Planar Bond Length Research Articles

A comparative study on the local structures of the V4+ and Cu2+ centers in WO3

Abstract The local structures and electron paramagnetic resonance (EPR) parameters (the g factors g i and the hyperfine structure constants A i , with i = x, y, z) of the V4+ and Cu2+ centers in WO3 were theoretically investigated based on the perturbation formulas of the EPR parameters for 3d1 and 3d9 ions under rhombically compressed and elongated octahedra, respectively. In these formulas, the adopted crystal-field parameters (CFPs) are obtained from the superposition model which enables connection of the CFPs and hence the EPR parameters to the rhombic distortion. Based on the calculations, the impurity-ligand distances of the studied [VO6]8− (or [CuO6]10−) cluster are found to suffer the axial compression (or elongation) δz of about 0.098 Å (or 0.132 Å) along the z-axis, and the additional planar bond length variation δr (≈0.052 or 0.047 Å) along x- and y-axis, respectively, due to the Jahn–Teller (JT) effect.

Studies of local structures for Cu2+ centers in M2Zn(SO4)2·6H2O (M = NH4 and Rb) crystals

Abstract The electron paramagnetic resonance (EPR) parameters (g factor g i , and hyperfine structure constants A i , with i = x, y, z) and local structures for Cu2+ centers in M2Zn(SO4)2·6H2O (M = NH4 and Rb) are theoretically investigated using the high order perturbation formulas of these EPR parameters for a 3d 9 ion under orthorhombically elongated octahedra. In the calculations, contribution to these EPR parameters due to the admixture of d-orbitals in the ground state wave function of the Cu2+ ion are taken into account based on the cluster approach, and the required crystal-field parameters are estimated from the superposition model which enables correlation of the crystal-field parameters and hence the studied EPR parameters with the local structures of the Cu2+ centers. Based on the calculations, the Cu–H2O bonds are found to suffer the axial elongation ratio δ of about 3 and 2.9% along the z-axis, meanwhile, the planar bond lengths may experience variation ratio τ (≈3.8 and 1%) along x- and y-axis for Cu2+ center in (NH4)2Zn(SO4)2·6H2O and Rb2Zn(SO4)2·6H2O, respectively. The theoretical results show good agreement with the observed values.

Theoretical studies of the g factors and local structures of the Ni3+ centers in Na2Zn(SO4)2·4H2O and K2Zn(SO4)2·6H2O crystals

The local structures and the g factors gi (i = x, y, z) for Ni3+ centers in Na2 Zn(SO4 )2 ·4H2 O (DPPH) and K2 Zn(SO4 )2 ·6H2 O (PHZS) crystals are theoretically studied by using the perturbation formulas of the g factors for a 3d7 ion with low spin (S = 1/2) in orthorhombically compressed octahedra. In these formulas, the contributions to g factors from both the spin-orbit coupling interactions of the central ion and ligands are taken into account, and the required crystal-field parameters are estimated from the superposition model and the local geometry of the systems. Based on the calculations, the Ni-O bonds are found to suffer the axial compression δz (or Δz) of about 0.111 Å (or 0.036 Å) along the z-axis for Ni3+ centers in DPPH (or PHZS) crystals. Meanwhile, the Ni-O bonds may experience additional planar bond length variation δx (≈0.015 Å) along x- and y-axes for the orthorhombic Ni3+ center in DPPH. The theoretical g factors agree well with the experimental data. The obtained local structural parameters for both Ni3+ centers are discussed.

Investigations on the g Factors for the Tetragonal and Orthorhombic Cu2+ Centers in YBa2Cu3O7–δ

The local structures and the g factors of two Cu2+ centers in YBa2Cu3O7–δ are theoretically studied by using the perturbation formulas of the g factors for a tetragonal and an orthorhombically elongated octahedral 3d9 clusters. The crystal-field parameters in these formulas are determined from the superposition model and the local distortions due to the Jahn–Teller effect. Based on the calculations, for the tetragonal Cu2+ center in YBa3Cu2O7, the Cu-O bonds are found to suffer the axial elongation of about 0.032 A along the c-axis due to the Jahn–Teller effect. For the orthorhombic Cu2+ center in Y2BaCuO5, the Cu-O bonds is found to experience the axial elongation δZ (≈ 0.078 A) along the c-axis and the planar bond length variation δX (≈ 0.012 Ǻ) in the perpendicular (ab) plane, responsible for the axial anisotropy Δg (= gz − (gx + gy)/2 = 0.168) and the perpendicular anisotropy δg (= gx – gy = 0.081). The theoretical g factors are in good agreement with the experimental data. The local structures of both Cu2+ centers are discussed.

