The local environment of cations in Bi12.5Er1.5ReO24.5

The local environments of cations in a recently reported δ-Bi₂O₃-related phase stabilised by erbium and rhenium, and which shows exceptionally high oxide ion conductivity at low temperatures, have been examined by Bi L_III-, Er L_III- and Re L_III- edge extended X-ray absorption fine structure (EXAFS) measurements. The Bi L_III- and Er L_III- edge data recorded from the compound Bi_12.5Er_1.5ReO_24.5 revealed a high level of oxygen disorder and the adoption by bismuth and erbium of different local environments than those previously observed in erbium-doped δ-Bi₂O₃. The Re L_III edge EXAFS recorded from Bi_12.5Er_1.5ReO_24.5 endorsed the highly disordered nature of the oxygen system and showed rhenium to adopt fourfold oxygen coordination and to be significantly different from that of the local environment of rhenium in Bi₂₈Re₂O₄₉.     

Crystallographic and optical properties of rare-earth chromium sulfides RCrS3

Rare earch chromium sulfides RCrS₃; R=Y, Gd, Dy, Ho and Er have been synthesized by high temperature reaction of the respective oxides, RCrO₃, with CS₂. Single crystals have been grown by mineralization of the polycrystalline powders using halogen as a mineralizer. The crystal structures were determined from X-ray rotation and Weissenberg photographs. RCrS₃, with R=Y, Gd, Ho and Er, were found to have an isostructural monoclinic structure, while DyCrS₃ was found to have an orthorhombic structure, closely related to the monoclinic compounds. The optical absorption of these compounds, as well as of LaCrS₃ whose structure was previously reported, was measured over the photon energy range 0·15～1·7 eV. For all the compounds, a sharp absorption edge was found at 1·20～1·30 eV at room temperature.This absorption edge shifts toward higher energy upon cooling from room temperature. The infrared absorption of DyCrS₃, HoCrS₃ and ErCrS₃ was negligible, while that of YCrS₃, LaCrS₃ and GdCrS₃ increased markedly, with decreasing photon energy.     

Novel photoluminescence properties of InAlO3（ZnO）m superlattice nanowires

One-dimension InAlO 3 (ZnO) m superlattice nanowires were successfully synthesized via chemical vapor deposition.Transmission electron microscopy measurements reveal that the nanowires have a periodic layered structure along the 0001 direction.The photoluminescence properties of InAlO 3 (ZnO) m superlattice nanowires are studied for the first time.The near-band-edge emissions exhibit an obvious red shift due to the formation of the localized tail states.The two peaks centered at 3.348 eV and 3.299 eV indicate a lever phenomenon at the low-temperature region.A new luminescence mechanism is proposed,combined with the special energy band structure of InAlO 3 (ZnO) m.     

Optical anisotropy of LiNbO3 and KNbO3 in the interband transition region

The reflectivity of single domain crystals of LiNbO₃ and KNbO₃ has been measured using linearly polarized light up to 10 eV. The Kramers-Kronig analysis reveals a large anisotropy of the optical constants in the interband transition region. It is shown that the large electro-optic effects observed in these compounds are due to transitions in a relatively narrow spectral region above the band edge only.     

Electroabsorption in TlInS2

Electroabsorption spectra of single crystals have been studied near the fundamental absorption edge at 77 and 300 K. At 300 K two positive peaks (2.34 and 2.42 eV) and a negative peak (2.38 eV) are observed in the electroabsorption spectrum. At liquid-nitrogen temperature a fine structure corresponding to the formation of a parabolic exciton (2.503 eV) is observed.Values of the width of the forbidden gap E_g, the n = 1 exciton positions, the exciton activation energy ΔE_b, the effective Bohr radius a_exc, the reduced effective mass of an electron-hole pair μ, and the exciton ionization field F(E_g = 2.535 eV, E_exc = 2.503 eV, E_b = 32 meV, $\text{a}{}_{\mathrm{<mtext>exc</mtext>}}\text{=}\text{28}\text{A}\text{A;}\text{;}$;, μ = 0.15 m₀, and F = 1.2 × 10⁵ V cm^-1) have been determined from the electroabsorption spectrum.     

