyzed using density functional theory and multireference wave-function based methods. We report the trends in electronic structure and bonding for An2B8, and found that (d-π)π and (d-p)δ are the most important factors in the stability of An2B8. The (f-p)δ bond provides extra stabilization for Pa2B8 and U2B8, owing to the extensive interactions of An-B8-An, resulting in a short distance for the Pa-Pa and U-U bonds.

Density Functional Theory Combined with Thermodynamics Exploration of Novel 2D Materials Created Using Aqueous Exfoliation

During the past decade, two-dimensional (2D) nanomaterials have provided a broad platform for a wide variety of applications and have been the focus of numerous studies due to their unique properties. The most common 2D materials are of hexagonal or trigonal symmetry, with structures that can be grown on inexpensive substrates like Si and sapphire. The ability to grow 2D materials on additional substrates with a larger variety of symmetries and surface terminations, such as metals, glass, and complex metal oxides, can be regarded as a big and necessary step in the design of new functional materials. However, there is a dearth of knowledge on 2D materials beyond hexagonal and trigonal symmetries, their synthesis, and epitaxy. Here, we present a procedure to rationalize the aqueous exfoliation of known 3D materials as a route to create stable tetragonal 2D materials with tunable lattice parameters and electronic structures. We develop a model using first-principles density functional theory plus thermodynamics to propose an ecofriendly synthesis of 2D materials using aqueous media and use it to investigate a class of previously synthesized tetragonal ABX compounds. Then, we predict the thermodynamics of their exfoliation in water by combining surface slab models, their subsequent surface transformations, and experimentally determined changes in free energy. The atomistic results obtained here are correlated to periodic properties and are used to elucidate trends in electronic structure and the thermodynamics of aqueous exfoliation.

Band Gap Engineering and 14 Electron Superatoms in 2D Superoctahedral Boranes B4X2 (B, N, P, As, Sb)

Band gap tuning in 2D monolayers is one of the most attractive approaches in design and production of new atomically thin semiconductors. Following our recent computational design of stable two-dimensional octahedral boranes, we present comprehensive computational study of electronic structure, stability and properties of members belonging to family B4X2 (X = B, N, P, As, Sb). We select 15 group atoms (pnictogens) as apical substituents to stabilize quasi-octahedral units by delocalized bonding and create interlayer “pressure” caused by vertically aligned lone pairs. We first substitute apical B atoms by N and then go down the 15 group to capture electronic structure trends. B→N substitution opens band gap, while further substitution consistently narrows band gap. We revealed elegant band gap trend which is inversely proportional to the size of octahedral units. Thus, old as world isoelectronic substitution could be used for band gap engineering in superoctahedral 2D boranes and other monolayers. We also discovered superatomic bonding in B4As2 and B4P2 octahedra which agrees with their exceptional stability (up to 800 K) and magnetic response properties. Remarkably, both B4As2 and B4P2 units have 14 valence electrons, making them exceptional examples of stable nonconventional electron count of spherical aromaticity. Alongside with a considerable negative formation energy and phonon stability, B4As2 has extremely low exfoliation energy of 0.02 eV/atom implying probability of synthetic route from a putative bulk material. It gives a strong basis to believe in possibility of experimental fabrication of B4As2 monolayer which can serve as alternative to MoS2 and silicon.

Electronic Structure of Bi-Activated Luminescent Compounds and Pure Bismuth Photocatalytic Compounds

The trends in electronic structure of Bi3+ and Bi2+ as luminescent dopant in wide bandgap inorganic compounds and that of pure Bi-compounds for photocatalytic splitting of water are explored by determination of vacuum referred electron binding energies. Spectroscopic data combined with the chemical shift model from the luminescence field and data on flatband potentials in electrochemistry provide most of the needed data. Occasionally data from photoelectron spectroscopy is used. The trends in VRBE of Bi3+ and Bi2+ ground state and excited state levels resemble that of Ce3+ and Eu2+ lanthanides. To some extend a pure Bi-compound can be regarded as a 100% Bi3+ doped La-compound. This all will be demonstrated and an overview on Bi electronic structure is presented. Comparison of electronic structure for Bi as luminescent dopant in inorganic compounds with pure Bi-compounds entails also a comparison of two disciplines in electro-chemistry, that of luminescence materials and of photo-catalytic compounds.