A new donor-acceptor crosslinkable l-shape chromophore for NLO applications

A new nonlinear-optical (NLO) chromophore containing L-Shape 2-benzofuranyldiazene in π-electron bridge has been synthetized and its chemical and thermal properties examined. Evaluation of the second order NLO properties was performed by EFISH measurements of μβ on chloroform solution and by SHG measurements on amorphous spin-coated thin films obtained from a blend of polymer and chromophores crosslinked “in situ”. The value of 830∙10−48 esu for μβ measured on the dye and of 10 pm/V for the d33 for the poled network are in medium range for similar systems, confirming the proper employ of the molecule to build NLO active networks. The crystal structures dye and its precursor examined by single-crystal analysis show a planar geometry and bond lengths pattern confirm the push pull L-shape nature of the D-π-A systems.

Studies on EPR Parameters and Local Structures for the Rhombic Cu2+ Center in the ZnGeO4 · 6H2O Crystal

The electron paramagnetic resonance (EPR) parameters (g factors and hyperfine structure constants) and local structures for the impurity Cu2+ center in ZnGeO4 · 6H2O crystal are theoretically studied using the perturbation formulas of these parameters for a 3d9 ion in rhombically elongated octahedra. These formulas are based on a two-spin-orbit-parameter model in which the contributions to the EPR parameters due to both the spin-orbit parameter of the central 3dn ion and that of ligand ion are included. Based on the calculation, the relative axial elongation ratio ρ (≈7.3%) along z-axis and the relative planar bond length variation τ (≈4.3%) along x- and y-axes are found for [Cu(H2O)6]2+ cluster in ZnGeO4 · 6H2O crystal. The above local rhombic distortions of the Jahn–Teller nature around Cu2+ can suitably account for the axial and perpendicular anisotropies of the observed EPR parameters.

Investigations of g Factors and Local Structures for Cu(en) $$_{3}^{{2 + }}$$ Groups in Cu(en)3SO4 and Cu(en)3Cu(CN)3 Polycrystalline Powders

From the perturbation formulas for g factors for an orthorhombically compressed octahedral 3d9 group, g factors, and local structures for the [Cu(en)3]2+ groups in Cu(en)3SO4 and Cu(en)3Cu(CN)3 powders are theoretically investigated. The local structure of Cu2+ in Cu(en)3Cu(CN)3 is evaluated by analyzing the experimental g factors and found to exhibit moderate axial compression ΔZ (≈0.018 A) and planar bond length variation ΔR (≈0.015 A) related to an ideal octahedron due to the Jahn–Teller effect. The larger experimental axial anisotropy Δg (= (gx + gy)/2 − gz) of Cu(en)3SO4 than Cu(en)3Cu(CN)3 is mainly ascribed to the larger ΔZ (≈0.053 A) in the former. The calculated g factors for both systems show reasonable agreement with experiment by including the charge transfer (CT) contributions to gz, which have the same sign and much larger magnitude compared to the conventional crystal-field ones and should be included.

Theoretical studies of the EPR parameters and local structures for the two Cu2+ centers in Cd(HCOO)2 ·2H2 O.

Based on the studies of the electron paramagnetic resonance parameters for two types of the Cu2+ centers in Cd(HCOO)2 ·2H2 O by using the high-order perturbation formulas for a 3d9 ion in a rhombically elongated octahedron, local structure of the doped copper ion is determined. Research suggests that the impurity Cu2+ replaces the host Cd2+ and undergoes the local rhombic elongation distortion, characterized by the axial elongation ratios of 4.1%, and 3.8% along the z-axis and the planar bond length variation ratios of 3.8%, and 3.1% along the x-axis and y-axis, for Cu2+ Centers, I and II, respectively. The above slightly different axial elongation ratios and planar bond length variation ratios may suitably account for the slightly dissimilar axial g anisotropies Δg (≈0.351 and 0.339) and perpendicular g anisotropies δg (≈0.028 and 0.022) of the two centers, respectively.

Analysis on the local structures for 3d1 impurities (Ti3+ and V4+ ) in KTiPO4.

