Effects of rare-earth substitution in the oxyarsenides REFeAsO (RE=Ce, Pr, Nd, Sm, Gd) and CeNiAsO by X-ray photoelectron and absorption spectroscopy

X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) have been applied to examine the electronic structure of the rare-earth transition-metal oxyarsenides REFeAsO (RE=Ce, Pr, Nd, Sm, Gd) and CeNiAsO. Within the metal-arsenic layer [MAs], the bonding character is predominantly covalent and the As atoms are anionic, as implied by the small energy shifts in the M 2p and As 3d XPS spectra. Within the rare-earth-oxygen layer [REO], the bonding character is predominantly ionic, as implied by the similarity of the O 1s binding energies to those in highly ionic oxides. Substitution with a smaller RE element increases the O 1s binding energy, a result of an enhanced Madelung potential. The Ce 3d XPS and Ce L₃-edge XANES spectra have lineshapes and energies that confirm the presence of trivalent cerium in CeFeAsO and CeNiAsO. A population analysis of the valence band spectrum of CeNiAsO supports the formal charge assignment [Ce³⁺O²⁻][Ni²⁺As³⁻].     

The enhancement mechanism of dielectric properties of Pb(Zr,Ti)O3 via (Mg2+,Sb3+) incorporation for supercapacitors

Due to the extraordinary versatility of the perovskite structure in accommodating different dopant ions in its structure, in recent years a huge number of multifunctional perovskite materials have been developed. In this work we aim to obtain high temperature-stable and huge dielectric constant materials for supercapacitors by doping divalent Mg²⁺ and trivalent Sb³⁺ ions into the octahedral sites, and divalent Sr²⁺ ions into the dodecahedral sites of lead zirconate-titanate perovskite. The resulting (Pb_0.95Sr_0.05)(Zr_0.425Ti_0.45Mg_0.042Sb_0.083)O_3-δ is examined by X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), dielectric spectroscopy (DS) and resonance dielectric spectroscopy (RDS) in order to correlate composition, local structure, ion valence and chemical environment of the doped material with the dielectric properties. HRTEM evidences that a composite structure, with co-existent ferroelectric domains and relaxor nanodomains, is formed by doping. XPS shows that Sb³⁺ and Mg²⁺ substitute for the Ti⁴⁺/Zr⁴⁺ ions, pointing to these strong defects as the main cause for the appearance of the relaxor phase. DS and RDS found that the ferroelectric lead zirconate-titanate transforms into a re-entrant relaxor-ferroelectric composite with a huge dielectric constant of about 10⁴ which remains stable (within ±10%) in the high temperature range up to 250 °C, pointing to this mechanism of relaxor phase re-entrance below the normal ferroelectric phase transition, as being responsible for the enhancement.     

Complex Fluids Based on the Flexible One-Dimensional Mineral Polymers [K(MPS4)]∞ (M=Ni,Pd): Autofragmentation to Concave,Cyclic (PPh4)3[(NiPS4)3]

A remarkable autofragmentation/rearrangement sequence results in the unprecedented formation of inorganic concave cyclic anion [(NiPS₄)₃]³⁻ (structure shown on the right) upon dissolving the potassium salt of the charged mineral polymer ¹_∞[NiPS₄]⁻ in DMF; the initial complex fluid has a transient anisotropic texture that can be identified by optical microscopy under polarized light. In contrast, the complex fluid that results upon dissolving ¹_∞[PdPS₄]⁻ is stable up to 323 K as persistent, flexible, charged chains.     

X-ray photoelectron spectroscopy study of neodymium niobate and tantalate precursors and thin films

Neodymium niobate NdNbO₄ (NNO) and tantalate NdTaO₄ (NTO) thin films (~100 nm) were prepared by sol-gel/spin-coating process on Al₂O₃ substrate with LaNbO₄/PbZrO₃ interlayer and annealing at 1000°C. Surface chemistry was investigated by X-ray photoelectron spectroscopy (XPS). The core-level XPS studies of sol-gel NNO and NTO were performed for the first time. The binding energy differences Δ(O―Nb) and Δ(O―Ta) were used to characterize average energies of Nb―O bonding in NNO (322.9 eV) and Ta―O bonding in NTO (504.2 eV). The XPS demonstrated single valence state of Nd (Nd³⁺) in precursors. Nd concentration (at. %) decreases from 22% in precursors to 7% in films considering the substrate contains C, Al, Si, Pb, and Zr elements (37%) at Nb or Ta (5%) and O (51%). The X-ray diffraction analyses verified formation of the monoclinic (M-NdNbO₄ or M′-NdTaO₄), orthorhombic (O-NdNbO₄) and tetragonal (T-NdTaO₄) phases in precursors and films. Single valence state of Nd³⁺ was confirmed in these films designed for the application in environmental electrolytic thin film devices.     