Electronic Structure Trends Across the Rare-Earth Series in Superconducting Infinite-Layer Nickelates

The recent discovery of superconductivity in oxygen-reduced monovalent nickelates has raised a new platform for the study of unconventional superconductivity, with similarities and differences with the cuprate high temperature superconductors. In this paper we investigate the family of infinite-layer nickelates $R$NiO$_2$ with rare-earth $R$ spanning across the lanthanide series, introducing a new and non-trivial "knob" with which to tune nickelate superconductivity. When traversing from La to Lu, the out-of-plane lattice constant decreases dramatically with an accompanying increase of Ni $ d_{x^2-y^2}$ bandwidth; however, surprisingly, the role of oxygen charge transfer diminishes. In contrast, the magnetic exchange grows across the lanthanides which may be favorable to superconductivity. Moreover, compensation effects from the itinerant $5d$ electrons present a closer analogy to Kondo lattices, indicating a stronger interplay between charge transfer, bandwidth renormalization, compensation, and magnetic exchange. We also obtain the microscopic Hamiltonian using Wannier downfolding technique, which will provide the starting point for further many-body theoretical studies.

Density Functional Theory Calculations of Structural, Electronic, and Magnetic Properties of the 3d Metal Trifluorides MF3 (M = Ti-Ni) in the Solid State.

We employ density functional theory with Hubbard U correction or hybrid functionals to study the series of magnetic 3d metal trifluorides MF3 (M = Ti-Ni). Experimental lattice parameters are reproduced with an error margin of 0.5%-4.3%. Cooperative Jahn-Teller distortions are reproduced for MnF3 , but also found in TiF3 and CoF3 at smaller levels compared to MnF3 . Trends in electronic structure with respect to positions of the d bands are linked to the magnetic properties where M = Ti-Cr are weak magnetic Mott-Hubbard insulators, M = Fe-Ni are strong magnetic charge-transfer insulators and MnF3 falls in between. Our work contributes to the characterization of the relatively unknown NiF3 , since FeF3 and CoF3 have similar electronic and magnetic properties. However, NiF3 does not show a Jahn-Teller distortion as present in CoF3 . © 2019 Wiley Periodicals, Inc.

Bandgap profiling in CIGS solar cells via valence electron energy-loss spectroscopy

A robust, reproducible method for the extraction of relative bandgap trends from scanning transmission electron microscopy (STEM) based electron energy-loss spectroscopy (EELS) is described. The effectiveness of the approach is demonstrated by profiling the bandgap through a CuIn1-xGaxSe2 solar cell that possesses intentional Ga/(In + Ga) composition variation. The EELS-determined bandgap profile is compared to the nominal profile calculated from compositional data collected via STEM-based energy dispersive X-ray spectroscopy. The EELS based profile is found to closely track the calculated bandgap trends, with only a small, fixed offset difference. This method, which is particularly advantageous for relatively narrow bandgap materials and/or STEM systems with modest resolution capabilities (i.e., >100 meV), compromises absolute accuracy to provide a straightforward route for the correlation of local electronic structure trends with nanoscale chemical and physical structure/microstructure within semiconductor materials and devices.

Computational Study of Halide Perovskite-Derived A2BX6 Inorganic Compounds: Chemical Trends in Electronic Structure and Structural Stability

The electronic structure and energetic stability of A2BX6 halide compounds with the cubic and tetragonal variants of the perovskite-derived K2PtCl6 prototype structure are investigated computationally within the frameworks of density-functional-theory (DFT) and hybrid (HSE06) functionals. The HSE06 calculations are undertaken for seven known A2BX6 compounds with A = K, Rb, and Cs; and B = Sn, Pd, Pt, Te, and X = I. Trends in band gaps and energetic stability are identified, which are explored further employing DFT calculations over a larger range of chemistries, characterized by A = K, Rb, Cs, B = Si, Ge, Sn, Pb, Ni, Pd, Pt, Se, and Te; and X = Cl, Br, I. For the systems investigated in this work, the band gap increases from iodide to bromide to chloride. Further, variations in the A site cation influences the band gap as well as the preferred degree of tetragonal distortion. Smaller A site cations such as K and Rb favor tetragonal structural distortions, resulting in a slightly larger band gap. For varia...

