Xiao-Yu Kuang

Theory of covalent magnetic exchange interaction for diiron(III) core in the active site of ribonucleotide reductase

A covalent magnetic exchange interaction formula for describing the transition-metal ion pairs in strong covalent complex molecules has been derived in terms of the Girerd-Journaux-Kahns magnetic theory and the Cuire-Barth-Cannys covalent theory. By this formula the relationship between the magnetic exchange coupling parameter J and the covalent parameter N has been studied. It is shown that the exchange interaction depends sensitively on the covalent effect, i.e., \J\ increases rapidly with decreasing N. For the diiron(III) core in the active site of ribonucleotide reductase as well as in its TPA model molecules, our calculations revealed that: (1) about 40% of the antiferromagnetic contribution comes from the covalent effect; (2) the theoretical values J = -98.6 similar to -103.9 cm(-1) of diiron(III) core for N = 0.89 and R = 3.58-3.60 A (R = R-1 + R-2 is the sum of the two bond lengths of Fe-III(1)-O and Fe-III(2)-O in Fe-III(1)-O-Fe-III (2) cluster) are in good agreement with the experimental findings J = -108 cm(-1) for Fe(III) pairs in the active site of ribonucleotide reductase and J = -101 similar to -119 cm(-1) in its TPA series model molecules, respectively

Pressure-induced structural transition and thermodynamic properties of NbN and effect of metallic bonding on its hardness

Using first- principles calculations, the elastic constants, thermodynamic properties and structural phase transition of NbN under high pressure are investigated by means of the pseudopotential plane- waves method, in addition to the effect of metallic bonding on its hardness. Three candidate structures are chosen to investigate NbN, namely, rocksalt (NaCl), NiAs and WC types. On the basis of the third- order Birch- Murnaghan equation of states, the transition pressure Pt (Pt = 200.64GPa) between the WC phase and the NaCl phase of NbN is predicted for the first time. Elastic constants, formation enthalpies, shear modulus, Youngs modulus, and Poissons ratio of NbN are derived. The calculated results are found to be in good agreement with the available experimental data and theoretical values. According to the quasi-harmonic Debye model, the Debye temperature under high pressure is derived from the average sound velocity. Moreover, the effect of metallic bonding on the hardness of NbN is investigated and the hardness shows a gradual decrease rather than increase under compression. This is a quantitative investigation on the structural and thermodynamic properties of NbN, and it still awaits experimental confirmation. Copyright (C) EPLA, 2010

Theoretical investigation on the high-pressure structural transition and thermodynamic properties of cadmium oxide

A theoretical investigation on the structural and thermodynamic properties of CdO under high pressure is performed by employing the pseudopotential plane-wave method in the framework of the density functional theory. Some structural parameters of CdO in both B1 and B2 phases are reported. According to the third-order Birch-Murnaghan equation of states, the transition pressure P(t) of CdO from the B1 structure to the B2 structure is determined. The calculated results are found to be in good agreement with the available experimental data and theoretical values. Based on the quasi-harmonic Debye model, the Debye temperature of CdO under high pressure is derived from the average sound velocity. This is a quantitative theoretical prediction of the elastic and thermodynamic properties of CdO and it still awaits experimental confirmation. Copyright (C) EPLA, 2010

Theoretical study of the dependence of EPR spectra on local structure for octahedral Cr(3+) centres in oxides and fluorides series

A theoretical method for investigating the inter-relation between the EPR parameters and local structure has been established on the basis of the complete energy matrices for 3d3 configuration ions in both the trigonal and tetragonal ligand fields. By means of this method, the local structure of the octahedral Cr3+ centres in double molybdates series and spinels series as well as the perovskite-type fluorides series has been studied systematically. Furthermore, the dependence of the EPR zero-field splitting parameter D on the local structure parameters in both trigonal and tetragonal ligand-fields has been revealed, simultaneously. The inter-relation between the EPR parameters D and Δg(g //  − g ⊥) is also elucidated.

Theoretical studies of EPR spectra and defect structure for Er3+ center in lithium niobate

The optical and EPR spectra of octahedral Er(3+) center in LiNbO(3) have been studied by diagonalizing 364 x 364 complete energy matrices. The new set of crystal-field parameters that can well account for the Stark levels and EPR parameters have been obtained for Er(3+) ions in LiNbO(3). Simultaneously, by simulating the most reliable six-order parameter B(60) obtained, we have presented the evidence that the Er(3+) ions do not occupy the actual Li(+) site, but have a displacement along the C(3)-axis away from the Li(+) center by about 0.0454 nm. The conclusion is well in accord with that drawn by earlier workers.

