Electronic structure and term energy calculations for cubic and axial iron defects in SrTiO3

The spin-unrestricted Xα method is used to obtain the molecular orbital diagrams relevant to cubic Fe²⁺, Fe³⁺ and axial Fe⁺ ?V₀ to Fe³⁺ ?V₀ defects in SrTiO₃, where V₀ is an oxygen vacancy. The relative energies of the lower spin multiplets are determined through the transition-state model of Slater. The electronic structures and the term patterns allow to discuss the stability and the ground term of each center trapped in cubic SrTiO₃. The mechanisms of ligand-metal and intervalence charge transfers are examined and compared to the abundant experimental data provided by EPR and optical measurements. The different behaviours of ferroelectrics like BaTiO₃: Fe and LiNbO₃: Fe in regard to the mechanisms implied in the photorefractive effects are reviewed according to the theoretical electronic structures with a special emphasis to the Fe²⁺ ? Fe³⁺ charge transfer.     

Charge exchange reaction 14C(6Li, 6He)14N

Angular distributions of the charge exchange reaction ¹⁴C(⁶Li, ⁶He)¹⁴N leading to the 1⁺ ground state and 3.95 MeV 1⁺, and 5.20 MeV 2^? excited states at the 34 MeV incident beam energy were analyzed and measured. The 62 MeV data of Goodman et al. were also reanalyzed. The direct one-step charge exchange caused by the spin-isospin dependent term in the two-body interaction can account well for the observed data. The strength of spin-isospin dependent effective interaction (gaussian form with a range parameter of 1.8 fm) was extracted to be 18.5 MeV.     

Fe deep level optical spectroscopy in InP

The optical cross-section σ_n⁰(hv) and σ_p⁰(hv) associated with the (Fe³⁺ ? Fe²⁺) deep level have been measured by Deep Level Optical Spectroscopy in n-type Fe doped samples of InP. Optical transitions are interpreted as transitions from the Fe²⁺ ground state to the Γ and L point minima of the conduction band for σ_n⁰(hv) and from the valence band to the ground and excited state for Fe²⁺ for σ_p(hv). A theoretical model which accounts for the main features of the experimental data is proposed.     

Autoionization resonances in the photoabsorption spectra of Fe<Superscript><Emphasis Type="Italic">n</Emphasis>+</Superscript> iron ions

The photoabsorption cross sections of a neutral iron atom, as well as positive Fe⁺ and Fe²⁺ ions, are calculated in the relativistic random-phase approximation with exchange in the energy range 20–160 eV. The wavefunctions of the ground and excited states are calculated in the single-configuration Hartree–Fock–Dirac approximation. The resultant photoabsorption spectra are compared with experimental data and with the results of calculations based on the nonrelativistic spin-polarized version of the random-phase approximation with exchange. Series of autoionization resonance peaks, as well as giant autoionization resonance lines corresponding to discrete transitions 3p → 3d, are clearly observed in the photoabsorption cross sections. The conformity of the positions of calculated peaks of giant autoionization resonances with experimental data is substantially improved by taking into account additionally the correlation electron–electron interaction based on the model of the dynamic polarization potential.     

Crystal field and exchange interactions in the SmFe3(BO3)4 multiferroic

The optical spectra of oriented SmFe₃(BO₃)₄ single crystals are studied in the region of the f-f transitions in the Sm³⁺ ion by Fourier spectroscopy. The energies, the symmetry properties, and the exchange splittings of the Stark sublevels of the ground and 17 excited multiplets of the Sm³⁺ ion in a crystal field of symmetry D ₃ are determined from the measured temperature dependences of polarized-radiation absorption spectra. The parameters of the crystal field acting on samarium ions and the parameters of the exchange interaction between Sm³⁺ and Fe³⁺ ions are found. The anisotropy of the effective exchange interaction is shown to be substantially stronger than the magnetic anisotropy, due to a strong crystal-field-induced mixing of the ground and excited multiplets.     

