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.     

Lattice relaxation and charge-transfer optical transitions due to self-trapped holes in nonstoichiometric LaMnO3 crystal

We explore the role of electronic and ionic polarization energies in the physics of “colossal” magnetoresistive (CMR) materials. We use the Mott-Littleton approach to evaluate polarization energies in the LaMnO₃ lattice associated with holes localized on both the Mn³⁺ cation and the O^2?anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O site is estimated at 2.4 eV, which is appreciably greater than that of 0.8 eV for a hole localized at the Mn site, indicating a strong electron-phonon interaction in the former case. The ionic relaxation around the localized holes differs for anion and cation holes. The relaxation associated with Mn⁴⁺ is approximately isotropic, whereas ionic displacements around O^? holes show axial symmetry with the axis directed towards the apical oxygens. Using the Born-Haber cycle, we examine thermal and optical energies of the hole formation associated with the electron ionization from Mn³⁺, O^2?, and La³⁺ions in the LaMnO₃ lattice. For these calculations, we derive a phenomenological value for the second electron affinity of oxygen in the LaMnO₃ lattice by matching the optical energies of the La⁴⁺ and O^? hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn⁴⁺ state in the LaMnO₃ host lattice, but the energy of a hole in the O^? state is only higher by a small amount, 0.75 eV, suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn⁴⁺ and O^? in the LaMnO₃ lattice. The reasonable agreement of our predicted energies, linewidths, and oscillator strengths with experimental data leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with the O(2p)-Mn(3d) transition of a charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn⁴⁺ and/or O^? self-trapped holes in the nonstoichiometric LaMnO₃ compound.     

Electronic structure of dichalcogenide and dipnictide anions

The Self-Consistent-Field Xα Scattered Wave Cluster MO method is used to calculate molecular orbital energy level diagrams for O₂, O₂^?, O₂^2?, S₂, S₂^2?, Se₂^2?, P₂^4?, As₂^4? and AsS^3?. Calculated energies are in good agreement with those measured by photoemission spectroscopy for O₂, S₂, S₂^2? in ZrS₃, Se₂^2? in ZrSe₃ and P₂^4? in TiP₂. For O₂^? the calculated energy of the π_u → π_g transition agrees fairly well with that of the UV absorption observed in LiO₂. The predicted photoemission spectrum of O₂^2? is consistent with a preliminary study on BaO₂. Calculated valence region widths in peroxides are found to be significantly greater than the calculated oxygen atomic valence _s-p separation while valence widths for the other anions closely match atomic valence s-p splittings. This effect arises from the instability of the peroxide antibonding orbitals which correlates with the absence of dsun peroxides and Superoxides. Calculations on a Cin2v geometry FeO₆su9? cluster show that the difference in energy of O2p nonbonding and Fe3d crystal field type orbitals drops sharply as an O-O edge is contracted, providing a mechanism for Fe³⁺ → Fe²⁺ reduction at high pressure.     

Collective electron excitations in 3D graphite clusters

Group-theoretical analysis and subsequent quantum-chemical calculations based on the molecular orbital method applied to a cyclic model of 3D semimetallic graphite lead to a multiplet of spectroscopic combinations of Slater determinants. The transition energies ΔE between terms of the multiplet are interpreted as the energies of collective electron mesoscopic excitations ?ω in the entire set of electron states characterizing the metal-type conductivity of a cluster. The estimate ?ω～0.2ΔE(N ₀/1000)^2/3 is obtained for a cluster consisting of N ₀ primitive cells. Depending on the thermal processing, N ₀=(0.3–20)×10⁶ in pyrolytic graphite, and accordingly ?ω～(10–150) eV. In the case when the energy cannot be determined accurately, methods permitting the variation of an excitation over a wide range (such as the spectroscopy of synchrotron radiation absorption and the characteristic energy losses of charged particles) appear to be the most promising.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

Thermal depolarization in crystals of calcium fluoride doped with oxygen

The study of thermally induced depolarization (TD) in crystals of calcium fluoride doped with oxygen reveals the existence of nearest-neighbour (nn) dipolar complexes comprising substitutional oxide ions (O_s^2?) and fluoride ion vacancies (F_v^?) on nn sites. Evidence for this relaxation is seen in TD experiments both on pure calcium fluoride doped with oxygen and on Na⁺ doped CaF₂ crystals that had been heated in air. Similar measurements on CaF₂: Y³⁺, O^2? reveal six separate relaxations, two of which are due to Y_s^3??F_i^? complexes that do not involve oxygen, one is due to O_s^2??F_v^? dipoles, and one is t the T₁ complex, Y_s³⁺ (O^2?)₄(F_v^?)₃. The remaining two relaxations were not identified but are probably d larger defect clusters.     

