Role Of Covalency Research Articles

Unveiling the Geometric Site Dependence of Co‐Based Spinel Oxides in the Halogen Evolution Reaction

AbstractCobalt‐based spinel oxides, such as Co3O4, have emerged as promising electrocatalysts for chlorine and bromine evolution reactions (CER and BrER) in recent years. However, the role of Co valence in determining the exceptional performance of Co3O4 for both CER and BrER remains ambiguous due to the coexistence of both octahedrally coordinated Co3+ (Co3+Oh) and tetrahedrally coordinated Co2+ (Co2+Td) sites, despite their high catalytic activity and stability. Herein, combining experiment results and electrochemical data analysis, the Co3+Oh site functions as the primary active site for CER is demonstrated. In contrast, for BrER, both Co3+Oh and Co2+Td sites exhibit good catalytic activity, with Co3+Oh sites displaying better BrER catalytic performance than Co2+Td sites. To further enhance the CER catalytic activity of the Co3+Oh site, inert Co2+Td is replaced with Cu2+ cations. As expected, CuCo2O4 featuring an optimized Co3+Oh site demonstrates an overpotential of 24 mV at a current density of 10 mA cm−2 while exhibiting exceptional stability for ≈60 h, surpassing the performance of the majority of non‐noble and even noble metal‐based electrocatalysts reported to date. Therefore, the study elucidates the significance of geometric configuration‐dependent activity in electrocatalytic halogen evolution reactions.

The role of covalency in enhancing stability of Eu and Am complexes: a DFT comparison of BTP and BTPhen.

We compare the stabilities and bonding nature of [Eu/Am(BTPhen)2(NO3)]2+ complexes to those previously reported for [Eu/Am(BTP)3]3+, and investigate whether more accurately reflecting the reaction conditions of the separation process by considering [Eu/Am(NO3)3(H2O)x] (x = 3, 4) complexes instead of aquo complexes increases the selectivity of the separation ligands BTP and BTPhen for Am over Eu. The geometric and electronic structures of [Eu/Am(BTPhen)2(NO3)]2+ and [Eu/Am(NO3)3(H2O)x] (x = 3, 4) have been evaluated using density functional theory (DFT) and used as the basis for analysis of the electron density through the application of the quantum theory of atoms in molecules (QTAIM). Increased covalent bond character for the Am complexes of BTPhen over Eu analogues was found, with this increase more pronounced than that found in BTP complexes. BHLYP-derived exchange reaction energies were evaluated using the hydrated nitrates as a reference and a favourability for actinide complexation by both BTP and BTPhen was found, with the BTPhen ligand found to be more selective, with relative stability ≈0.17 eV greater than BTP.

In silico design of pseudo D5h actinide based molecular magnets: role of covalency in magnetic anisotropy

Actinide molecular magnets are of great interest in the area molecular magnetism as they possess strong covalency and spin-orbit coupling that their 4f congeners lack. Despite these known advantages, the actinide based molecular magnets have not been explored in detail due to the limited availability of actinide salts. While theoretical tools are proven to be useful in lanthanide chemistry towards prediction, they are still at an infant stage in actinide chemistry. In this manuscript, we have attempted to utilise CASSCF/RASSI-SO calculations to predict suitable pseudo D5h molecules possessing attractive magnetic properties. To begin with, we have undertaken an extensive benchmarking of the methodology employed by studying two sets of isostructural, [NdTp3], [UTp3], [Nd(COT″)2]-, [U(COT″)2]- {Tp− = trispyrazolylborate, COT″ = bis(trimethylsilyl)cyclooctatetraenyl dianion} complexes. The method assessment reveals that the methodology employed is reliable as this has reproduced the experimental observables. With this, we have moved forward with prediction where pseudo-D5h symmetric [L2M(H2O)5][I]3L2.(H2O) {M = Nd, U, Pu; L = tBuPO(NHiPr)2} systems are modelled from their Nd(III) X-ray structure. Our calculations reveal that the Uranium complex studied possess superior SIM characteristics compared to its lanthanide analogue. Plutonium complex has prolate electron density at the ground state, and hence the ligand environment is unsuitable for yielding SIM behaviour. The role of solvent molecules, counter anions and equatorial and axial ligand are explored and tantalizing prediction with a barrier height of 1403.3 and 989.5 cm−1 are obtained for [tBuPO(NHiPr)2-U-tBuPO(NHiPr)2]3+ and [Pu(H2O)5]3+ models, respectively and this paves the way for synthetic chemist to target such geometries in actinide based SIMs. Is pseudo D5h geometry suitable for Actinide SIMs? In this article, we have extensively studied uranium complex and compared it with its lanthanide analogue. The pseudo D5h symmetric uranium complex possesses oblate electron density which is favourable in the axial ligand field and henceforth it is predicted to have attractive SIM behaviour. Studies on models suggest the low-coordinate actinide chemistry may hold the key to further the blocking temperature beyond liquid N2 temperatures.

