Covalency Effects Research Articles

Cation Ordering in Natural and Synthetic (Cu1–xZnx)2CO3(OH)2 and (Cu1–xZnx)5(CO3)2(OH)6

In the present paper we report combined experimental and theoretical studies of the UV-vis-NIR spectra of the mineral compounds malachite, rosasite, and aurichalcite and of the precursor compounds for Cu/ZnO catalysts. For the copper species in the minerals the crystal field splitting and the vibronic coupling constants are estimated using the exchange charge model of the crystal field accounting for the exchange and covalence effects. On this basis the transitions responsible for the formation of the optical bands arising from the copper centers in minerals are determined and the profiles of the absorption bands corresponding to these centers are calculated. The profiles of the absorption bands calculated as a sum of bands of their respective Cu species are in quite good agreement with the experimental data. In agreement with crystal chemical considerations, the Zn ions were found to be preferentially located on the more regular, i.e., less distorted, octahedral sites in zincian malachite and rosasite, suggesting a high degree of metal ordering in these phases. This concept also applies for the mineral aurichalcite, but not for synthetic aurichalcite, which seems to exhibit a lower degree of metal ordering. The catalyst precursor was found to be a mixture of zincian malachite and a minor amount of aurichalcite. The best fit of the optical spectrum is obtained assuming a mixture of contributions from malachite (0% Zn) and rosasite (38% Zn of [Zn + Cu]), which is probably due to the intermediate Zn content of the precursor (30%).

Optical and magnetic properties of transition-metal ions in tetrahedral and octahedral compounds

This paper presents the complete energy matrix of the 3d2 system containing the electron-electron interaction, the ligand-field interaction, the spin-orbit coupling interaction, and the Zeeman interaction, in which the optical spectra and g-factor of V3+and Ti2+ ions in the series of tetrahedral AIIBVI (AII=Zn, Cd, BVI=S, Se, Te) semiconductor materials are determined. In the investigation of the optical and magnetic properties of these transition-metal ions in the tetrahedral coordination complexes, we compared the data obtained from the transition-metal ions in the tetrahedral coordination complexes with those obtained from the corresponding ions in the octahedral ones, and found that the tetrahedral complexes have weaker crystal-field strength, inverse energy level ordering and stronger covalence effect.

Covalent Bond Cluster Effects and Phase Transitions in Superconductor Mg<sub>1-x</sub>Al<sub>x</sub>B<sub>2</sub> Compounds

First-principle calculations were performed for the study of the superconductor MgB2 and AlB2. The doping effect of compound Mg1-xAlxB2 was analyzed by supercell method. Band structure, electronic bands structure, total and partial densities of states were calculated and analyzed in detail. Covalent bond effects (CBC) appeared in the electronic bands structure when doping Al to MgB2. It is found that this CBC effects on Mg1-xAlxB2 samples have prominent relations to superconductivity. The study of the density of state indicates that the superconductivity decreases with the increase of compound Al. In particular, there exists the transition of superconductor to non-superconductor with the change of x. The theoretical predictions agreed well with experimental measurements.

Study of effect of covalency in heavy rare-earth monoantimonides upto 31 GPa

In the present paper, we have investigated the high-pressure structural phase transition of rare earth antimonides (HoSb and TmSb). A modified interaction potential model (MIPM) (including the covalency effect) has been developed. Phase transition pressures are associated with a sudden collapse in volume. At compressed volumes, these compounds are found in CsCl phase. The phase transition pressures and associated volume collapses obtained from present potential model show a generally good agreement with available experimental data and others. The elastic constants and bulk modulus are also reported. Our results are in general in good agreement with experimental and theoretical data where available, and provide predictions where they are unavailable.

High pressure phase transition and elastic properties of covalent heavy rare-earth Antimonides

In the present paper, we report an investigation into the high-pressure structural phase transition of rare earth antimonides (DySb and ErSb). A modified interaction potential model (MIPM) (including the covalency effect) has been developed. Phase transition pressures are associated with a sudden collapse in volume, indicating the occurrence of a first order phase transition. At compressed volumes, these compounds are found in the CsCl phase. The phase transition pressures and associated volume collapses obtained from the potential model developed here show a generally better agreement with available experimental data than others available in the literature. The elastic constants and bulk modulus are also reported. Our results are, in general, in good agreement with experimental and theoretical data where available, and provide predictions where data are unavailable.

