Moderate Covalency Research Articles

Investigations on the local structure and the spin-Hamiltonian parameters for the tetragonal Cu2+ centre in ZnGeF6⋅6H2O crystal

The spin-Hamiltonian parameters (g factors g ∥, g ⊥ and hyperfine structure constants A ∥, A ⊥) and the local structure for the tetragonal Cu2+ centre in trigonal ZnGeF6⋅6H2O crystal are theoretically studied using the perturbation formulae of these parameters for a 3 d 9 ion in tetragonally elongated octahedra. In the calculations, the contributions to the spin-Hamiltonian parameters from ligand orbital and spin-orbit coupling are included on the basis of the cluster approach in view of moderate covalency of the studied systems, and the required crystal-field parameters are obtained using the superposition model and the local structures of the studied [Cu(H2O)6] 2+ cluster. According to the calculations, the ligand octahedra around Cu2+ suffer relative elongation τ (≈ 0.085 A) along the [0 0 1] (or C 4) axis for the tetragonal Cu2+ centres in ZnGeF6⋅6H2O crystal, due to the Jahn–Teller effect. The calculated results show good agreement with the experimental data.

Investigations on the local structure and the EPR parameters for the tetragonal Cu2+ center in ZnSeF6·6H2O crystal

The electron paramagnetic resonance (EPR) parameters (g factors g∥, g⊥ and hyperfine structure constants A∥, A⊥) and the local structure of the tetragonal Cu2+ center in trigonal ZnSeF6·6H2O crystal are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion in tetragonally elongated octahedra. In the calculated formulas, the contributions to the EPR parameters from ligand orbital and spin–orbit coupling are included on the basis of the cluster approach in view of moderate covalency of the studied systems, the required crystal-field parameters are estimated from the superposition model, which enables correlation of the crystal-field parameters and hence the EPR parameters with the tetragonal distortion of the studied [Cu(H2O)6]2+ cluster. According to the calculations, the ligand octahedra around Cu2+ are suggested to suffer relative elongation τ (≈ 0.085 Å) along the [001] (or C4) axis for the tetragonal Cu2+ centers in ZnSeF6·6H2O crystal, due to the Jahn–Teller effect. The results are discussed.

Studies on the spin Hamiltonian parameters and the defect structures for the two interstitial tetragonal Cu2+ centers in the ZnO nanocrystals

The spin Hamiltonian parameters (g factors and hyperfine structure constants) are theoretically studied for the two interstitial Cu2+ centers I and II in the ZnO nanocrystals from the perturbation formulas of these parameters for a tetragonally elongated octahedral 3d9 cluster. The ligand spin–orbit coupling and orbital contributions are involved from the cluster approach in view of moderate covalency. The two Cu2+ centers demonstrate slight tetragonal elongation distortions (denoted by the relative elongation parameters ρ≈3.4% and 3.2% for centers I and II, respectively) due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters and three optical transition bands are in good agreement with the observed values. The differences in EPR and optical spectra for both centers can be ascribed to the weaker covalency (higher N), stronger tetragonal elongation distortion (larger ρ) and slightly larger core polarization constant κ as well as almost identical crystal-fields (nearly the same Dq) for center I as compared to center II. The above delicate discrepancies in spectroscopic behaviors may be attributed to the tiny differences in local structures and electronic states for Cu2+ at dissimilar parts of the ZnO nanocrystals.

Investigation on the EPR parameters and local structure for the Cu2+ center in ZnAl2O4 spinel

High-order perturbation formulas for a 3d9 ion in tetragonally elongated octahedral was applied to calculate the electron paramagnetic resonance (EPR) parameters (g factors g||, g⊥ and hyperfine structure constants A||, A⊥) of the tetragonal Cu2+ center in ZnAl2O4 spinel at the temperature T=4.2 K. In the calculations, the required crystal-field parameters are estimated from the superposition model which enables correlation of the crystal-field parameters and hence the EPR parameters with the tetragonal distortion (characterized by relative tetragonal elongation ratio ρ along the C4 axis due to the Jahn–Teller effect) of [CuO6]10− cluster, and the ligand orbital and spin–orbit coupling contributions are included on the basis of the cluster approach in view of moderate covalency for the studied systems. The calculated results show good agreement with the experimental data.