Beryllium Bonding in the Light of Modern Quantum Chemical Topology Tools.

We apply several modern quantum chemical topology (QCT) tools to explore the chemical bonding in well established beryllium bonds. By using the interacting quantum atoms (IQA) approach together with electron distribution functions (EDF) and the natural adaptive orbitals (NAdOs) picture, we show that, in agreement with orbital-based analyses, the interaction in simple σ and π complexes formed by BeX2 (X = H, F, Cl) with water, ammonia, ethylene, and acetylene is dominated by electrostatic terms, albeit covalent contributions cannot be ignored. Our detailed analysis proves that several σ back-donation channels are relevant in these dimers, actually controlling the conformational preference in the π adducts. A number of one-electron beryllium bonds are also studied. Orbital invariant real space arguments clearly show that the role of covalency and charge transfer cannot be ignored.

Hydrogen bonding from a valence bond theory perspective: the role of covalency.

A valence bond theory based method has been developed to decompose hydrogen bond energies into contributions from geometry, electrostatics, polarization and charge transfer. This decomposition method has been carried out for F-HFH, F-HOH2, F-HNH3, HO-HOH2, HO-HNH3, and H2N-HNH3. Localized valence bond self-consistent field (L-VBSCF) and localized breathing orbital valence bond (L-BOVB) calculations were performed at the PBEPBE/aug-cc-pVDZ optimized geometries. It is shown that inclusion of valence bond structures that explicitly include charge transfer account for at least 32% (likely over half) of the hydrogen bond energy of all systems studied, indicating the dominant role of covalency. This is in agreement with calculated bond lengths, geometry deformation energies, and polarization energies. Electrostatic effects were found to play only a minor role in contrast to some widely held ideas regarding the nature of hydrogen bonding.

Electronic and optical properties of magnesium and calcium hydroxides: the role of covalency and many-body effects

Magnesium and calcium hydroxides X(OH)2 (X = Mg, Ca) are multifunctional materials that have many important applications in industry, technology, and research. In solid-state electronics, the emerging applications of these compounds are related to photovoltaic devices. In the present paper we review electronic properties of X(OH)2, band gaps, work function, and features of chemical bonding and discuss theoretically predicted exciton effects.

Evolution of electromechanical properties of Bi1−xPrxFeO3 solid solutions across the rhombohedral–orthorhombic phase boundary: Role of covalency

Bi1−xPrxFeO3 ceramics of the compositions across the rhombohedral–orthorhombic phase boundary have been studied by X-ray diffraction and piezoresponse force microscopy. Charge density calculations and transport properties measurements have also been performed. Piezoresponse force microscopy measurements revealed a considerable increase of the piezoresponse signal for the x=0.125 compound with dominant polar rhombohedral phase and minor amount of antipolar orthorhombic phase. Electron density distribution study testified significant increase of covalency of this compound as compared with gradual delocalization of electronic wave functions observed for other Bi1−xPrxFeO3 compounds with higher praseodymium content. Alteration of charge distribution character has been confirmed by resistivity measurements.

Phase Stability, Physical Properties, and Hardness of Transition-Metal Diborides MB2 (M = Tc, W, Re, and Os): First-Principles Investigations

Using first-principles calculations, the structural stability, elastic strength, and formation enthalpies of four diborides MB2 (M = Tc, W, Re, and Os) are investigated by means of the pseudopotential plane-waves method, as well as the roles of covalency and bond topology in the phase incompressibility. Three candidate structures of known transition-metal diborides are chosen to probe. The calculated lattice parameters, elastic properties, Poisson’s ratio, and B/G ratio are derived. It is observed that the ReB2-type structure containing well-defined zigzag covalent chains exhibits an unusual incompressibility along the c axis comparable to that of diamond. Formation enthalpy calculations demonstrate that the ground-state phase is synthesizable at low pressure, whereas the other phase can be achieved through the phase transformation. Moreover, according to Mulliken overlap population analysis, a semiempirical method to evaluate the hardness of multicomponent crystals with a partial metallic bond is presented. The predicted hardness of WB2–WB2, ReB2–ReB2, and OsB2–OsB2 is in reasonable agreement with experiment data. Both strong covalency and a zigzag topology of interconnected bonds underlie the ultraincompressibilities. In addition, the superior performance and largest hardness of ReB2–ReB2 indicate that it is a superhard material. This work provides a useful guide for designing novel borides materials having excellent mechanical performances.