The infrared spectrum of OH-compensated defect sites in C-doped MgO and CaO single crystals

The IR spectra of OH-compensated point defects in MgO (and CaO) single crystals of various purity grades were reinvestigated. Three distinct groups of IR bands appear in the O-H stretching region: A, B and C around 3550 cm^?1 (3650 cm^?1), 3300 cm^?1 (3450 cm^?1) and 3700cm^?1 (3750cm^?1). They are assigned as follows: band A to the fully compensated, band B to the half compensated and band C to the overcompensated cation vacancies, [O?V”_catH?]^×, [O?V”_cat], and [O?O?V”_catH?$\text{]}\text{?}$, respectively.Upon cooling to 80 K the band A shows a complex behavior partly due to the formation of Ha molecules by charge transfer and concommittant O^? formation: [? (H₂)”_cat?]^×. The O^? represent defect electrons or positive holes in the O^2? matrix.Bands A and B show a characteristic multiplet splitting which is caused by local lattice strains coming from carbon atoms on near-by interstitial position. The intensity ratios between the multiplet components remain constant regardless of temperature pretreatments up to 1470 K, but strong variations of the integral intensities are observed. These are caused by the highly mobile C atoms entering and leaving reversibly the cation vacancy sites as a function of temperature and of the quenching speed. When the C atoms push the H₂ molecules onto interstitial sites, an H-H stretching signal appears around 4150cm^?1.     

Structure study of 12B from the elastic scattering of neutrons from 11B

Differential cross sections for neutrons scattered from ¹¹B have been measured for 2.2 MeV < E_n < 4.5 MeV. The differential cross section σ(θ) is fitted reasonably well by R-matrix parameters for broad states in ¹²B with assignments 1 ^? and (1) ⁺ at excitation energies E_x = 5.8 and 6.8 MeV respectively. The broad 1 ^? state has not been previously observed and is believed to be the 1 ^? member of the $\text{1}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{1}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ particle-hole multiplet predicted to exist by earlier shell model calculations. Its existence completes the identification of all of the levels of this multiplet (3 ^?, 2 ^?, 4 ^?, 1 ^?). The broad (1)⁺ level at E_x = 6.8 MeV has not been previously observed. States at excitation energies E_x = 5.61, 5.73 and 6.6 MeV have been assigned spins and parities of 3⁺, 3^? and (1)⁺ respectively. These states had previously been assigned spins of 2, 3 and $\text{≧ 1}$ respectively. Work on T = 1 states in ¹²C¹ has been compared with the present work.     

Oxygen ion conduction in 12CaO·7Al2O3: O2− conduction mechanism and possibility of O− fast conduction

12CaO·7Al₂O₃ (C12A7) with a unique nano-porous structure and free O^2? ions entrapped in sub-nanometer-sized cages is a fast oxygen-ion-conducting material. These free O^2– may be replaced by various oxygen-related species, OH^?, O^2? and O^?, by tuning the atmosphere during the heat treatment. We examined the conduction mechanism for stoichiometric C12A7 (C12A7:O^2?), in which O^2? ions exist as counter anions in sub-nanometer-sized cages, by Raman measurement of C12A7:O^2? annealed in a dry ¹⁸O₂ atmosphere. It was revealed that the primary ion conducting species is an O^2? ion which diffuses via exchange with O^2? in the cage wall. An experimental result on the sample containing O^? ions implied that O^? is more mobile than O^2? in C12A7. Ab initio calculations on the diffusion paths of O^2? and O^? ions in C12A7 supported the above experimental results.     

Photoemission study of oxygen adsorbed on coldly deposited silver films

UPS spectra of coldly deposited silver films differ from those of films deposited at room temperature by electronic states localized at surface defects with an energy about 4.2 eV below E_F. Changes after exposure at 140 K to oxygen only occur in the presence of these defects, demonstrating that oxygen is only adsorbed at defects. Raman vibrational spectroscopy shows that oxygen is adsorbed nominally as O₂^? and O₂^2?. Possible assignments of the oxygen related UPS structures are discussed.     