Effect of precursors on the growth of carbon filaments onto clay surface

The successful growth of carbon filaments on two different precursors, i.e., the pristine sodium-montmorillonite (Na⁺MMT), which undergoes reflux at 100 °C (r-MMT), and the Na⁺MMT exchanged with Fe³⁺ ions (MMT(Fe)), was attained through chemical vapor deposition (CVD). The products obtained were characterized by X-ray diffraction, thermogravimetry, scanning electron microscopy, and transmission electron microscopy. Refluxing can make the Fe³⁺ ions in the octahedral layer of Na⁺MMT migrate to the interlayer and exchange with Na⁺ ions. Furthermore, through calcination at 500 °C, the Fe³⁺ ions migrate again to the surface of the clay layer and form iron oxides, which can serve as precursors for the deposition of carbon. Although r-MMT contained less iron than the MMT(Fe), the ultimate yield of carbon components grown was almost the same, indicating that the iron species in r-MMT possess higher catalytic activity. However, on the surface of r-MMT, CVD hardly generated carbon nanotubes with a clear hollow structure but that those with a carbon fiber structure instead.     

Study of the ground state doublet in57Fe

The⁵⁶Fe(d,p)⁵⁷Fe reaction has been studied at an incident energy of about 10 MeV. The ground state doublet of⁵⁷Fe has been resolved using the Heidelberg Q 3 D magnetic spectrograph and the Minnesota split-pole spectrograph. The ratio of the spectroscopic strengths (2J+1)S for the transitions to the first excited state at 14.4 keV excitation energy (J ^π=3/2^?) and the ground state (J ^π=1/2^?) has been determined to be 5.80±0.5. This number is of special interest with respect to a possible deformation of the nucleus⁵⁷Fe and in connection with the corresponding analogue resonances in⁵⁷Co.     

Heterometallic fullerides of Fe and Cu groups with the composition K2MC60 (M=Fe+2, Fe+3, Co+2, Ni+2, Cu+1, Cu+2, Ag+1)

Heterometallic fullerides with composition K₂MC₆₀, synthesized by exchange chemical reaction of K₅C₆₀ or K₄C₆₀ with chlorides of metals Fe and Cu groups have been investigated by X-ray diffraction, magnetic resonance, Raman and Mössbauer spectroscopy. Magnetization and susceptibility measurements have also been carried out. Metal chlorides from Fe and Cu groups enable to cover the whole range of electronic configuration of metal from d⁵ to d¹⁰. Heterometallic fullerides with M=Cu⁺², Fe⁺², Fe⁺³ and Ni⁺² appeared to be superconductors with T_c=13.9–16.5 K. Ferromagnetism and superconductivity coexist in investigated fulleride K₂Fe⁺³C₆₀.     

A study of the reaction12C(π +,π + +p)11B

We have measured the cross section for the reaction¹² C(π ⁺,π ⁺+p)¹¹B in a coincidence experiment with an incident pion energy of 100 MeV and find that the residual¹¹B nucleus remains predominantly in the ground state. A test of the nucléon charge exchange model by Hewson [3] is discussed.     

Ferromagnetic ordering of iron impurities in the valence-fluctuating semiconductor SmB6

The electron paramagnetic resonance (EPR) of the valence-fluctuating semiconductor SmB₆ doped by 1 at % Fe is studied. The EPR measurements are performed on a SmB₆ single crystal in a temperature range of 1.6–300.0 K. A number of resonance lines whose g factors indicate the presence of iron ions in the Fe⁰, Fe⁺, Fe²⁺, and Fe³⁺ states have been detected. The iron ions are ferromagnetically ordered below a Curie temperature T = 100 K, and this ordering can be caused by the exchange interaction of impurity ions due to matrix polarization (a similar mechanism is observed in PdFe alloys). This exchange interaction is estimated to be significantly higher than that in PdFe; this fact can result from a very high density of states in the narrow f band, which is characteristic of a valence-fluctuating material.     