Photoelectron spectra of F3SiC2H4Si(CH3)3 molecule using monochromatized synchrotron radiation

A variety of photoelectron spectra for gas phase F₃SiC₂H₄Si(CH₃)₃ molecule have been measured using monochromatized undulator radiation and a hemispherical electrostatic analyzer. Valence photoelectron spectrum shows many peaks for ionization from shallow and deep molecular orbitals in the binding energy region of 9–40 eV. A calculation of ionization energies using the outer valence Green's function method indicates energies in agreement with experimental results below 17.5 eV. Spectra for Si L-shell electron emission show chemical shifts of Si atoms induced from different chemical environments around two Si atoms and also exhibit spin–orbit splitting for 2p photoelectrons. Further photoelectron spectra for C K-shell and F K-shell are discussed in comparison with those of related molecules.     

Ionic implantation of N2+ in Ni(110) at 300 K

The present work gives results of a preliminary investigation, carried out by SES, AES, LEED and ELS, on the implantation of nitrogen ions in Ni(110) as a function of ion dose and subsequent surface heat treatment at different temperatures. The fine structure in the SES spectrum is the most sensitive to implantation: peaks at 9, 17.5 and 31.5 eV are shifted towards lower energies by E = 1 eV for the first two peaks and 2.8 eV for the last. At high nitrogen doses a disordered layer is observed by LEED. The p(2 × 3) structure is obtained when the crystal is heated to 750 K. The two electron loss peaks of 4.8 and 10 eV arise from an induced electron N_2p level situated 4.8 eV below the Fermi level.     

Faraday rotation and surface excitons in PbI2

The fine structure of exciton spectrum in thin films of PbI₂ on NaCl substrate is investigated with the help of the Faraday rotation. The narrow negative peaks B and B' situated at 2.535 and 2.528 eV (90°K) correspond, as it is shown, to the surface excitons which are excited in the molecular layers of PbI₂ bordering with vacuum and NaCl surface.     

Inelastic effects and structure in the auger electron emission spectra of V2O5(010) and V(100) surfaces: Study of chemical shifts

Characteristic ionization losses and plasma losses occurring in the AES spectra of V₂O₅ (010) and V(100) surfaces are discussed. The former are used to evaluate chemical shifts of the VL₂ and VL₃ levels in V₂O₅. Values of approximateley 2.5 eV are found. These are smaller than the values obtained with ESCA (± 5 eV). It is described how the electron beam used in AES is thought to be responsible for this effect. ESCA data from partly reduced V₂O₅ samples tend to confirm the proposed model, based on oxygen loss and decomposition of V₂O₅ single crystals under the influence of the electron beam.The plasma losses of the larger Auger peaks are discussed. It is shown how some fine structure in the spectra can be partly explained by their presence. The plasma losses were simulated with numerical techniques based on the use of a signal averager.Signal averaging, curve fitting and related numerical techniques improve the resolution of AES spectra. Spectra of V₂O₅(010) and V(100) obtained in this manner are discussed. With respect to the transitions involving the valence band it is shown that the complex valence band structure is one of the causes of the observed discrepancies between theoretical and experimental AES data. Furthermore there is an uncertainty concerning the way in which ionization correction should be applied in this case. This correction is thought to increase with the degree of localization of the valence electrons.Auger peak intensities in function of the primary energy were found to show a maximum at about 3.5 times the critical potential, as was expected from theory if a moderate amount of backscattering is taken into account. Finally the intensity variations of the Auger peaks under continuous electron bombardment show the rate of oxygen loss at a V₂O₅ surface due to the primary beam.     