Structural phase stability and elastic behavior of III–V scandium pnictides

A modified interaction potential (MIPM) model (including the covalency effect) has been developed and applied for the first time to investigate the high-pressure structural phase transition of scandium pnictides (ScAs and ScSb). Phase transition pressures are associated with a sudden collapse in volume indicating the occurrence of first order phase transition. The phase transition pressures and associated volume collapses obtained from present potential model show a generally better agreement with available experimental data than others. The elastic constants and their pressure derivatives are also reported. Moreover, the thermo physical properties have also been obtained successfully. Our results are in good agreement with available experimental and theoretical data.

The net charge at interfaces between insulators

The issue of the net charge at insulating oxide interfaces is briefly reviewed with theambition of dispelling myths of such charges being affected by covalency and relatedcharge density effects. For electrostatic analysis purposes, the net charge at suchinterfaces is defined by the counting of discrete electrons and core ion charges,and by the definition of the reference polarization of the separate, unperturbedbulk materials. The arguments are illustrated for the case of a thin film ofLaAlO3 overSrTiO3 in the absence of free carriers, for which the net charge is exactly0.5e per interface formula unit, if the polarization response in both materials is referred to zerobulk values. Further consequences of the argument are extracted for structural andchemical alterations of such interfaces, in which internal rearrangements are distinguishedfrom extrinsic alterations (changes of stoichiometry, redox processes), only the latteraffecting the interfacial net charge. The arguments are reviewed alongside the proposal ofStengel and Vanderbilt (2009 Phys. Rev. B 80 241103) of using formal polarization valuesinstead of net interfacial charges, based on the interface theorem of Vanderbilt andKing-Smith (1993 Phys. Rev. B 48 4442–55). Implications for non-centrosymmetricmaterials are discussed, as well as for interfaces for which the charge mismatch is an integernumber of polarization quanta.

Synthesis, Crystal Structure, and Electron Paramagnetic Resonance Investigations of Heteronuclear CoII/ZnII and CoII/CdII Coordination Polymers

The crystal structures of five new Co(II), Zn(II), and Cd(II) coordination polymers based on pyridine-substituted triazolyl carboxylates are reported. The two isomorphous compounds (∞)³[M(Me-3py-pba)₂] (M = Zn, Co) possess {6⁶} topology (dia). In order to obtain heteronuclear compounds, we synthesized Co(II)-substituted Zn(II) and Cd(II) coordination polymers. At T = 5 K, the powder samples of the diamagnetically diluted Co(II)/Zn(II) and Co(II)/Cd(II) systems [Co/(Zn,Cd) ≈ 0.01] show intense electron paramagnetic resonance spectra, which were analyzed with an effective spin of S' = ½. The g tensor as well as the ⁵⁹Co hyperfine tensor A(Co) are strongly anisotropic. The g tensor components are used to gain information about the symmetry of the Co(II) coordination sphere and covalency effects. Differential thermal analysis/thermogravimetry-mass spectrometry and temperature-dependent powder X-ray diffraction studies reveal high thermal stability of the three-dimensional coordination polymers up to 390 °C.

Magnetic ground state of pyrochlore oxides close to metal-insulator boundary probed by muon spin rotation

Magnetism of ruthernium pyrochlore oxides A2Ru2O7 (A = Hg, Cd, Ca), whose electronic properties within a localized ion picture are characterized by non-degenerate t2g orbitals (Ru5+, 4d3) and thereby subject to geometrical frustration, has been investigated by muon spin rotation/relaxation (muSR) technique. The A cation (mostly divalent) was varied to examine the effect of covalency (Hg > Cd > Ca) on their electronic property. In a sample with A = Hg that exhibits a clear metal-insulator (MI) transition below >> 100 K (which is associated with a weak structural transition), a nearly commensurate magnetic order is observed to develop in accordance with the MI transition. Meanwhile, in the case of A = Cd where the MI transition is suppressed to the level of small anomaly in the resistivity, the local field distribution probed by muon indicates emergence of a certain magnetic inhomogeneity below {\guillemotright} 30 K. Moreover, in Ca2Ru2O7 that remains metallic, we find a highly inhomogeneous local magnetism below >>25 K that comes from randomly oriented Ru moments and thus described as a "frozen spin liquid" state. The systematic trend of increasing randomness and itinerant character with decreasing covalency suggests close relationship between these two characters. As a reference for the effect of orbital degeneracy and associated Jahn-Teller instability, we examine a tetravalent ruthernium pyrochlore, Tl2Ru2O7 (Ru4+, 4d4), where the result of muSR indicates a non-magnetic ground state that is consistent with the formation of the Haldane chains suggested by neutron diffraction experiment.