Structural, spectral characterization and electron paramagnetic resonance studies of Ni(2+) ions in various compounds: KZnF(3), CdCl(2), CdBr(2) and CsMgI(3)

The electron paramagnetic resonance (EPR) parameters (zero-field splitting D, and g-factors g ∥, g ⊥) for d 8 Ni2+ ions in tetragonal ligand fields KZnF3 : Ni2+ or in trigonal ligand fields CdCl2(CdBr2 and CsMgI3) : Ni2+ complexes have been systematically studied on the basis of the 45 × 45 complete energy matrices. The local structural distortion parameters of the crystals obtained are ΔR 1 = 0.0355 Å, ΔR 2 = 0.0296 Å for KZnF3 : Ni2+, ΔR = −0.1730 Å, Δθ = −1.5695○ for CdCl2 : Ni2+, Å, Δθ = −2.6236○ for CdBr2 : Ni2+ and ΔR = −0.0267 Å, Δθ = −1.8967○ for CsMgI3 : Ni2+. The calculated results show that the local structures exhibit elongation distortion for KZnF3 : Ni2+ and compression distortions for Ni2+ : CdCl2 (CdBr2 and CsMgI3) systems. Further, the relationships between ZFS parameter D and structural parameters (θ, R) are studied and the influence of the orbit reduction factor k on g-factors (Δg, g) are discussed.

Exploration of the Structural, Electronic and Tunable Magnetic Properties of Cu4M (M = Sc-Ni) Clusters

The structural, electronic and magnetic properties of Cu4M (M = Sc-Ni) clusters have been studied by using density functional theory, together with an unbiased CALYPSO structure searching method. Geometry optimizations indicate that M atoms in the ground state Cu4M clusters favor the most highly coordinated position. The geometry of Cu4M clusters is similar to that of the Cu5 cluster. The infrared spectra, Raman spectra and photoelectron spectra are predicted and can be used to identify the ground state in the future. The relative stability and chemical activity are investigated by means of the averaged binding energy, dissociation energy and energy level gap. It is found that the dopant atoms except for Cr and Mn can enhance the stability of the host cluster. The chemical activity of all Cu4M clusters is lower than that of Cu5 cluster whose energy level gap is in agreement with available experimental finding. The magnetism calculations show that the total magnetic moment of Cu4M cluster mainly come from M atom and vary from 1 to 5 μB by substituting a Cu atom in Cu5 cluster with different transition-metal atoms.

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.

Theoretical study of local structure for Ni2+ ions at tetragonal sites in K2ZnF4:Ni2+ system.

A theoretical method for studying the local lattice structure of Ni2+ ions in (NiF6)(4-) coordination complex is presented. Using the ligand-field model, the formulas relating the microscopic spin Hamiltonian parameters with the crystal structure parameters are derived. Based on the theoretical formulas, the 45 x 45 complete energy matrices for d8 (d2) configuration ions in a tetragonal ligand-field are constructed. By diagonalizing the complete energy matrices, the local distortion structure parameters (R perpendicular and R || ) of Ni2+ ions in K2ZnF4:Ni2+ system have been investigated. The theoretical results are accorded well with the experimental values. Moreover, to understand the detailed physical and chemical properties of the fluoroperovskite crystals, the theoretical values of the g factor of K2ZnF4:Ni2+ system at 78 and 290 K are reported first.

Equilibrium Geometries, Stabilities, and Electronic Properties of the Bimetallic Ag2-doped Sin (n=1 – 11) Clusters: A Density-Functional Investigation

An ab initio method based on the density functional theory has been employed to investigate the behaviours of the bimetallic Ag2-doped silicon clusters at a size of n = 1 - 11. The possible geometrical configurations, growth-pattern behaviours, stabilities, energy gaps, and electronic properties are presented and discussed. The optimized geometries reveal that the silicon atom surface-capped and silver atom substituted 3D structures are dominant growth patterns. The calculated averaged binding energy, fragmentation energy, and the second-order difference of energy manifest that the most stable structures of Ag2Sin (n = 1 - 11) clusters are Ag2Si2 and Ag2Si5 isomers, which is in qualitative agreement with the AgSin clusters. In addition, the gap between highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) exhibits that the Ag2Si3 and Ag2Si5 isomers have dramatically enhanced chemical stability. Natural population analysis shows that the charge-transfer phenomena are coincidence with the AgSin clusters but different from Mo2Sin systems. Furthermore, the dipole moments of stable Ag2Sin (n = 1 - 11) display a pronounced odd-even oscillation with the number of silicon atoms.