Strain Tuning of Tin–Halide and Lead–Halide Perovskites: A First-Principles Atomic and Electronic Structure Study

Using density functional theory (DFT)-based calculations, we investigate the effects of biaxial strain and the accompanying structural distortions on the energy landscapes, band gaps, and band edges of the perovskite photovoltaic materials CsSnI3 and CsPbI3. We show that biaxial strains within ±3% of the respective cubic lattice parameters can alter band gaps by several tenths of an electronvolt, mainly through the tuning of antibonding interactions in the valence band maximum, while temperature-controlled octahedral rotations further widen band gaps. Notably, we predict that biaxial strain, particularly tensile strain at low temperature, has the potential to induce ferroelectric polarization in these materials. Throughout this work, we rationalize trends in electronic structure based on the character and symmetry of band-edge crystal orbitals and discuss their implications with respect to broader classes of materials.

General Trends in Electronic Structure, Stability, Chemical Bonding and Mechanical Properties of Ultrahigh Temperature Ceramics TMB2 (TM = transition metal)

The electronic structure, stability, chemical bonding and mechanical properties of 3d, 4d and 5d transition metal diboride TMB2 were investigated using first-principles calculations based on density functional theory. All the primary chemical bonds, i.e., metallic, ionic and covalent have contributions to the bonding of TMB2. The number of valence electrons of transition metals or the valence electron concentration (VEC) of TMB2 has strong effects on the lattice parameters, stability and mechanical properties of TMB2. Both lattice constants a and c decrease with VEC, but c decreases faster than a, which is attributed to the enhanced TM d–B p (sp2) bonding. Bulk modulus B of TMB2 increases continuously with VEC due to the enhanced TM d–B p (sp2) and TM dd bonding. Shear modulus G increases with VEC, reaching a maximum at VEC = 3.33, and then decreases with further increase of VEC. YB2 and MnB2 have low Young's modulus and are predicted to have good thermal shock resistance. According to Pugh's criterion (G/B < 0.571), MnB2, MoB2 and WB2 are predicted as ductile or damage tolerant ultrahigh temperature ceramics (UHTCs).

Trends in electronic structures and structural properties of MAX phases: a first-principles study on M2AlC (M = Sc, Ti, Cr, Zr, Nb, Mo, Hf, or Ta), M2AlN, and hypothetical M2AlB phases

MAX phases are a large family of layered ceramics with many potential structural applications. A set of first-principles calculations was performed for M2AlC and M2AlN (M = Sc, Ti, Cr, Zr, Nb, Mo, Hf, or Ta) MAX phases as well as for hypothetical M2AlB to investigate trends in their electronic structures, formation energies, and various mechanical properties. Analysis of the calculated data is used to extend the idea that the elastic properties of MAX phases can be controlled according to the valence electron concentration. The valence electron concentration can be tuned through the various combinations of transition metal and nonmetal elements.

The Family of LiCo6P4 Type Compounds – Trends in Electronic Structure and Chemical Bonding

AbstractSelected ternary compounds of the family of AT6X4 (LiCo6P4‐type) intermetallics were studied by ab initio methods with respect to their electronic structures and chemical bonding. The bonding characteristics highlight the role of the three‐dimensional [T6X4] polyanionic networks which, within the hexagonal channels, accommodate loosely bonded A cations. In alkali metal based compounds, when T is replaced by indium (e.g. in KIn6Au4) the reverse occupancy leads to positively charged In (ca. +0.5) and negatively charged Au (ca. –0.9). Peculiar magnetic behavior is traced out whereby a magnetic order is identified only when T is a 3d element, which polarizes and induces a negative small moment on A (U) as in UCr6P4 oppositely to CeRh6X4 for X = Si, Ge.