Theory of the ligand-correlated covalent effect on spin orbit coupling for Cr3+ ion doping in a variety of oxide crystals

By using the complete energy matrix and taking into account the contributions from the spin orbit coupling of the central ions as well as those of the ligand ions, the local structure distortion have been studied for Cr(3+) ions doping in KAl( MoO(4)), Y(3)Ga(5)O(12), Y(3)Al(5)O(12) and ZnAl(2)O(4) crystals. The results show that the local structure exhibit elongation distortions for the doped systems. Moreover, the influence of the local structure parameter h on the EPR parameter D are studied. The relationships between D and (zeta, zeta') as well as the average parameter zeta(1) (zeta(1) = (zeta + zeta')/2) and the different parameter zeta(2) (zeta(2) = (zeta - zeta')/2) are discussed. (C) 2010 Elsevier B.V. All rights reserved.

Thermo-elastic and structural properties of thorium chalcogenides: A high pressure study

The pressure induced phase transition and thermo-elastic properties of Th-chalcogenides have been investigated by means of two different models (i) modified charge-transfer potential model which consists of Coulomb and Coulomb screening due to delocalization of f-electron of the rare-earth atom and modified by covalency and zero-point energy effects and repulsive interactions, and (ii) charge-transfer model which excludes covalency and zero-point energy effects in the previous model. These materials lie in F m 3 ¯ m phase at ambient conditions and transform to P m 3 ¯ m phase under pressure due to first-order phase transition ( P T = 24.75 and 14.34 GPa) and their equation of state show volume collapse of 8.92 and 9.13%, respectively for ThS and ThSe while ThTe retains P m 3 ¯ m phase up to 50 GPa. Our results are in good agreement with the experimental data. The elastic and thermal properties of these compounds have also been computed at normal and high pressures. Both the models are capable of explaining the Cauchy’s discrepancy, elastic, phase transition and thermophysical properties successfully.

Investigations of the spin-Hamiltonian parameters and tetragonal distortions due to Jahn–Teller effect for the monovalent d 9 (Ni +, Pd +, Pt +) impurity centers in AgCl crystals

When the monovalent md 9 ions Ni + (3d 9), Pd + (4d 9) and Pt +(5d 9) substitute for the Ag + ions in AgCl crystal, the cubic chlorine octahedron surrounding Ag + in the host AgCl crystal distorts into the tetragonally elongated octahedron in the md 9 impurity center owing to the Jahn–Teller effect. The spin-Hamiltonian parameters ( g factors g ||, g ⊥ and hyperfine structure constants A ||, A ⊥) of the tetragonal md 9 centers in AgCl are calculated from the high-order perturbation formulas based on the cluster approach, in which the admixture between d orbitals of d n ion and p orbitals of ligand ion via covalence effect are taken into account. From the calculations, the spin-Hamiltonian parameters of these tetragonal md 9 centers in AgCl are explained reasonably and the tetragonal distortions (characterized by the tetragonal elongation Δ R = R || − R ⊥, where R || and R ⊥ denote the metal–ligand distances parallel with and perpendicular to the tetragonal axis) of these centers due to Jahn–Teller effect are obtained. The results are discussed.

