Charge-transfer transitions and optical spectra of vanadates

Specific features of the charge-transfer states and O2p → V3d transitions in the (VO₆)^9? octahedral complex are studied using the cluster approach. The reduced matrix elements of the electric-dipole transition operator are calculated for many-electron wave functions corresponding to the initial and final states of a charge-transfer transition. Using a parameterization of the results, the relative intensities of the allowed charge-transfer transitions are calculated disregarding the mixing of different configurations of the same symmetry. The Tanabe-Sugano theory is used with inclusion of this mixing to calculate the energies of many-electron charge-transfer transitions and their actual intensities. Modeling of the optical spectrum of LaVO₃ reveals a complicated charge-transfer transition band consisting of 81 lines. The main maxima of the band are in the range 6.3–7.3 eV. There are also additional maxima in the regions of ≈3 and ≈8–9 eV. The bandwidth is ≈10 eV. The results of model calculations are in agreement with experiments and demonstrate the weakness of the widely used assumption that the spectrum of charge-transfer transitions has a simple structure.     

Charge-transfer transitions and optical properties of cobaltites

Specific features of the charge-transfer states and transitions of the O 2p → Co 3d type in octahedral complexes (CoO₆)^9? and (CoO₆)^10? are considered in the cluster approach. The reduced matrix elements of the electric dipole transition operator are calculated on the many-electron wave functions of the complexes corresponding to the initial (high-or low-spin) state and the final state at a charge-transfer transition. The energies of the many-electron charge-transfer transitions and their intensities are calculated within the Tanabe-Sugano theory taking into account the mixing of different configurations of the same symmetry. Simulation of the optical spectrum of cobaltites showed the presence of a wide band consisting of many lines due to the charge-transfer transitions. The results of the simulation are in agreement with experiment and demonstrate the limited validity of the generally accepted concepts of a simple structure of the spectrum of charge-transfer transitions.     

绿宝石晶体中(CrO6)9－团簇局域极化现象的理论研究

基于配位场理论,建立d³组态离子在三角晶场中的完全能量矩阵,采用完全能量矩阵的对角化计算方法,研究了绿宝石晶体Cr³⁺：Be₃Al₂(SiO₃)₆的光谱和EPR谱,理论值与实验值符合得很好.通过分析绿宝石晶体中(CrO₆)^9-团簇的光谱和EPR谱,研究了配体在络合物中的极化现象.结果表明由于周围配位环境的影响,绿宝石晶体中(CrO< 关键词： 3+：Be₃Al₂(SiO₃)₆体系')" href="#">Cr³⁺：Be₃Al₂(SiO₃)₆体系 光谱 EPR谱 局域结构极化     

Optical spectrum, local lattice structure and EPR g factors for Cr impurity ions in MgTiO3 and LiTaO3

On the basis of the 120×120 complete energy matrices for a d³ configuration ion in a trigonal ligand field, for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃, the local structures and EPR g factors of the octahedral (CrO₆)⁹⁻ clusters have been studied, respectively. By simulating the calculated optical spectra and the EPR spectra data to the experimental results, local structure parameters are obtained. The calculated results show that although the local lattice structures around the M (M=Mg²⁺, Ta⁵⁺) ions are obviously different, after Cr³⁺ replacing the M, the local lattice structures around the Cr³⁺ ions are quite similar and close to those of the Cr₂O₃. This may be ascribed to the fact that the octahedral Cr³⁺ center in MgTiO₃:Cr³⁺ and LiTaO₃:Cr³⁺ systems and that in Cr₂O₃ exhibit similar octahedral (CrO₆)⁹⁻ clusters. Moreover, the corresponding theoretical values of the optical spectra have been reported. It is also found that the orbital reduction factor k is very important to understand the EPR g factors for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃.     

Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr₂O₃ before and after surface reduction by 2 keV Ar⁺ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr₂O₃ exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar⁺ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr²⁺ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr₂O₃ surface exhibit features characteristic of both Cr⁰ and Cr₂O₃, indicating that Ar⁺ sputtering reduces Cr₂O₃. The fact that neither the surface- nor the bulk-plasmon features of Cr⁰ can be observed in the ELS spectra obtained from sputtered Cr₂O₃ while the loss features due to Cr⁰ interband transitions are clearly present indicates that Cr⁰ atoms form small clusters lacking a bulk metallic nature during Ar⁺ bombardment of Cr₂O₃.     

