Spectroscopic fingerprints of valence and spin states in manganese oxides and fluorides

We performed a systematic study of soft X-ray absorption spectroscopy in various manganese oxides and fluorides. Both Mn L-edges and ligand (O and F) K-edges are presented and compared with each other. Despite the distinct crystal structure and covalent/ionic nature in different systems, the Mn-L spectra fingerprint the Mn valence and spin states through spectral lineshape and energy position consistently and evidently. The clear O- and F-K pre-edge features in our high resolution spectra enable a quantitative definition of the molecular orbital diagram with different Mn valence. In addition, while the binding energy difference of the O-1s core electrons leads to a small shift of the O-K leading edges between trivalent and quadrivalent manganese oxides, a significant edge shift, with an order of magnitude larger in energy, was found between divalent and trivalent compounds, which is attributed to the spin exchange stabilization of half-filled 3d system. This shift is much enhanced in the ionic fluoride system. This work provides the spectroscopic foundation for further studies of complicated Mn compounds.     

The electronic structures of the core and valence ion states of metal oxides

SCF-Xα SW MO calculations on metal core ion hole states and X-ray emission (XES) and X-ray photoelectron (XPS) transition states of the non- transition metal oxidic clusters MgO₆^10?, AlO₄^5? and SiO₄^4? show relative valence orbital energies to be virtually unaffected by the creation of valence orbital or metal core orbital holes. Accordingly, valence orbital energies derived from XPS and XES are directly comparable and may be correlated to generate empirical MO diagrams. In addition, charge relaxation about the metal core hole is small and valence orbital compositions are little changed in the core hole state. On the other hand, for the transition metal oxidic clusters FeO₆^10?, CrO₆^9? and TiO₆^8? relative valence orbital energies are sharply changed by a metal core orbital or crystal field orbital hole, the energy lowering of an orbital increasing with its degree of metal character. Consequently O 2p nonbonding → M 3d-O 2p antibonding (crystal field) energies are reduced, while M 3d bonding → O 2p nonbonding and M 3d-O 2p antibonding → M 4s,p-O 2p antibonding (conduction band) energies increase. Charge relaxation about the core hole is virtually complete in the transition metal oxides and substantial changes are observed in the composition of those valence orbitals with appreciable M 3d character. This change in composition is greater for e _g than for t_2g orbitals and increases as the separation of the e_g crystal field (CF) orbitals and the O 2p nonbonding orbital set decreases. Based on the hole state MO diagrams the higher energy XPS satellite in TiO₂ (at about 13 eV) is assigned to a valence → conduction band transition. The UV PES satellites at 8.2 eV in Cr₂O₃ and 9.3 eV in FeO are tentatively assigned to similar transitions to conduction band orbitals, although the closeness in energy of the crystal field and O 2p nonbonding orbitals in the valence orbital hole state prevents a definite assignment on energy criteria alone. However the calculations do clearly show that charge transfer transitions of the e_g bonding → e_g crystal field orbital type would generally occur at lower energy than is consistent with observed satellite structure.A core electron hole has little effect upon relative orbital energies and is only slightly neutralized by valence electron redistribution for MgO and SiO₂. For the transition metal oxides a core hole lowers the relative energies of M3d containing orbitals by large amounts, reducing O → M charge transfer and increasing M 3d crystal field → conduction band energies. Large and sometimes overcomplete neutralization of the core hole is observed, increasing from CrO₆^9? to FeO₆^10? to TiO₆^8?. as the O → M charge transfer energy declines.High energy XPS satellites in TiO₂ may be assigned to O 2p nonbonding → conduction band transitions while lower energy UV PES satellites in FeO and Cr₂O₃ arise from crystal field or O 2p nonbonding → conduction band excitations. Our “shake-up” assignment for FeO₆^10?, CrO₆^9? and TiO₆^8? are less than definitive because no procedure has yet been developed to calculate “shake-up” intensities resulting from transitions of the type described. However the results do allow a critical evaluation of earlier qualitative predictions of core and valence hole effects. First, we find that the comparison of hole or valence state ionic systems with equilibrium distance systems of higher nuclear and/or cation charge (e.g. the comparison of the FeO₆^10? Fe 2p core hole state to Co₃O₄) is dangerous. For example, larger MO distances in the ion states substantially reduce crystal field splittings. Second, core and CF orbital holes sharply reduce O → M charge transfer energies, giving 2e_g → 3e_g energy separations which are generally too small to match observed satellite energies. Third, highest occupied CF-conduction band energies are only about 4–5 eV in the ground states, but increase to about 7–11 eV in the core and valence hole states of the transition metal oxides studied. The energetic arguments presented thus support the idea of CF and/or O 2p nonbonding → conduction band excitations as assignments for “shake-up” satellites, at least in oxides of metals near the beginning of the transition series.     