Theoretical investigations of the local structure distortion and relationship between the EPR parameter and spin-orbit coupling coefficients for CsCdX3:Ni2+ (X = Cl, Br) systems

The EPR zero-field splitting parameters and structural distortion of Ni2+-doped CsCdX3 (X = Cl, Br) systems have been studied on the basic of 45f × 45 complete energy matrices for a configuration ion in trigonal ligand field, in which the contributions from the spin-orbit coupling coefficients of the central ions and ligands are taken into account simultaneously. It is shown that the local structure exhibits compression distortions for CsCdX3 (X = Cl, Br):Ni2+ systems, and the contribution from the s.o. coupling coefficient of ligands to the covalent effect should not be neglected. Simultaneously, the relationship between the EPR parameter D and the spin-orbit coupling coefficients (ζ, ) as well as the average parameter ζ1() and the divergent parameter ζ2() have been discussed.

Stability of the various crystallographic phases of the multiferroic (1−x)BiFeO3–xPbTiO3 system as a function of composition and temperature

In this report, we have studied the stability of various crystallographic phases of (1−x)BiFeO3–xPbTiO3 as a function of composition and temperature. The structure of BF-xPT is reconfirmed to be tetragonal and monoclinic in the P4mm and Cc space groups for x>0.31 and 0.10≤x≤0.27, whereas the two phases coexist in the morphotropic phase boundary (MPB) region 0.27<x<0.31. A recent proposition of an orthorhombic phase in the MPB region has been ruled out comprehensively. From the high temperature powder x-ray diffraction (XRD) data it is shown that the MPB in this system is tilted towards the tetragonal side, a unique feature among the family of PbTiO3 based MPB systems. We have established accurately the room temperature phase diagram for this solid solution. By comparing the observed bond lengths between oxygen and other cations, obtained from Rietveld analysis of the room temperature powder XRD data with expected ionic bond lengths, we have shown that the very high c/a ratio in the tetragonal phase of this system is linked with the covalency effects for bonding between both A and B site cations with oxygen.

Theoretical investigations of the electron paramagnetic resonance for Cr3+ ions at tetragonal sites in K2ZnF4 crystal

Abstract In tetragonal ligand-field, based on the Zhao’s semi-SCF d-orbit wave functions model, the Macfarlane’s perturbation loop method and point-charge crystal field model, the third-order perturbation formulas of electron paramagnetic resonance parameters (zero-field splitting D and g- factors) for 3d 3/7 configuration ions have been constructed. In these formulas, the contributions from configuration interaction, covalency effect, Racah–Trees corrections and local structure distortion (Δ R ) have been considered. By using these formulas, the electron paramagnetic resonance parameters of Cr 3+ ion at tetragonal site in K 2 ZnF 4 crystal have been calculated. The theoretically calculated values are consistent with the experimentally observed.

Bonding in Classical and Nonclassical Transition Metal Carbonyls: The Interacting Quantum Atoms Perspective

Chemical bonding in simple transition metal carbonyls is examined under the interacting quantum atoms approach (IQA), which provides an energetic viewpoint within the quantum theory of atoms in molecules (QTAIM). We have studied both classical and nonclassical isoelectronic series of complexes, with different coordinations and geometries and studied the evolution of the IQA interatomic interactions, using several levels of theory. Our results in classical carbonyls are compatible with the standard Dewar−Chatt−Duncanson model, although multicenter bonding may have an important role in some complexes. The increase (decrease) in the CO distance upon bonding is faithfully coupled to a decrease (increase) in the CO covalent energy, although the main energetic change in the CO moiety is electrostatic and due to charge transfer and/or polarization of its electron density. The metal−ligand interaction energy is dominated by covalent effects and depends strongly on the total net charge of the complex, being larger for negatively charged molecules, where π-back-donation is very important. The electrostatic (ionic-like) metal−ligand interaction energy is small in general, although it becomes more and more stabilizing with increasing coordination number.

Effect of covalency, zero-point energy and charge transfer on the phase-transition, elastic and thermophysical properties of Ca-chalcogenides under compression

The pressure induced phase-transition, elastic and thermophysical properties of Ca-chalcogenides have been investigated by means of many body potential. The modified charge transfer potential consists of long-range Coulomb and charge-transfer interactions modified by covalency and short-range overlap repulsion extended up to second neighbours and zero-point energy effects. Another charge-transfer model excludes covalency and zero-point energy effects. These chalcogenides undergo first-order phase-transition at P T = 39.23, 36.30 and 31.20 GPa and their equation of state show volume collapse of 10.12, 7.61 and 4.55% for CaS, CaSe and CaTe, respectively, which are in good agreement with the experiments. The elastic and thermophysical properties of these compounds have also been computed at normal and high pressures. Both the models are capable of explaining the Cauchy-discrepancy (C12 ≠ C44), elastic, phase-transition and thermophysical properties successfully.