Crystal field analysis of energy level structure of the Cr2O3 antiferromagnet

A detailed analysis of the energy level structure of the six-fold coordinated Cr³⁺ ion in the chromium oxide Cr₂O₃ is performed using the exchange charge model of the crystal field theory. Parameters of the crystal field acting on the Cr³⁺ optical electrons are calculated from the crystal structure data for the [CrO₆]⁹⁻ impurity center. The energy levels obtained are compared with the experimental absorption spectra for the considered crystal; a good agreement with experimental data is demonstrated. One possible explanation for the ultraviolet p₁ absorption band is proposed based on the results of crystal field calculations.     

Nonradiative decay from 5d states of rare earths in crystals

Nonradiative decay from 4f^n?1 5d states was investigated for trivalent rare earths in Y₃Al₅O₁₂. The rates of both 5d→4f and 5d→5d transitions were determined from measurements of the lifetimes and intensities of 5d fluorescence from Ce³⁺ and Pr³⁺. Because of the stronger ion-lattice coupling, nonradiative decay rates for transitions involving 5d states are much faster than those between 4f states. Decay rates are dependent upon the temperature and the energy gap to the next-lower level. The temperature dependences of the 5d fluorescence lifetimes from 77 to 700°K are reported.     

六配位中铬离子不同价态的MS-Xα计算

本文采用自旋极化的MS-X_α方法,以铬离子与氧六配位形成的(CrO₆)铬离子为模型,计算铬离子三种价态(Cr²⁺,Cr³⁺,Cr⁴⁺的电子结构,在D_4k群下给出单电子轨道本征值和本征函数、能级分布和光学跃迁,对计算结果作了详细的讨论。 关键词：     

A study of final state structure in the x-pe spectra of the rare earth oxides,Part III: Ionisation of a 4d electron

Metal 4d photoelectron spectra of the lanthanoid oxides Ln₂O₃ are reported. The 4d signals are complex, showing subsidiary structure due both to electrostatic interactions within the 4d⁹4?ⁿ configurations and to accompanying O 2p → Ln 4? charge-transfer excitations (shake-up). The effects of electrostatic coupling, including configuration interaction in the final state, are analysed in terms of a simple model.     

Electroreflectance of bound excitonic states in semiconductors

While first-derivative spectroscopy (thermoreflectance, piezoreflectance, wavelength derivative modulation) has a general validity, no matter whether one is studying interband or excitonic transitions (involving also bound states), things are more complex in the case of electroreflectance (ER). As a matter of fact, Aspnes and Rowe's third-derivative theory does not include bound excitonic states. Using a phenomenological approach one can see that only in the case of a strong mixing between d³ε_r/dE³ and d³ε₁/dE³ it is possible to observe a qualitative agreement between Δε and d³ε/dE³ in some particular cases where Wannier excitonic series gives a predominant contribution to the optical spectra.     

Study of mechanisms of formation of X-ray emission bands in crystals by the density functional method: The Mg <Emphasis Type="Italic">L</Emphasis><Subscript>2,3</Subscript> bands in metal and in MgO

Using the density functional method in the pseudopotential approximation, we calculate the probabilities of X-ray emission transitions and shapes of the L _2,3 X-ray emission bands of Mg in crystals of metallic magnesium and monoxide MgO. We have paid special attention to the study of mechanisms by which these bands were formed; therefore, along with the total intensities, we determine the partial s- and d-contributions. In addition, the intensities of the L _2,3 bands of Mg have been separated into contributions from direct (intra-atomic) transitions and charge-transfer crossover transitions. We show that an unexpectedly large contribution of partial d-transitions in magnesium oxide is caused by crossover transitions of electrons of valence states of ligands to the core 2p-level of Mg, i.e., charge-transfer transitions. In metallic magnesium, the contribution of these transitions proved to be insignificant. An appreciable contribution of crossover transitions has also been revealed in s-contributions of X-ray emission bands. To estimate the calculation accuracy, we have compared the shapes of theoretical spectra with the shapes of experimental L _2,3 X-ray emission bands of Mg.     

Optical investigation of charge-transfer transitions in spinel Co3O4

Optical transitions in normal-spinel Co₃O₄ have been identified by investigating the variation of its optical absorption spectrum with the replacement of Co by Zn. Three optical-transition structures were located at about 1.65, 2.4, and 2.8 eV from the measured dielectric function of Co₃O₄ by spectroscopic ellipsometry. The variation of the absorption structures with the Zn substitution (Zn_xCo_3−xO₄) can be explained in terms of charge-transfer transitions involving d states of Co ions. The 1.65 eV structure is assigned to a d-d charge-transfer transition between the t_2g states of octahedral Co³⁺ ion and t₂ states of tetrahedral Co²⁺ ion, t_2g(Co³⁺)→t₂(Co²⁺). The 2.4 and 2.8 eV structures are interpreted as due to charge-transfer transitions involving the p states of O²⁻ ion: p(O²⁻)→t₂(Co²⁺) for the 2.4 eV absorption and p(O²⁻)→e_g(Co³⁺) for the 2.8 eV absorption. The observed gradual reduction of the 1.65 and 2.4 eV absorption strength with the increase of the Zn composition for Zn_xCo_3−xO₄ can be explained in terms of the substitution of the tetrahedral Co²⁺ sites by Zn²⁺ ions. The crystal-field splitting Δ_Oh between the e_g and the t_2g states of the octahedral Co³⁺ ion is estimated to be 2 eV.     