Ab initio and XPS studies of pyrite (1 0 0) surface states

Ab initio quantum chemical modelling (GGA, CASTEP and B3LYP, CRYSTAL03) is used to predict differences in electronic structure between the (1 0 0) surface and bulk of pyrite. Experimental X-ray photoelectron spectroscopic (XPS) data for the S 2p core lines show the presence of two types of S surface states: surface S²⁻ monomers at a S 2p_3/2 binding energy (BE) of 161.2 eV, and (S–S)²⁻ surface dimer states at a S 2p_3/2 BE of 162.0 eV, compared to the S 2p_3/2 BE of bulk pyrite at 162.7 eV. The Fe 2p surface XPS displays several multiplets (implying high spin configuration) at higher BE than the bulk Fe 2p signal, which can be ascribed to surface state contributions. The quantum chemical simulation predicts an S 2p core level shift of 0.69 eV between the S bulk and S surface dimers, in good agreement with the 0.6 eV found in XPS measurements. A Mulliken population analysis confirms the conjectured charge distribution on the surface, which leads to the two different S surface states, as well as the surface high spin configuration responsible for the high BE Fe multiplets. Evidence for surface Fe²⁺ and Fe³⁺ surface states can be seen in the Fe projected valence band density of states, confirming the interpretation of the photoemission spectra.     

Bonding of the chain structure for novel boranes B3k+pH−5k+p+3

The molecular orbitals obtained from conventional quantum chemistry calculations, are expressed in terms of symmetrized valence bond functions of fragment, and a direct picture of chemical bonding can be drawn easily. This method is utilized, together with extended Huckel calculations, to interpret the bonding properties of a centipede-like chain structure for novel laser-producing boranes B_3k+pH^?_5k+p+3 which is constructed from the repeated unit B₃H₅ halted to each other by three B—H—B bonds.     

Electronic structure of lanthanum transition-metal oxyarsenides LaMAsO (M = Fe,Co, Ni) and LaFe1−xM′xAsO (M′ = Co,Ni) by X-ray photoelectron and absorption spectroscopy

The electronic structures of the quaternary oxyarsenides LaMAsO (M = Fe, Co, Ni) were examined with X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Interpretation of the metal 2p_3/2 and arsenic 3d_5/2 binding energies, as well as a satellite feature in the Co 2p XPS spectrum, suggests charges that are much less extreme than expected (i.e., not M²⁺ and As^3?) because of the strong covalent character within the M–As bonds. As M is varied, the differing degrees of charge transfer from M to As atoms within these bonds are manifested by shifts in the As 3d_5/2 binding energies and changes in the As K-edge intensities. This charge transfer is isolated within the [MAs] layer and does not influence the O 1s and La 3d XPS spectra. Fitting the experimental valence band spectra of these oxyarsenides LaMAsO yielded electron populations of states that support the formal charge assignment [La³⁺O^2?][M²⁺As^3?]. The mixed-metal series LaFe_1?xM′_xAsO (M′ = Co, Ni) was examined by XANES; analysis of the metal K- and L-edges, as well as of the Co 2p XPS satellite feature, revealed that no metal–metal charge transfer takes place.     