The U4Re7Si6 type – Trends in electronic structure and chemical bonding

The electronic structures and chemical bonding of selected ternary compounds of the A4T7X6 family (of U4Re7Si6-type) intermetallics have been studied by ab initio methods. The calculations for two series: Mg4Rh7P6, Sc4Co7Ge6, Ti4Co7Ge6 and uranium containing U4Re7Si6, U4Ru7Ge6, and U4Ru7As6 show common bonding characteristics pertaining to main T1–X and T2–X interactions (T = transition metal and X = p-element) due to the peculiar crystal chemistry with T1@X6 and T2@X4 coordination polyhedra. The uranium compounds are found to be stabilized in a spin polarized ferromagnetic configuration, especially for U4Ru7Ge6 (in agreement with experiment) and U4Ru7As6.

Electronic structure trends of Möbius graphene nanoribbons from minimal-cell simulations

Investigating topological effects in materials requires often the modeling of material systems as a whole. Such modeling restricts system sizes, and makes it hard to extract systematic trends. Here, we investigate the effect of Möbius topology in the electronic structures of armchair graphene nanoribbons. Using density-functional tight-binding method and minimum-cell simulations through revised periodic boundary conditions, we extract electronic trends merely by changing cells’ symmetry operations and respective quantum number samplings. It turns out that for a minimum cell calculation, once geometric and magnetic contributions are ignored, the effect of the global topology is unexpectedly short-ranged.

Electronic Structure Trends in N‐Heterocyclic Carbenes (NHCs) with Varying Number of Nitrogen Atoms and NHCTransition‐Metal Bond Properties

Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the σ-lone pair at Ccarbene and the π-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the pπ population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metalNHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHCgold and the NHCtitanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.

Nonlinear variations in the electronic structure of II–VI and III–V wurtzite semiconductors with biaxial strain

Using first-principles calculations within many-body perturbation theory, we predict effects of biaxial strain on electronic band gaps and band edges of wurtzite III–V and II–VI semiconductor compounds. We find strain-induced changes in band gaps are large and highly nonlinear. Under both compressive and tensile biaxial strains, II–VI chalcogenide band gaps are predicted to decrease by as much as 0.6 eV for 10% strain; in contrast, III–V nitrides attain maximum gaps for compressive strains near 4%. Whereas nitrides tend to preserve covalent bond angle, more ionic chalcogenides tend to preserve bond length and volume, leading to qualitatively different trends in electronic structure.

ChemInform Abstract: Computational Chemistry of Modified [MFe3S4] and [M2Fe2S4] Clusters: Assessment of Trends in Electronic Structure and Properties

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Chemical Bonding in Metallic Rutile-type Oxides TO2 (T = Ru, Rh, Pd, Pt)

Abstract Synthesis routes to rutile-type oxides with 4d and 5d transition elements are summarized. Trends in electronic structure have been established through an analysis in the framework of density functional theory presenting the band structure, the density of states and the properties of chemical bonding. The metal-oxygen bond is found to play the major role in bonding of the system in the valence band. Throughout the series 4d → 5d (RuO2, RhO2, PdO2 and PtO2) the crystal field analysis of the band structure shows a lowering of eg towards t2g manifolds and a broadening of the overall density of states. In the vicinity of the Fermi level the role of the antibonding metal-oxygen character is investigated in the context of instability towards possible magnetic polarization, especially for RuO2.

Special Issue Current Trends in Electronic Structure: Real‐space, Embedding and Linear Scaling Techniques

AbstractThis Special Issue of physica status solidi (b) is devoted to the topic of first‐principles simulation of solids, liquids and complex matter and presents articles on the progress in linear scaling methods, electronic transport, simulation of STM images, and time‐dependent DFT methods.The cover picture is taken from the paper by Torsti et al. and shows an isosurface of the positron density localized at a Ga vacancy in GaN [1]. The positron state is calculated by the real‐space code Mika/doppler based on the Rayleigh‐quotient multigrid method. The code predicts positron lifetimes and momentum densities of annihilating electron–positron pairs measured in positron annihilation spectroscopy for defects in solids.