Highly excited,normally inaccessible vibrational levels by sub-doppler modulated gain spectroscopy: The Na2 A1Σ+u state

Modulated gain spectroscopy is a sensitive, widely applicable, rovibronically state selective, sub-Doppler, triple resonance method for examining excited vibronic levels which are Franck—Condon inaccessible from thermally populated levels of the electronic ground state. A cw optically pumped molecular laser (OPL) prepares a steady-state population in a selected, vibrationally highly excited, rotation-vibration level of the electronic ground state (the lower level of the OPL transition). An intensity-modulated, single frequency dye laser excites part of this intracavity OPL-prepared population to the level of interest, thereby causing an increase in the OPL population inversion density, and, in turn, its output power. As the frequency of the dye laser is scanned, resonances are selectively detected by the appearance of modulation on the OPL output power; discrimination against dye laser excitations out of levels unconnected with the OPL is nearly perfect. Sub-Doppler (≈300 MHz FWHM) transitions are observed, thereby extending knowledge of the Na₂A¹Σ⁺_u state from v=44 to 62.     

Mössbauer study of Na0.82Co0.99Fe0.01O2

We studied by Mössbauer spectroscopy the Na_0.82CoO₂ compound using 1% ⁵⁷Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe³⁺ and Fe⁴⁺. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co⁺³, Co⁺⁴ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe⁴⁺ and all of Fe³⁺, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.     

The (18O, 20Ne) reaction on the even Ni isotopes and the mass of 62Fe

The (¹⁸O, ²⁰Ne) reaction on the even Ni isotopes has been studied at 63.0 MeV with ΔE-E time-of-flight telescopes. From the measured ground-state Q-value for the ⁶⁴Ni(¹⁸O, ²⁰Ne)⁶²Fe reaction, ?1.97±0.20 MeV, a mass excess ?58.87±0.20 MeV is obtained for the ⁶²Fe nucleus. This result is in good agreement with a recent measurement of the β-endpoint energy. Angular distributions for the transitions to the Fe ground states, leaving ²⁰Ne in its ground and 1.63 MeV 2⁺ excited state, yield relative spectroscopic strengths in fair agreement with DWBA calculations based on simple shell-model estimates.     

Using the 57m Fe3+ Mössbauer probe to determine the EFG tensor parameters at CuO cation sites

Mössbauer emission spectroscopy of the ⁵⁷Co(^57m Fe) isotope has shown that the impurity iron atoms appearing at the CuO-lattice cation sites after the decay of ⁵⁷Co²⁺ are donors and can become stabilized in two charge states, ^57m Fe³⁺ and ^57m Fe²⁺, and that the population ratio of these states depends on the Fermi level, whose position is governed by native CuO-lattice defects. A satisfactory agreement between the calculated and experimental values of the quadrupole splitting in Mössbauer spectra has been obtained for the ^57m Fe³⁺ centers. This permits one to consider the results obtained in the ⁵⁷Co(^57m Fe) Mössbauer emission spectroscopy study of cuprates as reliable experimental data on the crystal-field EFG tensor parameters at copper sites.     

Application of X-ray absorption spectroscopy to the investigation of charge states of iron ions in iron borate nanoceramics

The method of X-ray absorption spectroscopy has been used for the investigation of charge states of iron ions in iron borate nanoceramics prepared by shear deformation under pressure. The experimental Fe 2p X-ray absorption spectra have been presented in comparison with the calculation of atomic multiplets of iron ions taking into account the charge transfer from the 2p orbitals of oxygen to the 3d orbitals of iron and the crystal-field splitting of the 3d orbitals of iron. Our results indicate that, in addition to iron ions in the ground charge state Fe³⁺, nanostructured FeBO₃ contains a few percent of Fe²⁺ ions. It has been found that an increase in the degree of plastic deformation (the rotation angle of the anvils) leads to a decrease in the size of crystallites and to an increase in the concentration of Fe²⁺ ions without the formation of new phases. The results of this work agree with the magnetic and optical measurements and confirm high defectness of FeBO₃ nanoceramics.     