Theoretical studies of the spin Hamiltonian parameters and local structures for the tetragonal Cu2+ and Ni3+ centers in Mg2TiO4

The local structures of the two impurity Cu2+ (and Ni3+) centers with low spin (S=1/2) in Mg2TiO4 are theoretically studied by using the perturbation formulas of the spin Hamiltonian parameters for 3d9 (and 3d7) ions in tetragonally elongated octahedra. In these formulas, the tetragonal field parameters are quantitatively determined using the superposition model and the local structures of the impurity Cu2+ (and Ni3+) centers, and the ligand orbital and spin–orbit coupling contributions are included on the basis of the cluster approach in view of moderate covalency for the studied systems. The [CuO6]10− and [NiO6]9−clusters on the substitutional Mg2+ site are found to suffer relative elongations by about ΔZCu (≈3.2%) and ΔZNi (≈0.7%) for the impurity Cu2+ and Ni3+ centers, respectively, along the C4 axis due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters based on the above Jahn–Teller elongations show good agreement with the observed values. The results are discussed.

Effect of Bi2O3 Addition on Electron Paramagnetic Resonance, Optical Absorption, and Conductivity in Vanadyl-Doped Li2O−K2O−Bi2O3−B2O3 Glasses

Glasses with composition 15Li(2)O-15K(2)O-xBi(2)O(3)-(65 - x)-B(2)O(3)/5V(2)O(5) (3 ≤ x ≤ 15) have been prepared by the conventional melt quench technique. The electron paramagnetic resonance spectra of VO(2+) in these glasses have been recorded in the X-band frequency (≈9.3 GHz) at room temperature. The spin Hamiltonian parameters and covalency rates were evaluated. It was found that the V(4+) ions exist as vanadyl (VO(2+)) ions and are in an octahedral coordination with a tetragonal compression. The covalency rates (1 - α(2)) and (1 - γ(2)) indicate moderate covalency for the σ- and π-bonds. It was observed that the spin-Hamiltonian parameters depend slightly on the relative concentration of Bi(2)O(3). The optical properties of this glass system are studied from the optical absorption spectra recorded in the wavelength range 200-800 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The values of optical band gap for indirect allowed transitions have been determined using available theories. The direct current electrical conductivity, σ, has been measured in the temperature range 373-573 K. The conductivity decreases with the increase in Bi(2)O(3) concentration. This has been discussed in terms of the decrease in the number of mobile ions and their mobility. An attempt is made to correlate the EPR, optical, and electrical results and to find the effect of Bi(2)O(3) content on these parameters.

Theoretical study of the electron paramagnetic resonance parameters and local structure for the tetragonal Ir2+ centre in NaCl

The electron paramagnetic resonance (EPR) parameters (the g factors, hyperfine structure constants and the superhyperfine parameters) for the tetragonal Ir(2+) centre in NaCl are theoretically investigated from the perturbation formulas of these parameters for a 5d(7) ion in tetragonally elongated octahedra. This impurity centre is attributed to the substitutional [IrCl(6)](4-) cluster on host Na(+) site, associated with the 4% relative elongation along the C (4)-axis due to the Jahn-Teller effect. Despite the ionicity of host NaCl, the [IrCl(6)](4-) cluster still exhibits moderate covalency and then the ligand orbital and spin-orbit coupling contributions should be taken into account. In addition, the theoretical EPR parameters based on the Jahn-Teller elongation show good agreement with the observed values.