Soft X-ray appearance-potential spectroscopy studies of DyFe2 and ErFe2 intermetallics

The soft X-ray appearance-potential spectra (SXAPS) of the M_4,5 levels of Dy and Er and the L_2,3 levels of Fe in pure metals and their intermetallics DyFe₂ and ErFe₂ have been obtained in the energy range of 700–1500 eV. SXAPS probes the binding energies of the core levels and the distribution of local unfilled conduction-band states. Both the constituents of DyFe₂ show negative shifts with respect to the pure metals while those of ErFe₂ show a negative shift for Er and a positive shift for Fe. The widths of the Dy and Er peaks increase and that of the Fe decreases on alloying. The results are interpreted in terms of the Fermi level changes and the charge transfer from the rare earth to Fe. The discrepancy in Fe chemical shifts has been correlated to the electronic specific heat capacity coefficient reported for these intermetallics.     

Electronic structure of AgNbO3 and NaNbO3 studied by X-ray photoelectron spectroscopy

The XPS examinations of the AgNbO₃ and NaNbO₃ single crystals and ceramics allowed estimate their average composition as Ag_1.1Nb_0.9O₃ and Na_1.2Nb_0.9O_2.9. The valence bands of the AgNbO₃ compound, formed mainly of the Nb 4d, Ag 4d and O 2p states, show an energy gap about 3 eV while for the NaNbO₃ compound consist of the O 2p states hybridized with the Nb 3d states and show an energy gap about 4 eV. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. The broadening of the core level lines of AgNbO₃ suggests a stronger structural disorder in comparison with NaNbO₃ compound.     

5f Localization in light actinide compounds studied by L3-edge x-ray absorption

L₃-edge x-ray absorption spectroscopy was applied to electronic structure studies in compounds of U, Np, and Pu. Single-peaked white lines are observed at the L₃ thresholds, which exhibit shifts up to 6 eV to lower energies for the monopnictides and monochalcogenides relative to the tetravalent dioxides. These chemical shifts increase systematically from U to Np and Pu, reflecting an increasing localization of the 5f states.     

Electro- and thermoreflectance of Bi2Te3 and Bi2Se3

Electroreflectance measurements in Bi₂Te₃ and Bi₂Se₃ with the electric field vector of the incident light both inclined and perpendicular to the C-axis have been made at room temperature. The structures found by other workers in the reflection measurements are observed in the present experiment, together with new structures at 0.91 eV, 1.18 eV, 1.78 eV, and 2.61 eV in Bi₂Se₃ which are not related to formerly observed transitions. From these measurements, the selection rules for direct optical transitions in Bi₂Te₃ and Bi₂Se₃ are studied. Thermoreflectance measurements are also made at both room and liquid-nitrogen temperatures. The positions of the peaks obtained in the present work are compared with the electroreflectance and reflection data.     

Chemical shifts in strontium compounds by X-ray spectroscopy

The positions of the X-ray K-absorption edge of strontium in various compounds and in aqueous solutions have been measured. With the help of modified Moseley diagram the wavelength of the X-ray K-absorption edge of Strontium in metal has been extrapolated. Chemical shifts of the K-absorption edge with respect to this extrapolated value have been presented. The fine structure has been observed on the high energy side of the main absorption edge both in Rubidium and Strontium compounds. Bond lengths for various compounds have been calculated from the maxima-minima separations. The chemical shifts of fluorescence Kα_1,2 lines with respect to their position in Sr²⁺ ion have also been studied. While the edge shift with respect to the edge in free ion is towards the low energy side, the line shift with respect to the line in free ion is towards the high energy side.     

Xanes study of trivalent and homogeneous mixed valent rare earth systems

We report for the first time a systematic study of X-ray absorption structures 20–30eV above the L_III edge (XANES) of homogeneous mixed valent (SmB₆, YbAl₂, CePd₃...) and trivalent reference Rare-Earth compounds (e.g. GdB₆, LaAl₂, NdPd₃). The XANES of integral valent compounds in simple structures (NaCl, AuCu₃...), which is due to multiple scattering of the photoelectron on the neighbouring atomic shells, is characterized by a single peak. On the contrary the XANES of mixed-valent materials presents a splitted structure which replicates the edge structures. We interpret this splitting as a direct evidence for the existence of two threshold energies and corresponding presumably to two different Rare-Earth metal distances in agreement with 4f fluctuations. Finally, we shall propose a consistent picture for core hole spectroscopies (L_III edge, XANES, EXAFS and XPS) in mixed-valent materials.     