The Role of Covalency in the Orbital‐Order of 3d1 Perovskites

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The role of covalency in the orbital-order of 3d 1 perovskites

We investigate the interplay between lattice distortion, bond formation and orbital ordering in A TiO 3 ( A = La , Y) perovskites using full-potential density-functional calculations and ELF analysis. It is shown that the lattice distortion implies a dramatic reorganization of the bonds between A cations and oxygen on the one hand, and titanium and oxygen on the other hand. Along the series cubic LaTiO 3, orthorhombic LaTiO 3, orthorhombic YTiO 3 an increasing asymmetrization of the environment of Ti is taking place. We show that changes in chemical bonding are relevant for lifting the degeneracy of Ti 3d orbitals across the metal–insulator transition and we sketch the use of the ELF to visualize orbital ordering.

Role of covalency in the ground-state properties of perovskite ruthenates: A first-principles study using local spin density approximations

We investigate the electronic structure of SrRuO3 and CaRuO3 using full potential linearized augmented plane wave method within the local spin density approximations. The ferromagnetic ground state in SrRuO3 could exactly be described in these calculations and the calculated spin magnetic moment is found to be close to the experimentally observed values. Interestingly, the spin polarized calculations for CaRuO3 exhibit large spin moment as observed in the experiments but the magnetic ground state has higher energy than that in the non-magnetic solution. Various calculations for different structural configurations indicate that Ca-O covalency plays the key role in determining the electronic structure and thereby the magnetic ground state in this system.

New class of holographic materials based on CdF2 semiconductor crystals with bistable centers. I. Role of covalence in the formation of bistable centers

Calculations of the electronic structure of In, Ga, and Al impurity centers in a CdF2 crystal in the cluster approximation using the method of scattered waves are made. The first two impurities form in additively colored crystals bistable centers having a ground two-electron (deep) state and a metastable hydrogen-like (shallow) state. A change in the nature of the chemical bond on doping a crystal with these impurities is traced, which consists in a considerable increase of its covalent component. A change for deep In and Ga centers is shown to be caused by the reconstruction of centers in their ground state, and a conclusion about the character of reconstruction is made. This conclusion agrees with recent calculations made for the center structure using the pseudopotential method. Conditions of formation of bistable centers in CdF 2 and their structure in different charge states are discussed.

New Class of Holographic Materials Based on CdF[sub 2] Semiconductor Crystals with Bistable Centers. I. Role of Covalence in the Formation of Bistable Centers

New Class of Holographic Materials Based on CdF[sub 2] Semiconductor Crystals with Bistable Centers. I. Role of Covalence in the Formation of Bistable Centers

Crystalline network influence on the variation of TC with the F-O substitution rate in oxyfluorides derived from octahedral-type ferroelectric oxides

Abstract Ferroelectric fluorides have been studied in the two last decades in relation to the performance of ceramics sintered at low temperature for various potential applications. Compositions derived from octahedraltype ferroelectrics by progressive replacement of oxygen by fluorine are particularly interesting. Those considered in the present work belong to perovskite, tetragonal potassium tungsten bronze TKWB and LiTaO3 families. The variation of the Curie temperature TC with the F-O substitution rate is strongly dependent on the crystalline network and on the ferroelectricity-type. The decrease of TC is maximal for TKWB-type materials due to the predominant role of covalency of the M-X (X = O, F) bonds in relation to the same direction for such bonds and for both the displacement vector of the M-atom from the octahedron-centre and the spontaneous polarization vector. Also the ferroelectricity is purely displacive for this type.

Prediction of ferroelectric properties in niobates and tantalates based on covalency considerations

The modulation of ferroelectric material properties using ionic substitutions is well known. The Curie temperature, in particular, was previously related to chemical bonding considerations. The present work deals with niobates and tantalates, ferroelectric families particularly interesting for applications. The metal-oxygen covalency is investigated using quantum chemistry methods. The role of covalency on the ferroelectric distortion is discussed and relations between covalency on one side, Curie temperature and microwave relaxation frequency on the other side, are evidenced. Covalency not only softens the short-range repulsions, but also stiffens and stabilizes the metal-oxygen network. This last effect appears to dominate in niobates and tantalates. As an unusual result, covalency tends to inhibit the ferroelectric distortion in these compounds. Covalency effects can be used in a predictive way to modulate chemically ferroelectric and dielectric properties. This approach is validated through various examples of niobates and tantalates.

