Yong-Qiang Xu

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.

Studies of impurity occupation and local angular distortions for Cu2+ in CdS films at different concentrations

In order to clarify impurity occupation and local structures for Cu2+ in the CdS films, the g factors and hyperfine structure constants (HSCs) and their concentration dependences are theoretically studied from the perturbation calculations for both the cases of tetragonally compressed tetrahedral (TCT) and tetragonally elongated octahedral (TEO) 3d9 clusters. Despite similar g anisotropies g // > g ⊥ > 2, the calculated g factors and HSCs and their concentration dependences based on the TCT [CuS4]6− cluster show much better agreement with the experimental data than those based on the previously assigned TEO [CuS6]10− cluster. So, the g factors and HSCs can be ascribed to the substitutional TCT [CuS4]6− cluster on Cd2+ site, and the possibility of the previous assignment of interstitial TEO [CuS6]10− cluster may be tentatively excluded. In the TCT [CuS4]6− clusters, the local Cu2+–S2− bond angles θ related to C4 axis are found to be about 1.54° larger than the ideal value θ 0 (≈54.736°) of a regular tetrahedron owing to the Jahn–Teller effect. The magnitudes of g factors and HSCs demonstrate the overall increasing trends with increasing Cu2+ concentration, attributable to the decrease of covalency (increase of covalency factor N) and the increases of the angular deviation Δθ (=θ – θ 0) and the core polarization constant κ.

Studies on the local structures for the trigonal Ni3+ centers in cathode materials LiAlyCo1-yO2 (y = 0, 0.1, 0.5, and 0.8)

The local angular distortions Δθ are theoretically studied for the various Ni3+ centers in LiAlyCo1–yO2 at different Al concentrations (y = 0, 0.1, 0.5, and 0.8) based on the perturbation calculations of electron paramagnetic resonance g factors for a trigonally distorted octahedral 3d7 cluster with low spin (S = 1/2). Due to the Jahn–Teller effect, the [NiO6]9– clusters are found to experience the local angular distortions (Δθ ≈ 5°–9°) along the C3 axis. The variation trend of Δθ with y is in accordance with that of anisotropy (Δg = g|| − g⊥). As the substitutions can weaken bond strengths between transition metal and oxygen and the structural stability plays an important role in cathode performances, detailed investigations on the structural properties of the cathode materials LiAlyCo1–yO2 can be practically helpful to understand the performances of these materials. The oxy‐redox properties of LiAlyCo1–yO2 systems are comprehensible in the framework of Ni3+/Ni4+ couples, and the trigonally compressed octahedral [NiO6]9– clusters are applicable to the clarification of the electrochemical properties of lithium nickel oxide batteries. It appears that LiAl0.8Co0.2O2 with the largest Al concentration may correspond to the smallest distortion among the mixing systems.

The DFT Calculations of Structures and EPR Parameters for the Dinuclear Paddle-Wheel Copper(II) Complex {Cu2(μ2-O2CCH3)4}(OCNH2CH3) as Powder or Single Crystal

Abstract Density functional theory (DFT) calculations of the structures and the Cu2+ g factors (gx, gy and gz ) and hyperfine coupling tensor A (Ax , Ay and Az ) were performed for the paddle-wheel (PW)-type binuclear copper(II) complex {Cu2(μ2-O2CCH3)4}(OCNH2CH3) powder and single crystal. Calculations were carried out with the ORCA software using the functionals BHandHlyp, B3P86 and B3LYP with five different basis sets: 6-311g, 6-311g(d,p), VTZ, def-2 and def2-TZVP. Results were tested by the MPAD analysis to find the most suitable functional and basis sets. The electronic structure and covalency between copper and oxygen were investigated by the electron localisation function and the localised orbital locator as well as the Mayer bond order for the [CuO5] group. The optical spectra were theoretically calculated by the time-dependent DFT module and plotted by the Multiwfn program for the [CuO5] group and reasonably associated with the local structure in the vicinity of the central ion copper. In addition, the interactions between the OCNH2CH3, NH3 and H2O molecules and the uncoordinated PW copper(II) complex were studied, and the corresponding adsorption energies, the frequency shifts with respect to the free molecules and the changes of the Cu–Cu distances were calculated and compared with the relevant systems.