Making use of the perturbation formulae for 3d1 ions (Ti3+ and V4+ ) under orthorhombically compressed octahedra, the spin Hamiltonian parameters (g factors: gx , gy , gz and hyperfine structure constants: Ax , Ay , Az ) and local structures of the 3d1 impurity centres C1 , C2 , and C3 in KTiOPO4 crystals are theoretically analyzed in a consistent way. The remarkable local distortions (i.e., the relative axial compression ratios 11.2%, 7.0%, and 5.5% along Z axis and the relative planar bond length variation ratios 15.9%, 7.0%, and 6.0%) are obtained for the [Ti2O6 ]9- cluster on Ti2 site and [VO6 ]8- clusters on Ti1 and Ti2 sites, respectively, in view of the Jahn-Teller effect. The above local orthorhombic distortion parameters in the impurity centres are found to be more significant than the host Ti1 and Ti2 sites in pure KTiOPO4 . The sequences (C1 >C2 >C3 ) of the local orthorhombic distortion parameters ρ and τ are in accordance with those of the axial and perpendicular anisotropies Δg and δg of g factors, respectively.

A-Site Ordered Double Perovskite CaMnTi2O6 as a Multifunctional Piezoelectric and Ferroelectric-Photovoltaic Material.

The double perovskite CaMnTi2O6, is a rare A-site ordered perovskite oxide that exhibits a sizable ferroelectric polarization and relatively high Curie temperature. Using first-principles calculations combined with detailed symmetry analyses, we identify the origin of the ferroelectricity in CaMnTi2O6. We further explore the material properties of CaMnTi2O6, including its ferroelectric polarization, dielectric and piezoelectric responses, magnetic order, electronic structure, and optical absorption coefficient. It is found that CaMnTi2O6 exhibits room-temperature-stable ferroelectricity and moderate piezoelectric responses. Moreover, CaMnTi2O6 is predicted to have a semiconducting energy band gap similar to that of BiFeO3, and its band gap can further be tuned via distortions of the planar Mn-O bond lengths. CaMnTi2O6 exemplifies a new class of single-phase semiconducting ferroelectric perovskites for potential applications in ferroelectric photovoltaic solar cells.

Safe-by-Design CuO Nanoparticles via Fe-Doping, Cu-O Bond Length Variation, and Biological Assessment in Cells and Zebrafish Embryos.

The safe implementation of nanotechnology requires nanomaterial hazard assessment in accordance with the material physicochemical properties that trigger the injury response at the nano/bio interface. Since CuO nanoparticles (NPs) are widely used industrially and their dissolution properties play a major role in hazard potential, we hypothesized that tighter bonding of Cu to Fe by particle doping could constitute a safer-by-design approach through decreased dissolution. Accordingly, we designed a combinatorial library in which CuO was doped with 1-10% Fe in a flame spray pyrolysis reactor. The morphology and structural properties were determined by XRD, BET, Raman spectroscopy, HRTEM, EFTEM, and EELS, which demonstrated a significant reduction in the apical Cu-O bond length while simultaneously increasing the planar bond length (Jahn-Teller distortion). Hazard screening was performed in tissue culture cell lines and zebrafish embryos to discern the change in the hazardous effects of doped vs nondoped particles. This demonstrated that with increased levels of doping there was a progressive decrease in cytotoxicity in BEAS-2B and THP-1 cells, as well as an incremental decrease in the rate of hatching interference in zebrafish embryos. The dissolution profiles were determined and the surface reactions taking place in Holtfreter's solution were validated using cyclic voltammetry measurements to demonstrate that the Cu+/Cu2+ and Fe2+/Fe3+ redox species play a major role in the dissolution process of pure and Fe-doped CuO. Altogether, a safe-by-design strategy was implemented for the toxic CuO particles via Fe doping and has been demonstrated for their safe use in the environment.

Theoretical investigations of the EPR g factors and the local structures for Ni3+ in LiAlyCo1–yO2 at various Al concentrations

The electron paramagnetic resonance (EPR) g factors and local structures for Ni3+ in LiAlyCo1–yO2 as the cathode materials for lithium-ion batteries are theoretically investigated at various Al concentrations y (= 0.1, 0.5, 0.8 and 1.0). The [NiO6]9– clusters are subject to the tetragonal elongation distortions along the C4 axis owing to the Jahn–Teller effect and the different degrees of distortions are characterised by the slightly increasing relative elongation ratio ρ (and also crystal-field strength and covalency) with y. In pure LiAlO2 (y = 1.0), the [NiO6]9– cluster exhibits the largest ρ (≈1.4%) and an additional relative variation ratio τ (≈1.4%) of the planar bond lengths, responsible for the perpendicular g anisotropy δg = (gx – gy). The number of impurity Ni3+ centres increases from one at y = 0 and 1.0 to two at y = 0.1 and 0.8 and reaches the maximum four at y = 0.5, in accordance with the increasing degree of disorder of the systems. The distinct impurity centres for the same y may be accompanied by the comparable energies of the ground and near excited states, similar to ‘resonant states’ with slightly different local elongation distortions and tetragonal level splittings. The present studies will be helpful to understand the relationships between local structures and performances of LiAlyCo1–yO2 type cathode materials.