EPR AND DSC STUDIES ON MgO–R2O–B2O3–V2O5 ALKALI GLASSES

EPR and DSC studies were carried out on MgO – R 2 O – B 2 O 3– V 2 O 5 glasses. The electron paramagnetic resonance spectra of V 4+ ion in the ternary glass system x MgO –(30-x) R 2 O –70 B 2 O 3 ( R = Li, Na, K ) were recorded at X-band frequencies at 303 K. The V 4+ ions in the present glass system exists as VO 2+ ions in octahedral coordination with tetragonal compression. The site symmetry of vanadyl is C 4v and the ground state of 3d1 ion is dxy. Tetragonality (Δg‖/Δg⊥) of V 4+ ion sites show a nonlinear variation with MgO content in the glasses. The covalency rates indicate moderate covalency for σ and π-bonds. The optical basicity values were also reported. Glass transition temperatures (Tg) of the glasses were found to increase linearly with increasing MgO content.

Strontium Tetraborate Glasses Doped with Transition Metal Ions: EPR and Optical Absorption Study

Electron paramagnetic resonance (EPR) and optical studies have been carried out on Cu(II)-, VO(II)- and Cr(III)-doped strontium tetraborate glasses to understand the distortion and substitution of these ions. The EPR results of Cu(II) glass indicate that g∥ > g⊥, typical for the tetragonally elongated octahedral site of the Cu(II) impurity. The evaluated covalency parameter 0.788 suggests a moderate covalency for the bonding. By correlating EPR and optical results, the in-plane π-bonding β12 is evaluated as 0.715. In the vanadium-doped glasses, the distortion must be a tetragonally elongated octahedron, similar to Cu(II). However, the EPR studies show that g⊥ > g∥ indicating the tetragonally compressed octahedral site for the ion. The site symmetry is C4V. Supported by the optical absorption, evaluated parameters propose a moderate covalency. The EPR and optical results for Cr(III) glass indicate the distorted octahedral site symmetry in the host lattice. These results further suggest that the bonding between Cr(III) and the ligands is covalent.

Transition metal quinone–thiosemicarbazone complexes 1: Evaluation of EPR covalency parameters and redox properties of pseudo-square-planar copper(II)–naphthoquinone thiosemicarbazones

EPR, optical and redox studies have been carried out on six copper(II) complexes of naphthoquinone–thiosemicarbazones having [CuLCl] stoichiometry to elucidate the electronic structure, nature of metal–ligand bonding and electrochemical features. EPR spectra, simulated with an axial spin-Hamiltonian, exhibit a four-line pattern with nitrogen super-hyperfine couplings originating from imine/hydrazinic nitrogen atoms. These planar complexes possess a significant amount of tetrahedral distortion leading to a pseudo-square-planar geometry as evidenced from EPR and optical properties. The evaluation of covalency parameters suggest that the unpaired electron, present in the d x 2 - y 2 orbital, spends about 42–45% of its time on the nitrogen donor site of the coordinated thiosemicarbazones, reflecting the π-acceptor property of the sulfur center as well as the charge accumulating character of the quinone molecules. The presence of a strong π-interaction leads to extensive delocalization thorough the entire chelate rings formed by the tridentate ligands. Moderate covalency is observed in σ-bonding while in plane π-bonding possess appreciable covalent character. IR spectral data indicate a tridentate ONS donor set for these ligands. A quasi-reversible Cu(II)/Cu(I) redox couple is observed at relatively higher potential (Δ E p = −0.45 to −0.66 V) as a consequence of structural reorganizations, while these complexes exhibit lower redox potentials for the Cu(II)/Cu(III) oxidation process. There exists a linear relationship between the degree of tetrahedral distortion f( α) with spectroscopic and redox parameters.