Electrical Resistivity, Magnetic Susceptibility, X-Ray Photoelectron Spectroscopy, and Electronic Band Structure Studies of Cu2.33−xV4O11

The electronic and physical properties of Cu_2.33V₄O₁₁ were characterized by electrical resistivity, magnetic susceptibility and X-ray photoelectron spectroscopy (XPS) measurements and by tight-binding electronic band structure calculations. Attempts to prepare Cu_2.33−xV₄O₁₁ outside its narrow homogeneity range led to a mixture of Cu_2.33V₄O₁₁, CuVO₃ and β-Cu_xV₂O₅. The magnetic susceptibility data show no evidence for a magnetic/structural transition around 300 K. The XPS spectra of Cu_2.33V₄O₁₁ reveal the presence of mixed valence in both Cu and V. The [Cu⁺]/[Cu²⁺] ratio is estimated to be 1.11 from the Cu 2p_3/2 peak areas, so [V⁴⁺]/[V⁵⁺]=0.56 by the charge balance. Our electronic structure calculations suggest that the oxidation state of the Cu ions is +2 in the channels of CuO₄ tetrahedra, and +1 in the channels of linear CuO₂ and trigonal planar CuO₃ units. This predicts that [Cu⁺]/[Cu²⁺]=1.33 and [V⁴⁺]/[V⁵⁺]=0.50, in good agreement with those deduced from the XPS study.     

A First-Principles Calculation of Electronic Properties of LiNH<Subscript>2</Subscript> and NaNH<Subscript>2</Subscript>

Band spectra, densities of states, total and deformation densities of α-LiNH₂ and α-NaNH₂ are calculated from the first principles using the density functional method in the all-electron approximation. The upper valence band is formed mostly by nitrogen p-states with a small admixture of metal states, the lower conduction bands are formed by the states of all atoms in α-LiNH₂ and mainly by sodium and nitrogen states in α-NaNH₂. The bottom of the conduction band appears in both crystals in the center of the Brillouin zone. α-LiNH₂ exhibits indirect-gap transitions at the absorption edge and three valence band extrema at a short distance of ~0.15 eV from each other. The top of the valence band in α-NaNH₂ appears in the center of the Brillouin zone with the competing maximum at the lateral point at a distance of ~0.06 eV. The electron density distributions testify that polar covalent bonding occur inside the amide anion and ionic bonding occurs between the metal and the amide ion.     

Hopping conduction in a mixed valence oxide system: Pr1−yGdyO1.5+δ

The electrical conductivity of σ-phase Pr_1?yGd_yO_1.5+δ (y = 0.0, 0.1, 0.2, and 0.4) was measured as a function of temperature and oxygen partial pressure. The dependence of the electrical conductivity of Pr_1?yGd_yO_1.5+δ on the composition was determined by combining gravimetric and conductivity data. The results are consistent with the small-polaron model of localized charge carriers hopping between two adjacent Pr ions in different valence states. The following equation was quantitatively examined: σ = ns(1?s)pek_h, where n is the concentration of total Pr ions, s is the fraction of Pr⁴⁺ ions in total Pr ions, p is the probability that cationic positions are occupied by mixed valence ions, and k_h is a constant characteristic of the charge-transfer process between different valence states. It was found that k_h increases slightly with increasing δ. This dependence is discussed in terms of the observed activation energy and the degree of delocalization of the 4f electrons.     

Formal Nickelate(−I) Complexes Supported by Group 13 Ions

Formal nickelate(?I) complexes bearing Group 13 metalloligands (M=Al and Ga) were isolated. These 17 e^? complexes were synthesized by one‐electron reduction of the corresponding Ni⁰→M^III precursors, and were investigated by single‐crystal X‐ray diffraction, EPR spectroscopy, and quantum chemical calculations. Collectively, the experimental and computational data support: 1) the strengthening of the Ni?M bond upon one‐electron reduction, and 2) the delocalization of the unpaired spin across the Ni and M atoms. An intriguing electronic configuration is revealed where three valence electrons occupy two σ‐type bonding interactions: Ni(3d )²→M and σ‐(Ni?M)¹. The latter is an unusual Ni?M σ‐bonding molecular orbital that comprises primarily the Ni 4p_z and M np_z/ns atomic orbitals.     