Emission characteristics of Dy3+ ions in lead antimony borate glasses

Glasses with the composition 30PbO–25Sb₂O₃–(45?x)B₂O₃–xDy₂O₃ for x=0 to 1 were prepared in steps of 0.2 by the melt-quenching method. Various physical parameters, viz., density, molar volume, and oxygen packing density, were evaluated. Optical absorption and luminescence spectra of all the glasses were recorded at room temperature. From the observed absorption edges optical band gap, the Urbach energies are calculated; the optical band gap is found to decrease with the concentration of Dy₂O₃. The Judd–Ofelt theory was applied to characterize the absorption and luminescence spectra of Dy³⁺ ions in these glasses. Following the luminescence spectra, various radiative properties, like transition probability A, branching ratio β and the radiative life time τ for different emission levels of Dy³⁺ ions, have been evaluated. The radiative lifetime for the ⁴F_9/2 multiplet has also been evaluated from the recorded life time decay curves, and the quantum efficiencies were estimated for all the glasses. The quantum efficiency is found to increase with the concentration of Dy₂O₃.     

Comparative electronic structure of a lanthanide and actinide diatomic oxide: Nd versus U

Using a modified version of the Alchemy electronic structure code and relativistic pseudopotentials, the electronic structure of the ground and low lying excited states of UO, NdO, and NdO⁺ have been calculated at the Hartree—Fock (HF) and multiconfiguration self-consistent field (MCSCF) levels of theory. Including results from an earlier study of UO⁺ this provides the information for a comparative analysis of a lanthanide and an actinide diatomic oxide. UO and NdO are both described formally as M⁺²O^?2 and the cations as M⁺³O^?2, but the HF and MCSCF calculations show that these systems are considerably less ionic due to large charge back-transfer in the π orbitals. The electronic states putatively arise from the ligand field (oxygen anion) perturbed f⁴, sf³, df³, sdf², or s²f² states of M⁺² and f³, sf² or df² states of M⁺³. Molecular orbital results show a substantial stabilization of the sf³ or s²f² configurations relative to the f⁴ or df³ configurations that are the even or odd parity ground states in the M⁺² free ion. The compact f and d orbitals are more destabilized by the anion field than the diffuse s orbital. The ground states of the neutral species are dominated by orbitals arising from the M⁺²sf³ term, and all the potential energy curves arising from this configuration are similar, which allows an estimate of the vibrational frequencies for UO and NdO of 862 cm^?1 and 836 cm^?1, respectively. For NdO⁺ and UO⁺ the excitation energies for the Ω states were calculated with a valence configuration interaction method using ab initio effective spin—orbit operators to couple the molecular orbital configurations. The results for NdO⁺ are very comparable with the results for UO⁺, and show the vibrational and electronic states to be interleaved.     

Electronic and ionic conductivities and point defects in ytterbium sesquioxide at high temperature

From the study of complex impedance diagrams applied to a symmetric cell Pt-Yb₂O₃-Pt, the authors have shown the mixed character of electrical conduction within the ytterbium sesquioxide. The measurements were performed at thermodynamic equilibrium in the temperature range from 1423 to 1623 K and the partial pressure of oxygen range from 10^?12 to 1 atm. The variations of ionic and electronic conductivity as a function of P_O2, were interpreted in terms of point defects e′, ?, V?_Yb and Yb_{$\text{I}\text{?}$}, in the general case of a Frenkel disorder. The relative contributions and the activation energies of conduction of these different defects were determined.     

Molecular orbital interpretation of X-ray emission and esca spectral shifts in silicates

An approximate molecular orbital theory is presented, tested, and applied to silicon and aluminum oxyanions. The orbital structure of SiO₄^?4 is calculated and is used to assign the X-ray emission spectra. Calculations are performed at several internuclear distances and semiquantitative agreement is found with experimentally observed trends in A1Kα and SiKβ spectra. Calculations on Si₂O₇^?6 and AlSiO₇^?7 yield fair agreement with experimental trends relating to degree of SiO₄ polymerization and to Al/Si ratio. Charges for bridging and non-bridging oxygens in Si₂O₇^?6, combined with point charge potentials from metal ions in the orthopyroxene structure yield Ols binding energies in agreement with ESCA results.     