Influence of Host Lattice in Interstital Dopant Sites: EPR Studies on Cu (II) Doped Sarcosine Cadmium Bromide Single Crystals

EPR study of Cu(II) doped in sarcosine cadmium bromide single crystals are carried out at room temperature. The impurity ions occupy the interstitial position in this crystal lattice. Crystalline field around the Cu(II) ion in this low symmetry crystal is rhombic. The transitions arise from a single paramagnetic site with g xx = 2.1082, g yy =2.0005, g zz = 2.2071, and A xx = -64 x 10 -4 cm -1 , A yy = -23 × 10 -4 cm -1 , A zz = -185x10 -4 cm -1 . The ground state is an admixture of d x2 -y2 and d z 2 states. The observed molecular orbital coefficient value α 2 = 0.85 reveals a moderate covalency of the σ bonding and β 2 0.967 indicates the weak π bonding. A strong interaction between Cu(II) and nitrogen ligands is found to exist.

Electron paramagnetic resonance spectra of VO 2+ ions in NaI–Na 2O–K 2O–B 2O 3 mixed alkali glasses

The electron paramagnetic resonance (EPR) spectra of VO 2+ ions in the 15 NaI–(15 − x)Na 2O– xK 2O–B 2O 3 mixed alkali glass system were analysed at X-band frequencies at 310 K. The spin-Hamiltonian parameters and covalency rates were evaluated. It was found that the V 4+ ions exist as vanadyl (VO 2+) ions and are in an octahedral coordination with a tetragonal compression. The covalency rates (1 − α 2) and (1 − υ 2) indicate moderate covalency for the σ- and the π-bonds. It was observed that the spin-Hamiltonian parameters depend slightly on the relative alkali ion concentrations of K and Na.

Nephelauxetic series of in-plane ligands for square-pyramidal nitridochromium(V) complexes as studied by means of ESR spectroscopy

The crystal structure of the nitridochromium(V) complex [CrN(salophen)]·MeCN [H 2salophen= N, N′-bis(salicylidene)- o-phenylenediamine] has been determined. The coordination geometry of this complex, similar to those of all the nitridochromium(V) complexes investigated so far, is square pyramidal with the nitride ion occupying the apex. The ESR parameters of the nitridochromium(V) complexes isolated so far were compared with one another to show an enormous expansion of the 3d electron cloud due to the strong CrN triple bonding. Moderate covalency in the in-plane bonds was also concluded. The nephelauxetic series of three types of the basal ligands is deduced to be salen≈salophen>bpb≈mebpb≈dmebpb>oep≈ttp (in nephlauxetic ratio) for the high-oxidation nitridochromium(V), H 2salen= N, N′-bis(salicylidene)ethane-1,2-diamine, H 2bpb=1,2-bis(pyridine-2-carboxamido)benzene, H 2mebpb=1,2-bis(pyridine-2-carboxamido)-4-methylbenzene, H 2dmbpb=1,2-bis(pyridine-2-carboxamido-4,5-dimethylbenzene, H 2oep=2,3,7,8,12,13,17,18-octaethylporphyrin and H 2ttp=5,10,15,20-tetra- p-tolylporphyrin.

The Jahn-Teller instability criterion for resonance impurity states

The possibility of Jahn-Teller distortions for the isolated impurity (or the defect) under the conditions of the virtual bound state (the impurity levels are degenerate in energy with the band) is considered. It is shown that the vibronic instability of the initial defect state is responsible for the diamagnetism of the defect and the appearance of the U −-center. In accordance with the obtained criterion the small density of states in the conduction band and moderate covalency of the defect promote this.

Electron Paramagnetic Resonance Study of Copper(II) Ions in Single Crystals of Thallium Magnesium Sulphate Hexahydrate

AbstractEPR investigations of the copper(II) ion is carried out in its paramagnetically dilute form in thallium magnesium sulphate hexahydrate (Tl2Mg(SO4)2 · 6 H2O) at 300 and 77 K at X‐band. Cu2+ ions are found to substitute for divalent cations exhibiting two symmetry related equivalent complexes. The g‐ and A‐tensors are found to be axially symmetric upto liquid nitrogen temperature (LNT) and are temperature dependent. The ground state is suggested to be mainly a d orbital. The molecular orbital coefficients are evaluated and indicate a moderate covalency of Cu2+ ion.