X-ray spectroscopy of lanthanum manganites: Nature of doping holes,correlation effects,and orbital ordering

The Mn3s X-ray photoelectron spectra of manganites were studied. It was shown that for the formal valence of manganese from 3+ to 3.3+, the doping holes are O2p in character; as the valence of manganese increases further, the Mn3d states acquire holes. For La_0.7Sr_0.3MnO₃, the Mn3p-3d resonance spectra provided information about the occupied and unoccupied Mn3d states, and the correlation energy U = 6.7 eV was determined experimentally. An analysis of X-ray dichroism on the L absorption spectra of three-dimensional La_7/8Sr_1/8MnO₃ showed that the cooperative Jahn Teller distortion of the orthorhombic phase at 240 K was related to (x ² ? z ²)/(y ² ? z ²) type orbital ordering.     

A review: Influence of divalent,trivalent, rare earth and additives ions on Ni–Cu–Zn ferrites

From several decades, researchers were fabricated the materials like ferrites, perovskites, ceramic, metal oxides, chalcogenides, etc. and investigated its properties for numerous useful applications. Among all the materials, ferrites are more useful in technological and industrial application purpose. In modern era of technology, ferrites in the form of a nano scales are be the soul of electronic and electrical components. Due to the noticeable versatile properties like high resistivity, moderate permeability, low dielectric loss and moderate sensitivity researcher's shifts their theoretical and technological point of view from other materials to ferrites. The scientists from chemistry, physics, material science and metallurgy are trying to achieve the desirable electric, magnetic, optical and sensing properties of nanoferrites by optimizing its parameters like chemical composition, synthesis methods, addition of impurity, sintering conditions etc. In this review we investigated an influence of substitution or doping of divalent (Ni²⁺, Cu²⁺, Zn²⁺, Mn²⁺, Mg²⁺ etc.), trivalent (Cr³⁺, Al³⁺, In³⁺, WO³⁺ etc.), rare earth (La³⁺, Sm³⁺, Y³⁺, Nd³⁺, Dy³⁺, Tb³⁺, Tm³⁺ etc.) metal ions and addition of additive (MnO₂, SiO₂, TiO₂, BaTiO₃, PbO, V₂O₅ etc.) in Ni–Cu–Zn ferrites. Researchers successfully transforms the Ni–Cu–Zn properties in a way that, industrialists utilized it for various applicative products such as in Li-Ion battery, super capacitors, EM filters, magnetic recording media, micro-inductors, biofuel production, gas sensors, humidity sensors etc. In nutshell, review is a complete combination of preparation methods, impact of divalent, trivalent, rare earth and additives on Ni–Cu–Zn nanoferrite with its useful applications.     

X-ray and UV photoemission studies of valence electronic structure of NiO

The X-ray and UV photoemission valence band spectra of NiO are interpreted using the molecular orbital theory for the NiO^10?₆ cluster and the sudden approximation (monopole selection rules). They exhibit the effects of crystal field splitting, multiplet splitting, electron shake-up (O 2pe^b_g→ Ni 3de^a_g). relaxation and Ni 3dO 2p hybridization. Shake-up satellite data indicate that the NiO optical absorption edge near 4 eV is associated with an O 2p → Ni 3d transition. The NiO valence electronic structure obtained in this work is compared with band structure models of Wilson and Mattheiss.     

XPS calibration study of thin‐film nickel silicides

This paper presents a systematic X‐ray photoelectron spectroscopy (XPS)study of the Ni silicides Ni₃Si, Ni₃₁Si₁₂, Ni₂Si, NiSi and NiSi₂ produced by annealing of sputtered thin films. The in situ XPS study focuses on both the core level peaks and Auger peaks. The peak positions, shapes, satellites as well as Auger parameters are compared for different silicides. The factors that influence the Ni core level peak shifts are discussed. The Ni 2p_3/2 peak shape and satellites are correlated with the valence band structure. The effect of argon ion etching on surface composition and chemical states is also investigated. Copyright © 2009 John Wiley & Sons, Ltd.     