Studies of the local distortions for nd1 (Mo5+ and W5+) impurities in KTP crystals

4d1 (Mo5+) and 5d1 (W5+) impurities doped into nonlinear optical crystals KTiOPO4 (KTP) can occupy the inequivalent Ti1 and Ti2 sites and exhibit orthorhombically compressed octahedral nd1 (n=4, 5) centres arising from the Jahn–Teller effect. Closely relevant to the electronic levels as well as optical and magnetic properties of these materials, the above local distortions can be suitably reflected by the anisotropic electron paramagnetic resonance (EPR) signals with the paramagnetic nd1 probes. This work stresses on the theoretical studies of the EPR parameters (g factors gx, gy, gz and hyperfine structure constants Ax, Ay, Az) and local distortions for the above nd1 impurity centres in a consistent way. The local distortions are characterised by the relative axial compression ratios ρ (≈0.040, 0.032 and 0.031) and the relative planar bond length variation ratios τ (≈0.030, 0.015 and 0.012) for Mo5+ centre I and W5+ centres II and III, respectively, which are larger than those of host Ti1 and Ti2 sites.

Theoretical investigations on the defect structures and spin Hamiltonian parameters for various orthorhombic Rh2+ centres in KTiOPO4 and KTiOAsO4

The defect structures and spin Hamiltonian parameters (SHPs) for three Rh2+ centres (denoted C1 in KTiOAsO4 and C2 and C3 in KTiOPO4) are theoretically investigated by utilising the perturbation formulae for a 4d7 ion under orthorhombically (D2h) elongated octahedra. The defect structures are characterized by the axial elongation ratios of 4.91%, 4.93% and 4.90% along the Z axis and the planar bond length variation ratios of 0.05%, 0.01% and 0.04% for centres C1, C2 and C3, respectively, owing to the Jahn–Teller effect. The nearly identical moderate axial elongation ratios and the slightly different tiny planar bond length variation ratios may suitably account for the comparable moderate axial g anisotropies ∆g (≈0.6087, 0.6124 and 0.6067) and the slightly dissimilar tiny perpendicular g anisotropies δg (≈0.0649, 0.0097 and 0.0509) of the three centres, respectively. All centres demonstrate similar strong crystal-field interactions and moderate covalence arising from the comparable short impurity-ligand distances.

A molecular dynamics study of bond strength and interface conditions in the [formula omitted] metal–ceramic composites

High ductility of metals as well as high strength of ceramics has made the metal/ceramic composites an attractive material for many applications requiring high strength to weight ratios. An important issue in using this material is the behavior of the material and its ceramic–metal interface under various loading, especially at high strain rate. To provide a better understanding of the interface conditions, in this work, a molecular dynamics study of the interface behavior in Al/α_Al2O3 composite as the result of tensile and shear loadings is presented. For this purpose, the reactive force field (ReaxFF) potential function is utilized. The effects of crystallographic orientations and atomic layers in the interface region on the deformation and fracture strength of the interface are examined. The radial distribution function, atomic planar density and mean bond length parameters as well as stress–strain curves are employed to identify the strength of the interface structures. The results show the highest tensile and shear strengths in the interface with O-termination of alumina, which can be explained by the formation of strong ionic bonds in the Al/Al2O1 interface. Moreover, the higher atomic planar density in (111) planes of aluminum and its similarity with that of alumina leads to a higher tensile and shear strength in the interface. Furthermore, the deformation mechanism in shear mode is shown to be controlled by planner sliding and local amorphization at the interface zone.

Studies on the EPR g factors for the oxygenated and nonoxygenated BaCuO2+x

The EPR g factors for the oxygenated and nonoxygenated BaCuO2+x are theoretically studied from the perturbation formulas of the g factors for an orthorhombically elongated octahedral 3d9 complex based on the cluster approach. The relative axial elongation ratios (≈1% and 0.6%) along c axis and the relative planar bond length variations (≈6.9% and 8.9%) are found for the oxygenated and nonoxygenated systems, respectively. The above local orthorhombic distortions of the Jahn–Teller nature around Cu2+ can suitably account for the axial and perpendicular anisotropies of the observed g factors. The present studies can be useful to understand the influences of this parasitic phase BaCuO2+x on the EPR behaviours and superconductivity of the R123 high Tc superconductors.