Research on the defect structure and g factors for the tetragonal (CrO4)3− impurity center in ZrSiO4: Cr5+ crystal

ABSTRACT

The EPR g factors g// and g⊥ for the tetragonal (CrO₄)^3? impurity center in ZrSiO4: Cr⁵⁺ crystal are studied from the high-order perturbation formulas based on the two-mechanism (the crystal field and charge-transfer mechanisms) model. The studies indicate that differing from the tetragonally-elongated host (SiO4)^4- tetrahedron, the dominant defect structure of the substitutional (CrO₄)^3? tetrahedron is tetragonally- compressed with the ground state |d_z² due to the Jahn-Teller distortion. Furthermore, the agreement of g factors between calculation and experiment requires a small admixture of the first excited state |d_x²_?y² to the ground state |d_z² due to the vibrational motion of ligands, which leads a compressed (CrO₄)^3? tetrahedron to become a twinkling elongated one, These results are discussed.     

Luminescence of impurity 3d and 4f metal ions in different crystalline forms of Al2O3

The ² E-⁴ A ₂ luminescence spectra of Cr³⁺ ions in Al₂O₃ are investigated in the course of transitions between the structural forms γ-δ-θ-α. The spectral lines observed are assigned to Cr³⁺ ions in these structural forms, which are identified by an X-ray powder diffraction analysis. The lifetimes of the Cr³⁺ excited states in transient forms of Al₂O₃ are measured. Investigations of the luminescence spectra of Al₂O₃: Eu³⁺ demonstrate that the Eu³⁺ ions can form regular centers only in α-Al₂O₃ and, unlike the Cr³⁺ ions, give no rise to similar centers in moderately ordered θ-Al₂O₃.     

Charge-transfer states and optical absorption

The intensities of the Laporte-forbidden transitions in complexes like RhCl₆ ^3-, IrBr₆ ^2-, etc. are connected to the energies of the initial, final and charge-transfer states by the approximate relation, intensity α(E_f - E_i ) (E_C - E_f )^-2. This is due to the mixing in of the odd, bonding states into the even, anti-bonding ones through the vibrations of the complex. An analysis of the electronic states of these strongly covalent complexes and of the allowed, charge-transfer transitions is also presented. From this it is concluded that the dipole moment of the transitions is largely due to the presence of the charges in the anti-bonding states on the ligand due to covalent bonding.     

Electronic and vibrational spectra of CrO2−4 in cesium halide lattices

The electronic spectra of CrO^2?₄ ions embedded in cesium halide lattices show two well defined bands due to dipole allowed transitions along with some weak bands due to forbidden transitions. All the above bands are interpreted in terms of transitions within molecu ar orbital levels using the Ballhausen and Liehr scheme. It is observed that superposed on the electronic transitions there is a vibrational fine structure which arises due to the coupling between the electronic transitions and the totally symmetric stretching frequency of the ion. IR absorption spectrum of Analar grade CsI :CrO^2?₄ shows seven lines, whereas Specpure CsI :CrO^2?₄ shows five lines. The above spectra are interpreted on the basis of different local symmetries of CrO^2?₄ ions in the lattice.     

Theoretical investigation of optical spectra and covalent effect of Cr in Y2Ti2O7 and Y2Sn2O7

In this work, the complete matrix of optical spectral levels in trigonal symmetry of 3d² (3d⁸) ions are established on basis of strong field coupling mechanism by using two spin–orbit coupling parameters model. The contribution of the spin–orbit coupling of ligand to the optical spectra has been included in these formulas. As an application, the optical spectra of Cr⁴⁺ in Y₂Ti₂O₇ and Y₂Sn₂O₇ have been studied by the complete diagonalization (energy matrix) method. The covalent effect has been studied and the difficulty about Dq parameter in explanation of optical spectra of Cr-doped Y₂Ti₂O₇ and Y₂Sn₂O₇ is removed. The theoretical results are in good agreement with observed data.