First-principles calculation of temperature-dependent electronic transitions mechanism in V or Nb substituted BiFeO3

Here, we present a simulation study of temperature-dependent electronic transitions in BiVO₃ (BVO) and BiNbO₃ (BNO) using density functional theory (DFT) together with generalized gradient approximation (GGA) and two-dimensional correlation analysis (2D-CA). The results indicate that heat accumulation can accelerate the degeneracy of V-3d orbital in BVO and the splitting of Nb-4d orbital in BNO at 750 K. We found changes in the type of d–p hybrid orbital as follows, for BVO: V-d_x²_+y² + d_Z²-O-2p_z → V-d_x²_+y²-O-2p_z; and for BNO: Nb-d_x²_+y²-O-2p_z → Nb-d_x²_+y² + d_Z²-O-2p_z. Furthermore, we found changes in the type of hybrid orbital leading to the following electron–electron interactions, for BVO: t_2g (V-d_Z²-O-2p_z) + e_g (V-d_x²_+y²-O-2p_z) → t_2g (V-d_x²_+y²-O-2p_z); and for BNO: t_2g + e_g (Nb-d_x²_+y² + d_Z²-O-2p_z) → t_2g (Nb-d_x²_+y²-O-2p_z) + e_g (Nb-d_z²-O-2p_z). The electronic transitions are determined by a charge-transfer from the occupied O-2p⁴ orbitals to the unoccupied V-3d³ (or Nb-4d³) and Bi-6p³ orbitals. Due to the temperature-dependent electronic structure closely related to these electronic transitions, this study provides a new perspective for the design and improvement of BFO-based temperature-sensitive devices.     

A comparative study of effective core potential and full-electron calculations in Mo compounds. I. An analysis of topological properties of bond charge distribution

Effective core potential (ECP) and full-electron (FE) calculations for MoS₄^?2, MoO₄^?2, and MoOCl₄ compounds were analyzed. Geometry parameters, binding energies, charge distributions, and topological properties of the electronic density were studied for Mo? L bonds (L = S, O, Cl). Results clearly indicate that those approaches that include valence plus 4s and 4p electrons (ECP2 methods) are able to reproduce the topological properties of Mo? L bonds, charge distributions, and geometries with respect to those obtained by FE methods. ECP methods that consider only the 4d and 5s valence electrons (ECP1) fail in the calculation of molecular properties. The use of 5p functions in ECP1 approaches produces a negative Mulliken charge on Mo. Bader's charges give more consistent results than Mulliken's ones. A new parameter for measuring the degree of ionicity is proposed. © 1994 by John Wiley & Sons, Inc.     

Biomimetic synthesis of the arachidic acid/Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>S nanocomposite films

Ag _x Cd _y S nanoparticles were obtained in arachidic acid (AA) monolayer containing Ag⁺ and Cd²⁺ under H₂S flow. The AA/Ag _x Cd _y S monolayers were deposited onto solid substrate to prepare LB films. The UV-vis spectrum showed that the LB film exhibited notable quantum-size effect. The small-angle X-ray diffraction revealed periodic structure of the LB films. The molar ratio of Ag to Cd in AA/Ag _x Cd _y S film was ca. 1 : 5 as measured by the XPS. TEM and FTIR spectroscopy showed that the head-groups of arachidic acid molecules controlled formation of Ag _x Cd _y S nanoparticles in the monolayer.     

Valence state of the Fe ions in Sr1−yLayFeO3

The Sr²⁺_1?yLa³⁺_yFeO₃ system with 0.1 ≦ y ≦ 0.6 has been studied mainly by the Mössbauer effect. The results are discussed referring to the Ca_1?xSr_xFeO₃ system. The following four kinds of electronic phases have been observed: the paramagnetic and the antiferromagnetic average valence phases and the corresponding mixed valence phases. Two kinds of Fe ions coexist, in general, in the mixed valence phases. In the antiferromagnetic mixed valence phase, typically at 4 K, the magnetic hyperfine field and the center shift each takes a wide range of value depending on the composition, while a beautiful correlation is kept between them. The extreme values are close to those expected for Fe³⁺ and Fe⁵⁺. The appropriate chemical formulas are, therefore, Ca_1?xSr_xFe^(3+Δ)+_0.5Fe^(5?Δ)+_0.5O₃ and $\text{Sr}{}_{\mathrm{1?y}}\text{La}{}_{\mathrm{y}}\text{Fe}{}^{\mathrm{(3+\delta )+}}{}_{\mathrm{<mtext>(1+y)</mtext><mtext>2</mtext>}}\text{Fe}{}^{\mathrm{(5?\delta )+}}{}_{\mathrm{<mtext>(1?y)</mtext><mtext>2</mtext>}}\text{O}{}_3$.