Studies on the g -factors of the copper(II)–oxygen compounds

The g factors for Cu2+ in meta-zeunerite (Cu(UO2)2(AsO4)2·3H2O), kroehnkite (Na2Cu(SO4)2·2H2O), copper benzoate (Cu(PhCO2)2·3H2O) and diaboleite (Pb2Cu(OH)4Cl2) of the tetragonal phase are uniformly treated by high order perturbation formulas for 3d9 ions in tetragonally elongated octahedra. The calculation results are in good agreement with the observed values and systematically analyzed in view of the local structures around Cu2+. The g anisotropies Δg (= g‖−g⊥) are largely ascribed to the local tetragonal elongations of the Cu2+ sites, characterized by the relative elongation ratios (R‖−R⊥)/R̅ ≈ 19%, 21%, 27% and 30% for metazeunertie, kroehnkite, copper benzoate and diaboletie, respectively. The anomalous valley (minimum) of relative g anisotropy for copper benzoate is attributed to the modification of the Cu2+ electronic states due to the phenyl ring. The ligand orbital contributions are found to be significant due to covalency, and should be taken into account. The present study would be helpful to the unified investigations of structures and properties of the copper oxygen compounds.

Theoretical studies of local structures and spin hamiltonian parameters for Cu2+ in alkali barium borate glasses

The local structures and spin Hamiltonian parameters are theoretically studied for Cu2+ in alkali barium borate glasses 20A2O ∙ 24.5BaO · 55B2O3 · 0.5CuO, where A = Li, Na and K by the quantitative calculations of these parameters for tetragonally elongated octahedral 3d9 clusters. The [CuO6]10‒ clusters are subject to the local relative tetragonal elongation ratios 7.8, 8.1 and 8.4% in Li, Na and K barium borate glasses, respectively, owing to the Jahn–Teller effect. The increasing (Li < Na < K) local relative elongation ratio and decreasing cubic field parameter and covalency factor are discussed in a consistent way.

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

Abstract 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.

Theory studies of the local lattice distortions and the EPR parameters for Cu2+, Mn2+ and Fe3+ centres in ZnWO4

The local lattice distortions and the electron paramagnetic resonance (EPR) parameters (g factors, hyperfine structure constants and zero-field splittings) for Cu2+, Mn2+ and Fe3+ in ZnWO4 are theoretically studied based on the perturbation calculations for rhombically elongated octahedral 3d9 and 3d5 complexes. The impurity centres on Zn2+ sites undergo the local elongations of 0.01, 0.002 and 0.013 Å along the C2 axis and the planar bond angle variations of 8.1°, 8.0° and 8.6° for Cu2+, Mn2+ and Fe3+, respectively, due to the Jahn–Teller effect and size and charge mismatch. In contrast to the host Zn2+ site with obvious axial elongation (∼0.31 Å) and perpendicular (angular) rhombic distortion, all the impurity centres demonstrate more regular octahedral due to the above local lattice distortions. The copper centre exhibits significant Jahn–Teller reductions for the spin-orbit coupling and orbital angular momentum interactions, characterised by the Jahn–Teller reduction factor J (≈0.29 ≪ 1). The calculated EPR parameters agree well with the experimental results. The local structures of the impurity centres are analysed in view of the corresponding lattice distortions.

Theoretical investigations of the g factors of orthorhombic Cu2+ site in oxycarbonate phase YBa2Cu2.95(CO3)0.35O6.6

The g factors of orthorhombic Cu2+(1) site in oxycarbonate phase YBa2Cu2.95(CO3)0.35O6.6 are theoretically investigated using the perturbation formulas of the g factors for an orthorhombically elongated 3d9 cluster. This pseudo-octahedral Cu2+ center is formed by two apical oxygen ligands and four coplanar oxygen ions belonging to the respective four neighboring carbonate groups. The [CuO6]10− cluster is found to suffer the relative axial elongation of about 0.04 A and the perpendicular bond length variation of about 0.12 A due to the Jahn–Teller effect. The theoretical g factors based on the local structural parameters show good agreement with the experimental data.