Role of covalence and of correlation in the dielectric polarization of oxides

The dynamical charges measure the macroscopic polarization induced by a relative sublattice displacement in a polar insulator. When the material has a mixed ionic/covalent character such charges present nontrivial features. Apparently similar oxides have strikingly different behaviors: some (like the ferroelectric perovskites) have giant dynamical charges, while others (like ZnO) are quite normal and display dynamical charges close to the nominal static ones. The rationale for such differences is found, upon closely scrutinizing the covalence mechanism. Furthermore, the effects of electron-electron interaction on the polarization of a model highly correlated dielectric are discussed.

The role of covalency and bridging ligands in two‐ion cooperative optical transitions in lanthanide ion‐coupled systems

AbstractThe influence of covalency and bridging ligands on cooperative optical effects in dimers with closely coupled lanthanide ions in insulating crystals and binuclear complexes is theoretically treated. It is shown that there is a specific interaction between Ln3+ ions (which may be called covalent coupling) in tight ion pairs originating from virtual electron hoppings between lanthanide ions via common bridging ligands. This interaction can be regarded as a generalization of the superexchange interaction mechanism. A general microscopic model and the corresponding formalism for the covalent coupling mechanism are developed. A general expression to estimate the intensity of electric dipole two‐ion cooperative optical transitions is derived. Both the nature of bridging ligands and the local geometry of the Ln3+ (A)–ligandLn3+ (B) dimer are found to affect strongly the intensities of cooperative transitions. Our model predicts rapid increase in the contribution of this mechanism to the intensities of two‐ion transitions when going from small “ionic” ligands (F− and O2−) to large “covalent” ligands (Br−, I−). Possible manifestations of the covalent coupling mechanism in fast energy transfer processes and unusually strong up‐conversion fluorescence observed in CsCdBr3 : Ln3+ systems and as well as in electronic spectra of the U2Cl10 binuclear compound are discussed.

Crystal Structure and the Role of Covalency in Eight-Layered Hexagonal Ba2CrTaO6

According to the Rietveld refinement, the perovskite-related Ba2CrTaO6 was found to have a P63/mmc space group with lattice constants of a=5.7383(1) and c=18.7832(2) Å. The Cr and Ta ions are completely ordered; the face-shared octahedra are occupied by Cr ions and the corner-shared octahedra by Ta ions. The increased covalent bonding nature of (Cr-O-Cr) and/or (Cr-Cr) bonds via π(εgπ-pπ-εgπ) and σ(a1g-a1g) would be the main driving force to stabilize the 8-layer hexagonal structure. The existence of such covalent bondings was confirmed by the ESR spectroscopic analysis and magnetic susceptibility measurement. The XANES spectra for Ta LIII-edge indicated that such a covalency induces a relatively weak covalency of competing (Ta-O) bond.

A MEG study of the olivine and spinel forms of Mg2SiO4

In this paper we present a theoretical investigation of the structures and relative stability of the olivine and spinel phases of Mg2SiO4. We use both a purely ionic model, based on the Modified Electron Gas (MEG) model of intermolecular forces, and a bond polarization model, developed for low pressure silica phases, to investigate the role of covalency in these compounds. The standard MEG ionic model gives adequate structural results for the two phases but incorrectly predicts the spinel phase to be more stable at zero pressure. This is mainly because the ionic modeling of Mg2SiO4 only accounts for 95 percent of the lattice energy. The remainder can be attributed to covalency and many-body effects. An extension of the MEG ionic model using “many-body” pair potentials corrects the phase stability error, but predicts structures which are in poorer agreement with experiment than the standard ionic approach. In addition, calculations using these many-body pair potentials can only account for 10 percent of the missing lattice energy. This model predicts an olivine-spinel phase transition of 8 GPa, below the experimental value of 20 GPa. Therefore, in order to understand more fully the stability of these structures we must consider polarization. A two-shell bond polarization model enhances the stability of both structures, with the olivine structure being stabilized more. This model predicts a phase transition at about 80 GPa, well above the observed value. Also, the olivine and spinel structures calculated with this approach are in poorer agreement with experiment than the ionic model. Therefore, based on our investigations, to properly model covalency in Mg2SiO4, a treatment more sophisticated than the two-shell model is needed.

Calculation of bulk moduli of diamond and zinc-blende solids.

Theoretical arguments on the role of covalency in determining the bulk moduli of diamond and zinc-blende semiconductors and insulators are shown to yield a surprisingly simple and accurate expression for determining the bulk moduli B of these materials. One resulting formula for compounds in the center of the Periodic Table depends only on the nearest-neighbor separation d. It has the form B=1761${d}^{\mathrm{\ensuremath{-}}3.5}$ for B in GPa and d in A\r{}.