Adsorption and valence electronic states of nitric oxide on metal surfaces

Among fundamental diatomic molecules, the adsorption of carbon monoxide (CO) and nitric oxide (NO) on metal surfaces has been a subject of intensive research in the surface science community, partly owing to its relevance to heterogeneous catalysis used for environmental control. Compared to the rather well-defined adsorption mechanism of CO, that of NO is less understood because the adsorption results in much more complex reactions. The complexity is ascribed to the open-shell structure of valence electrons, making the molecule readily interact with the metal surface itself as well as with co-adsorbed molecules. Furthermore, the interaction crucially depends on the local structure of the surface. Therefore, to elucidate the interaction at the molecular scale, it is essential to study the valence state as well as the bonding geometry for individual NO molecules placed in a well-defined environment on the surface. Scanning tunneling microscopy (STM) is suitable for this purpose. In this review, we summarize the knowledge about the interaction of NO with metal surfaces, mainly focused on the valence electronic states, followed by recent studies using STM and atomic force microscopy (AFM) at the level of individual molecules.     

Nd3+离子掺杂氧化物玻璃的光谱性质

高温熔融法制备了69.5B2O3-20Li2O-(10-χ)CaO-χPbO-0.5Nd2O3(χ=1,3,5,7,mol%)和68.5Li2O-(29-φ)CaO-φAl2O3-2La2O3-0.5Nd2O3(φ=3,7,10,15,mol%)玻璃,测量了玻璃样品的吸收和发射光谱(800nm激光二极管激发),从吸收光谱出发,应用Judd-Ofelt理论获得了Nd3+光学跃起的强度参数。根据强度参数和发射光谱,计算了Nd3+离子4F3/2→4I11/2跃迁的荧光发射截面σe。结果显示,在B2O3-Li2O-CaO-PbO-Nd2O3系统中随着CaO的减少和PbO的增加,Nd3+离子的强度参数Ω2增大,表明样品的对称性降低;强度参数Ω6也增大,说明玻璃样品中Nd-O键的共价性和键强降低;同时,Nd3+离子4F3/2→4I11/2跃迁的荧光发射截面σe的大小和荧光强度都减小。在Li2O-CaO-Al2O3-La2O3-Nd2O3系统中,随着CaO的减少和Al2O3的增加,Nd3+离子的强度参数Ω2减小,说明样品的对称性增加;强度参数Ω6减小,说明玻璃样品中Nd-O键的共价性和键强增强;同时,Nd3+离子4F3/2→4I11/2跃迁的荧光发射截面的大小和荧光强度也都减小。     

The electronic states of silicon monofluoride: Low-lying valence states

Hartree-Fock and configuration interaction calculations are reported for the lowest valence state of the SiF molecule. Deduced spectroscopic constants are in good agreement with the experimental values. The dipole moment calculated for SiF is consistent with a charge distribution corresponding to a Si⁺F^? configuration.     