Trends in Electronic Structure and Redox Energetics for Early-Actinide Pentamethylcyclopentadienyl Complexes

Detailed cyclic voltammetric and UV−visible−near-infrared electronic absorption spectral data have been obtained for a series of pentamethylcyclopentadienyl complexes of uranium(IV) and thorium(IV) of the general formula (C5Me5)2An(L1)(L2), where L1, L2 = Cl, SO3CF3, CH3, CH2Ph, imido (N-2,4,6-tBu3C6H2), hydrazonato (η2(N,N‘)-RNNCPh2; R = CH3, CH2Ph, Ph), ketimido (−NC(Ph)(R); R = CH3, CH2Ph, Ph) ligands, and for the hexavalent uranium bis(imido) complex (C5Me5)2U(NPh)2. The electrochemical and spectroscopic behavior of the tetravalent uranium complexes falls cleanly into distinct categories based on the nature of L1 and L2. If both ligands are simple σ-donors (Cl, SO3CF3, CH3, CH2Ph), a reversible U(IV)/U(III) voltammetric wave is the only metal-based process observed, and it occurs between ∼−1.8 and −2.6 V vs [(C5H5)2Fe]+/0. If either L1 or L2 is a nitrogen-donor ligand (imido, hydrazonato, ketimido), then both a U(IV)/U(III) reduction wave and a U(V)/U(IV) oxidation wave are observed. The reduction step occurs in the same potential region as for the σ-donor complexes, and the oxidation wave occurs in the range from ∼+0.2 to −0.7 V vs [(C5H5)2Fe]+/0. This oxidation wave is reversible, indicating that the unusual pentavalent uranium oxidation state is kinetically stable on a voltammetric time scale, and the potential of the oxidation step indicates that the pentavalent state is thermodynamically stabilized from interaction with the nitrogen-donor ligand(s). The separation between reduction and oxidation processes in these nitrogen-donor complexes remains nearly constant over the series of eight complexes, with an average value of 2.09 V. Additional ligand-based redox processes are also observed and assigned on the basis of the existence of corresponding voltammetric waves in the Th(IV) complexes and other cyclopentadiene complexes. The electronic absorption spectra for all U(IV) complexes are comprised of two distinct regions: a lower energy region (E < 15 000 cm-1) containing the narrow f−f transitions arising from within the 5f orbital manifold and a higher energy region (E > 15 000 cm-1) containing broad, unstructured, or poorly structured bands derived from both metal-localized 5f−6d transitions and molecular-based transitions including ligand-localized and metal-to-ligand charge-transfer transitions. A definite trend in intensities of these transitions is observed, depending on the nature of L1 and L2. If L1 and L2 are both simple σ-donor ligands, the f−f transition intensities are relatively weak (molar absorptivity ε ≈ 10−80 M-1 cm-1), consistent with observations for most classical coordination complexes of 5f2 electronic configuration, and the broad, higher energy bands have ε values in the 3000−5000 M-1 cm-1 range. If L1 and/or L2 is a hydrazonato ligand, the f−f transition intensities increase to ∼30−120 M-1 cm-1 and the broad, higher energy bands develop significantly greater intensities (ε ≈ 15 000−20 000 M-1 cm-1). Finally, for the imido and ketimido complexes of U(IV), the f−f transition intensities increase to ∼50−400 M-1 cm-1, and the broad, higher energy bands continue to carry substantial intensity (ε ≈ 10 000−15 000 M-1 cm-1) while also extending to lower energy. This interesting trend in f−f transition intensity is interpreted in the context of an intensity-stealing mechanism from the charge-transfer excited states that reflects an enhanced degree of covalent interaction between the actinide metal center and L1/L2 for the nitrogen-donor ligand systems.