Chemical shifts of the LIII absorption discontinuity of rhenium

Chemical shifts of the X-ray L_III absorption discontinuities of rhenium in some of its binary and ternary compounds have been studied using a bent crystal X-ray spectrograph. The chemical shifts (ΔE) are found to be governed by the effective charges (q) on the absorbing ions, which have been calculated using Suchet's theory. For the compound ReCl₅, however, the effective ionic charge cannot be calculated since the pentavalent radius of rhenium is not known. The ΔE, q plot has been used to determine the charges on the rhenium ions in ReCl₅ as well as in two ternary compounds KReO₄ and NaReO₄. Our work has enabled us to determine the ionic radius of pentavalent rhenium.     

K-absorption spectral investigations on some dimeric copper(II) carboxylate complexes

The X-ray K-absorption edge position and the extended fine structure of copper in some copper(II) carboxylate complexes involving metal-metal exchange interaction have been investigated using a 40 cm bentcrystal spectrograph. It has been found that the edge position in these complexes shifts towards the higher energy side as the pK_a value of the corresponding acids increases. Sharp and narrow main peaks have been observed in all these complexes except the monomeric complex, copper(II) trichloroacetate. Estimates of metal-ligand average bond distances have also been made.     

Inverse TMR in a nominally symmetric CoFe/AlOx/CoFe junction induced by interfacial Fe3O4 investigated by STEM-EELS

We have found inverse tunneling magnetoresistance (TMR) with a non-symmetric bias voltage dependence in a nominally symmetric Si (001)/Ag/CoFe/AlO_x/CoFe/IrMn/Ag magnetic tunnel junction after field cooling. The O K edge fine structure extracted from electron energy loss spectroscopy spectrum images taken at the interfaces of junctions with inverse TMR shows a thin, discontinuous Fe₃O₄ layer at the CoFe/AlO_x interfaces. The Fe L_2,3 edge core level shifts are also consistent with those of Fe₃O₄. We find no Fe₃O₄ layer in junctions with normal TMR. We believe this Fe₃O₄ layer is responsible for the inverse TMR.     

Valence band photoemission spectroscopy of a ternary layered semiconductor: ZnIn2S4

UV photoemission spectroscopy (UPS) experiments have been carried out on the layer compound ZnIn₂S₄ employing several different photon energies in the range $\text{h}\text{?}{}_{\mathrm{\omega }}\text{= 9.5?21.2}\text{eV}$. The energy distribution curves (EDC's) exhibit four valence band density of states structures besides the Zn 3d peak. These five peaks appear 0.90 eV, 1.6 eV, 4.3 eV, 5.8 eV and 8.7 eV respectively below the top of the valence band, E_v. The atomic orbital character of the shallowest peak A appears different from that of the three deeper valence band peaks B, C and D and this is discussed in terms of the more or less pronounced ionic character of the intralayer chemical bonds. These results demonstrate that an overall understanding of the electronic states in complex structures can be achieved by an approach based on photoemission experiments and chemical bonding considerations which has been widely used in the past to study simple binary layer compounds.     

The x-ray LIII absorption discontinuity of ytterbium in the metal and in some of its compounds

The shape and fine structure of the x-ray L_III absorption discontinuity of ytterbium (Z=70) has been studied in the pure metal and in several of its compounds. The shape and the near edge structure of the discontinuity in the metal reflects the band structure. A molecular orbital interpretation for the fine structure near the absorption edge has been proposed for the spectrum of the sesqui-oxide. The average bond lengths for the systems studied have been determined from the measurements on the fine structure employing Lytle’s and Levy’s methods and are compared with the available crystallographic data in the case of the metal, oxide and fluoride.