Monitoring by Mössbauer spectroscopy the thermal reduction of hematite into magnetite: the surface effect and charge disproportionality in iron oxide nanoparticles

Nanoparticles of magnetite Fe₃O₄ were synthesized by thermal reduction of hematite α-Fe₂O₃ powder in the presence of high boiling point solvent. The structural transformations and magnetic properties of the obtained nanoparticles were investigated by the ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at each step of the chemical and thermal treatment of the product. An increase of saturation magnetization with the reaction time correlates with an increase of concentration of magnetite in the samples. The electron hoping between Fe^2?+? and Fe^3?+? ions in the octahedral sites of the magnetite nanoparticles and Verwey phase transition were investigated. It was established that not all iron ions in the octahedral sites participated in electron hoping Fe^2?+????Fe^3?+? above the Verwey temperature T _V, and the charge distribution could be expressed as $\big( {{\rm Fe}^{3+}}\big)_{{\rm tet}} \big[ {{\rm Fe}_{1.85}^{2.5+} {\rm Fe}_{0.15}^{3+} }\big]_{{\rm oct}} {\rm O}_4$ .     

Low-lying states of trinuclear mixed-valence cluster: [Fe3S4]0

Low-lying energy states of the [Fe₃S₄]⁰ cluster have been calculated by taking into account the double exchange, superexchange and vibronic interaction. It was found that the adiabatic potential of the excited state withS=0 corresponds to the full delocalization of the “excess” charge. From the analysis of experimental data of Mössbauer spectroscopy and the temperature dependence of the magnetic susceptibility the double exchange parametert≥4000 cm^?1 and the vibronic interaction parameter λ₂/2k cm^?1 have been estimated.     

SU3 comparison of line reversed charge exchange and hyperchange exchange reactions

The recent data at 10.1 GeV/c on the hypercharge reactions π⁺p → K⁺Σ⁺ (1385) and the line reversed processes are compared with the SU₃ related charge exchange processes K⁺n → K^op and K⁺p →K^oΔ⁺⁺ and their line reversed partners in order to study the systematics of line reversal breaking. Allowing for a t- independent SU₃ breaking between charge and hypercharge exchange the data follow the SU₃ relations. We conclude that the line reversal breaking in the hypercharge exchange reactions is in agreement with the breaking in exchange degeneracy (in both flip and non-flip amplitudes) which is observed for the charge exchange processes.     

Low temperature Mossbauer study of Fe2+ impurities in ZnS: Source and absorber experiments

Fe²⁺ impurities in ZnS blende exhibit low temperature Mössbauer spectra which are remarkably different in source and in absorber experiments. In source experiments below 5K we observed a complex quadrupole structure, which is partially due to the slow relaxation contribution of the first excited triplet Λ₄ of Fe²⁺, whose population appears to be out of thermal equilibrium. The corresponding reduction factor q due to the dynamic Jahn—Teller coupling has been estimated to be about 0.91.     

Study of electric monopole transitions between the ground state and the first excited 0+ state in 40, 42, 44, 48Ca with high resolution inelastic electron scattering

Monopole transitions from the 0₁⁺ ground states to 0₂⁺ excited states at 3.353 MeV (⁴⁰Ca), 1.837 MeV (⁴²Ca), 1.884 MeV (⁴⁴Ca) and 4.272 MeV (⁴⁸Ca) have been investigated with high resolution inelastic electron scattering (FWHM ≈ 30 keV) at low momentum transfer (0.29 ≦ q ≦ 0.53 fm^?1). The respective monopole matrix elements are 2.53 ± 0.41 fm², 5.24 ± 0.39 fm², 5.45 ± 0.41 fm² and 2.28 ± 0.49 fm². These results are used together with known ground state charge radii and the average number of holes in the sd shell in the ground state to estimate the number of particle-hole excitations in the wave functions of the excited 0⁺ states.