Pressure dependent elastic and structural ( B3– B1) properties of Ga based monopnictides

By formulating an effective interionic interaction potential that incorporates the long-range Coulomb, the covalency effects, the charge transfer caused by the deformation of the electron shells of the overlapping ions, the Hafemeister and Flygare type short-range overlap repulsion extended up to the second neighbour ions and the van der Waals (vdW) interaction, the pressure dependent elastic and thermodynamical properties of the III–V semiconductors as Ga Y ( Y = N, P, As) are studied. The estimated values of phase transition pressure of Ga Y ( Y = N, P, As) are in reasonably good agreement with the available data on the phase transition pressures ( P t = 41, 22, 17 GPa). The vast volume discontinuity in pressure–volume phase diagram identifies a structural phase transition from zinc-blende ( B3) to rock salt ( B1) structure. Later on, the Poisson's ratio ν, the ratio R S/ B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and Debye temperature as functions of pressure is calculated. From Poisson's ratio and the ratio R S/ B it is inferred that Ga Y ( Y = N, P, As) is brittle [ductile] in zinc-blende ( B3) [Sodium Chloride ( B1)] phase. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Ga Y compounds and still awaits experimental confirmations.

Pressure induced structural phase transition and elastic properties in BSb, AlSb, GaSb and InSb compounds

By formulating an effective interionic interaction potential that incorporates the long-range Coulomb, the covalency effects, the charge transfer caused by the deformation of the electron shells of the overlapping ions, the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbour ions and the van der Waals (vdW) interaction, the pressure dependent elastic and thermodynamical properties of the III–V antimonide semiconductors as YSb (Y=B, Al, Ga, and In) are investigated. Estimated values of phase transition pressure of YSb antimonides are consistent with the available data on the phase transition pressures. The ratio R S / B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic wave velocity, average wave velocity and Debye temperature as functions of pressure is calculated. From the ratio R S / B it is inferred that YSb (Y=Al, Ga, and In) are ductile and BSb is brittle in zinc blende (B3) phase. To our knowledge this is the first quantitative theoretical prediction of the ductile (brittle) nature of YSb antimonides and still awaits experimental confirmations.

Covalent antithrombin-heparin effect on thrombin-thrombomodulin and activated protein C reaction with factor V/Va

Thrombomodulin (TM), which variably contains a chondroitin sulfate (+/-CS), forms an anticoagulant complex with thrombin (IIa). IIa-TM(+/-CS) converts protein C (PC) into activated PC (APC), which then inactivates activated factors V (FVa) and VIII (FVIIIa). This reduces prothrombinase and tenase complexes that generate IIa. Heparin (H) increases the rate of IIa-TM inhibition by antithrombin (AT) and enhances FV cleavage by APC. Our novel covalent AT-H (ATH) product, has superior anticoagulant activity compared to AT + unfractionated H (UFH). We studied mechanisms by which ATH versus AT + UFH inhibits IIa-TM(+/-CS), and ATH influences on APC cleavage of FV/FVa compared to UFH. Findings would determine how these reactions moderate ATH's overall effects as an anticoagulant. Discontinuous second order rate inhibition assays of IIa-TM(+/-CS) inhibition by AT + UFH or ATH were performed in presence or absence of human umbilical vein endothelial cells (HUVECs). FV/FVa cleavage by APC in the presence of UFH or ATH was analysed by Western blots. ATH increased IIa-TM(+/-CS) inhibition to a greater degree than AT + UFH, both on plastic and HUVEC surfaces. Unlike UFH, ATH did not accelerate FV cleavage by APC, but ATH did enhance FVa cleavage relative to UFH. Increased IIa-TM inhibition by ATH downregulates PC activation. However, ATH does accelerate downstream inactivation of FVa, which increases its potency for IIa generation inhibition compared to UFH. This trend holds true in the presence of APC's cofactor, protein S. Overall, ATH may have a balanced function towards inhibiting or accelerating PC pathway activities.