Study of complex impedance spectroscopic properties of the KFeP2O7 compound

The KFeP₂O₇ compound was prepared by the conventional solid-state reaction. The sample was characterized by X-ray powder diffraction. The AC electrical conductivity and the dielectric relaxation properties of this compound have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies and temperatures, 200 Hz–5 MHz and 553–699 K, respectively. Both impedance and modulus analysis exhibit the grain and grain boundary contribution to the electrical response of the sample. The temperature dependence of the bulk and grain boundary conductivity were found to obey the Arrhenius law with activation energies Eg?=?0.94 (3)?eV and Egb?=?0.89 (1)?eV. The grain-and-grain boundary conductivities at 573 K are 1.07?×?10^?4 and 1.16?×?10^?5 (Ω^?1 cm^?1). The scaling behavior of the imaginary part of the complex impedance suggests that the relaxation describes the same mechanism at various temperatures. The near value of the activation energies obtained from the equivalent circuit, conductivity data, and analysis of M″ confirms that the transport is through ion hopping mechanism.     

Charge transfer reaction of multi-charged oxygen ions with O2

The rate of transfer of electrons from O₂ to O²⁺ and O³⁺ has been measured at energies ? 2 eV using a stored ion technique. The rate for O²⁺ is k = 1.0(0.3) × 10^?9 cm³/s and for O³⁺, k = 2.5(0.3) × 10^?9 cm³/s, compared to calculated Langevin rates of 1.8 × 10^?9 cm³/s and 2.7 × 10^?9 cm³/s respectively.     

Charge exchange and excitation cross sections at collisions of alpha particles with be-like impurity oxygen ions in a plasma

The charge exchange and excitation cross sections at collisions of alphas with O⁴⁺(1s ²2s ²) impurity atoms in a hot plasma for striking energies E _c varying from 20 keV to 2 MeV are determined for the first time. The cross sections are calculated using the method of close-coupling equations with 13 singlet four-electron quasi-molecular states taken as a basis. The partial cross sections of charge transfer to the 1s, 2s, and 2p states of a He⁺ ion and for O⁴⁺(1s ²2s ²) → O⁴⁺(1s ²2lnl’) (n = 2, 3) electronic excitation of an oxygen ion are found. The maximal value of the charge exchange total cross section roughly equals 2.2 × 10^?16 cm² at E _c ≈ 0.7 MeV. The excitation total cross section has a maximum of ≈ 7.7 × 10^?16 cm² at E _c ≈ 80 keV for single-electron excitation and ≈6.5 × 10^?16 cm² at E _c ≈ 0.7 MeV for two-electron excitation.     

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of Fe_xO (x ~ 0.90–0.95); Fe₂O₃ (α and γ); Fe₃O₄; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). Fe^II and Fe^III states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below E_F, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for Fe_xO and Fe₃O₄. Fe surfaces (films, foils, (100) face) yield predominantly Fe^III species when exposed to high exposures of oxygen or air, though there is evidence for some Fe^II also. At low exposures the Fe^II/Fe^III ratio increases.     

Lithium-ionic diffusion and electrical conduction in the Li7La3Ta2O13 compounds

The electrical properties and the mechanism of lithium ionic diffusion in the Li₇La₃Ta₂O₁₃ compounds were investigated. The bulk and total conductivity at 300 K of the Li₇La₃Ta₂O₁₃ compound are about 3.3 × 10^? 6 S/cm and 2.6 × 10^? 6 S/cm, respectively. The activation energy of bulk and total conductivity is in the range of 0.38–0.4 eV. A prominent internal friction peak in Li₇La₃Ta₂O₁₃ compounds was observed around 280 K at 0.5 Hz, which is actually composed of two subpeaks (P₁ peak at lower temperature and P₂ peak at higher temperature). From the shift of peak position with frequency, the activation energy of 1.0 eV and the pre-exponential factor of relaxation time in the order of 10^? 18–10^? 21 s were obtained if one assumes Debye relaxation processes. These values of relaxation parameters strongly suggest the existence of interaction between the relaxation species (here lithium ions or vacancies). Based on the coupling model, the relaxation activation energies are deduced as 0.45 eV and the pre-exponential factor of relaxation time as 10^? 15 s. Judging from these relaxation parameters and the similarity of structure between Li₇La₃Ta₂O₁₃ and Li₅La₃Ta₂O₁₂ compounds, the P₁ and P₂ peaks are suggested to be related with the lithium ionic diffusion between 48g?48g and 24d?48g.     

Composition nanodefects in doped lithium fluoride crystals

This paper presents the results of investigations into the optical properties of LiF crystals doped with oxides of different metals (Li, W, Ti, Fe). It has been proved that, during the growth of crystals, nanodefects containing polyvalent dopant ions and oxygen in different states (O^2?, OH^?, O ₂ ^? ) are formed in the crystalline matrix. It has been shown that these nanodefects are sinks of electronic excitations and determine the direction and efficiency of radiation-induced processes.