Investigation of charge states and multiferroicity in Fe-doped h-YMnO3

Polycrystalline YMn1−xFexO3 (YMFOx) (0 ≤ x ≤ 0.1) compounds have been prepared in single phase and characterized by synchrotron X-ray diffraction, X-ray absorption near edge spectroscopy, magnetization, and dielectric measurements. Iron-substitution in hexagonal YMnO3 causes intra-lattice changes exceeding those of the lattice cell. X-ray absorption near edge structure provides mixed-valence Mn3+/Mn4+ and Fe4+ charge states in these manganites, consistent with the observed decrease of the effective magnetic moment with Fe-doping. Magnetization M(T) evidence antiferromagnetic ordering of the specimens with little weak ferromagnetism, and the metrices of exchange interaction suppress with Fe-doping, attributed to the lengthening of the Mn–O planar bond lengths. Dielectric ε′(T) results showing highly doping-dependent anomaly at TN indicate linear magneto-electric coupling.

Investigations on the g factors for the Cu2+ sites in YBa1.9Na0.1Cu3O7−δ and YBa2Cu3O7−δ

The anisotropic g factors gi (i=x, y, z ) for the six-fold coordinated orthorhombic Cu2+(1) site in YBa1.9Na0.1Cu3O7−δ and the five-fold coordinated tetragonal Cu2+(2) site in YBa2Cu3O7−δ are theoretically studied from the perturbation formulas of the g factors for a 3d9 ion in orthorhombically and tetragonally elongated octahedra, respectively. The orthorhombic (or tetragonal) field parameters are obtained using the superposition model and the local structures of the Cu2+ sites. In view of covalency, the contributions from the ligand orbital and spin-orbit coupling interactions are taken into account. For the Cu2+(1) site in YBa1.9Na0.1Cu3O7−δ, the axial anisotropy Δg[=gz−(gx−gy)/2] may be ascribed to the moderate (about 3%) elongation of the Cu–O distance along c axis, while the perpendicular anisotropy δg(=gx−gy) can be characterized by the planar bond length deviation difference δR(≈0.083Å) in the perpendicular (ab) plane. However, for the Cu2+(2) site in YBa2Cu3O7−δ, the anisotropy Δg(=g||−g⊥) is illustrated by the significant elongation of the [CuO5]8− cluster along c axis with about 25% longer axial Cu–O distance than the four planar ones. The theoretical g factors are in good agreement with the experimental data.

Investigations on the spin Hamiltonian parameters and the local structures for the orthorhombic [CuX4(H2O)2]2− centers in NH4X (X = Cl, Br)

The spin Hamiltonian parameters (the g factors and the hyperfine structure constants) and the local structures for the orthorhombic [CuX4(H2O)2]2− clusters in NH4X (X=Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion in orthorhombically compressed octahedra. The related molecular orbital coefficients are quantitatively determined from the cluster approach in a uniform way. The studied defect centers are attributed to the impurity Cu2+ occupying the interstitial site associated with two adjacent water molecules along the [100] (or C2) axis. The ligand octahedra are found to suffer the relative planar bond length variations of about 0.19 and 0.20Å along X and Y axes for X=Cl and Br, respectively, due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters based on the above local structures show reasonable agreement with the observed values for both the centers. The validity of the results is also discussed.

Theoretical studies on the local structure and spin Hamiltonian parameters for the orthorhombic Cu2+ center in LiNbO3

The local structure and spin Hamiltonian parameters (the g factors and the hyperfine structure constants) for the orthorhombic Cu2+ center in LiNbO3 are theoretically studied from the perturbation formulas of these parameters for a 3d9 ion in an orthorhombically elongated octahedron. This center is ascribed to Cu2+ occupying the Nb5+ site in LiNbO3, associated with one nearest neighbour oxygen vacancy VO along the Z axis. The planar bond lengths are found to suffer the relative variation of about 0.16Å by compressing and stretching the Cu2+–O2− bonds along the X and Y axes, respectively, due to the Jahn–Teller effect and the charge mismatching substitution of Nb5+ by Cu2+. Meanwhile, the effectively positive VO can make the central Cu2+ displace away from the VO along the Z axis by about 0.3Å. The theoretical spin Hamiltonian parameters based on the above local distortions show good agreement with the experimental data.