The electronic states of carbon monofluoride: Low-lying valence states

Multiconfiguration Hartree-Fock calculations are reported on the low-lying valence states, the X²Π, ⁴Σ^?, B²Δ, and ²Σ^± states, of carbon monofluoride. The wavefunctions describe dissociation of the molecule to the correct atomic limits and take account of the atomic 2s-2p near-degeneracy effect. For the ground state the calculations give (with experimental values in parentheses): a bond length of 1.286 Å (1.2667 Å), a fundamental frequency of 1292 cm^?1 (1308 cm¹), and a dissociation energy of 3.93 eV (5.5 ± 0.1 eV). A ⁴Σ^? state arising from the $\text{C}\text{(}{}^3\text{P) +}\text{F}\text{(}{}^2\text{P)}$ manifold is calculated to lie just 2.66 eV above the ground state. The B²Δ state, calculated adiabatic excitation energy 6.59 eV (6.12 eV), is found to dissociate to $\text{C}\text{(}{}^1\text{D) +}\text{F}\text{(}{}^2\text{P)}$ via a potential maximum. Calculations are also reported on a repulsive ²Δ state arising from ground state atoms.     

Fluorophosphate glasses containing manganese

New fluorophosphate glasses based on MnF₂, NaPO₃ and MF_n (M=Zn²⁺, Sr²⁺, Mg²⁺, Ba²⁺, Li⁺, Na⁺ and K⁺) have been synthesised and characterized. Large vitreous areas were observed. Samples of 4 mm in thickness have been obtained. These glasses are easy to prepare and stable in ambient air. Depending on the composition and the nature of the M cation, glass transition temperature, T_g, lies between 230 and 314 °C, crystallisation temperature, T_x is between 320 and 475 °C. These glasses are pink coloured, and infrared transmission extends up to 4.5 μm with extrinsic OH⁻ absorption band at 3200 cm⁻¹ and other bands around 2200 and 1600 cm⁻¹ that relate to PO₄ tetrahedron vibration. Other physical properties including density, microhardness, Young modulus, thermal expansion and refractive index were investigated and correlated to composition.     

Determination of manganese valence in complex LaMn perovskites

We describe a chemical method for determining the manganese concentration and average oxidation state in mixed-valence compounds, and in particular in complex LaMn perovskites. The method is based on two independent iodometric titrations, with amperometric dead-stop end-point detection. The oxygen stoichiometry and the molecular weight of the compounds can also be evaluated if secondary phases or multi-valent elements different from manganese are not present. The method was applied to the analysis of LaMn perovskites with the same cation stoichiometry but synthesized under different conditions. Significant variations of the Mn average valence were measured, which corresponded to different physical and structural properties of the materials.     

Elucidation of the crystallization kinetics for sodium-alumino-silicate glasses containing different amounts of manganese oxide

The glass samples [40SiO₂?+?5Al₂O₃?+?{55???x}Na₂O?+?xMnO₂] where x?=?0.05, 0.2, 0.4, 0.6, 0.8, and 1?mol% MnO₂ before and after being heat treated were subjected to X-ray diffraction. The diffraction lines provided clear evidence of the nucleation and growth, which are characteristic of sodium silicate phase. Crystallization studies were conducted using differential thermal analysis. Crystallization peak temperatures were identified and the transformed fractions were determined. Both the rate of growth, K ₀, and the activation energy, E, depend on the influence of manganese ions in the glass network as a modifier or as a former and the manganese content. The values of the Avrami parameter, n, were calculated using two methods and were in excellent agreement. The process of nucleation and growth rate depends on the manganese content.     

The valence states of NO+

Spectroscopic constants for the eight lowest electronic states of the NO⁺ ion are tabulated. These constants result from reanalysis of previously reported optical and photoelectron spectra and interpolation from corresponding states of the isoelectronic molecules CO and N₂. Similar spin-orbit perturbations of the A¹Π states of NO⁺ and CO are compared. An interpretation is given of previously reported emission from a beam of long-lived states of NO⁺. The intensities of ionizing transitions from NO X²Π (v = 0) observed in photoelectron spectra are compared with calculated Franck-Condon factors.     

Mossbauer investigations of corrosion environment influence on Fe valence states in oxide films of zirconium alloys

Mössbauer investigations about iron atom redistribution in oxide films of zirconium alloys subjected to corrosion at 500°C in pure oxygen and water pair have been analysed. The alloys were also subjected to autoclave conditions at a pressure of 10.0 MPa and autoclave conditions at 350°C and at a pressure of 16.8 MPa, using distilled water and water with additives of lithium and fluorine. It is shown that, depending on the corrosion environment, various compounds of iron, such as α-Fe₂O₃, Fe₃O₄, and FeO, as solid solutions of iron in ZrO₂ are formed in oxide films.     

Tunneling states in ferromagnetic glasses

We study the interaction between structural defects, represented by two level systems, and spin waves in a ferromagnetic glass. The damping and energy shift of the spin wave due to this interaction have a “resonant” and a “relaxation” contribution in analogy with the case of phonons. Ferromagnetic resonance in amorphous ferromagnets can provide useful information on structural relaxation in these materials.     

Mean energies of hybridized valence states

A method of determining the relative signs of J _AB and J _BX in nuclear spin systems of the type AB₂X _q is described and applied to the AB₂X₃ proton spin system of 2-butyn-1-ol. Variation of the relative chemical shift v _A—v _B (through changes in temperature or in the composition of the solvent) causes certain pairs of ‘labile’ transitions in the X spectrum to coalesce on one side or other of the X spectrum. The information about the relative signs of the coupling constants is obtained by noting the two different critical values of v _A-v _B at which these coalescences occur. Since the method remains applicable even if the third coupling constant J _AX is vanishingly small, it is particularly useful in the determination of the signs of long-range spin coupling constants where the usual double resonance methods often break down. It is concluded that the long-range coupling J(H-C-C≡C-C-H) of 2-butyn-1-ol has a positive sign.     

Mixed valence states in cobalt iron cyanide

Cobalt iron cyanide with both Co and Fe in mixed valence states were prepared and characterized. In this mixed valence system the cobalt atom is found both as high spin Co(2+) and low spin Co(III) while iron always appears in low spin state to form two solid solutions: Co(2+)Co(III) hexacyanoferrates (II,III), and Co(2+)Co(III) hexacyanoferrate (II). Such solid solutions have the following formula units: (Co²⁺)_x(Co^III)_1−xK[(Fe^II)_1−x(Fe^III)_x(CN)₆]·H₂O and (Co²⁺)_1.5x(Co^III)_1−xK[Fe^II(CN)₆]·yH₂O (0?x?1, 1?y?14). Compounds within these two series were characterized from Infrared, Mössbauer, X-ray diffraction and thermo-gravimetric data, and magnetic measurements at low temperature. A model for their crystal structure is proposed and the structure for a representative composition refined from XRD powder patterns using the Rietveld method. A simple and reproducible procedure to prepare these solid solutions is provided. Within hexacyanoferrates, such mixed valence states system in both metal centres shows unique features, which are discussed from the obtained data.     

Localization and symmetry of valence states in GaS

The valence electron states of layered semiconductor GaS were studied by polarizedK X-ray emission bands. The polarized gallium and sulphurK bands were calculated and GaK₂-bands were measured by the two crystal spectrometer. In the calculations the self-consistent pseudo-potential method was applied. Thep _x ,p _y p _z -character and localisation of valence electron states were identified. Comparison with the results of controversial interpretation of photoemission and tight binding calculations were done and analysed in detail.We are indebted to J. Mikkelsen from Xerox Research Center in Palo Alto for kindly sending us the GaS single crystals.     

The stability of metallic glasses under ion irradiation

Abstract

Metallic glasses of Fe₈₀B₂₀ and Fe₈₀P₁₃C₇ prepared by rapid quenching have been implanted with 40 keV Ar⁺, P⁺ and B⁺ ions at doses up to 2.10¹⁶ ions cm^?2 at 273 K. The structure of the glasses was examined by X-ray diffraction. Crystallites of α-Fe and Fe₃B, present in F₈₀B₂₀ samples, either become highly defective or are amorphised. Implantation also produces structural changes in the amorphous Fe₈₀B₂₀ (a-Fe₈₀B₂₀) phase as revealed by changes in the width β of the principal diffraction halo. Both disordering and relaxation effects are observed, dependent on the initial degree of order within the a-Fe₈₀B₂₀ phase before irradiation. Implantation of F₈₀p₁₃C₇ results in an increase in the content of γ-Fe and Fe₃P crystallites, already present before irradiation, and in the nucleation of α-Fe.