Valence States Of Co Research Articles

Co-doped Y 2O 3 optical functional nanoparticles and novel self-assembly squama-like aggregates

Co-doped Y 2O 3 optical functional nanoparticles with size of 4–10 nm have been synthesized by the plasma arc-discharge method. Meanwhile, novel three-dimensional squama-like macro-aggregates were self-assembled by disordered nanoparticles synthesized simultaneously in the arc-discharge process. The formation mechanism of the aggregates is ascribed to the periodic coagulation of Y nanoparticles. The valence states of Co in Y 2O 3 lattice can be determined as +2 from XPS spectrum with an etching depth of 6 nm. For Co-doped Y 2O 3 nanoparticles, upon excitation with 426 nm, a red emission (620 nm) has been observed which originates from 4A 2( 4F) → 4T 1( 4P) d–d transitions of Co 2+ ions in tetrahedral co-ordination.

Pb-for-Bi substitution for enhancing thermoelectric characteristics of [(Bi,Pb)2Ba2O4±ω]0.5CoO2

We report strongly enhanced thermoelectric characteristics for a misfit-layered oxide, [Bi2Ba2O4±ω]0.5CoO2, in a wide temperature range, as achieved through substituting up to 20% of Bi by Pb. The Pb substitution kept the thermal conductivity (κ) unchanged but decreased the electrical resistivity (ρ) and increased the Seebeck coefficient (S) simultaneously, such that a three-fold enhancement in the thermoelectric figure of merit, Z (≡S2∕ρκ), was realized. At the same time x-ray absorption near-edge structure data indicated that the valence and spin states of Co are not affected by the Pb-for-Bi substitution.

X-ray absorption near-edge spectroscopy study of Mn and Co valence states inLaMn1−xCoxO3(x=0–1)

A systematic study of the valence states of Mn and Co in the perovskite series ${\mathrm{LaMn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{3}\phantom{\rule{0.3em}{0ex}}(x=0\phantom{\rule{0.3em}{0ex}}\mathrm{to}\phantom{\rule{0.3em}{0ex}}1)$ by means of x-ray absorption near edge spectroscopy (XANES) at the K-edges is presented. The chemical shift and the evolution of the pre-edge features reveal a gradual increase of the average oxidation level of both, Mn and Co ions, with Co doping, which suggests that mixed valence states of ${\mathrm{Co}}^{2+}∕{\mathrm{Co}}^{3+}$ and ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$ exist in the whole solid solution range. The relation of the results to the magnetic properties of the compounds is discussed.

Electronic structure ofCa3CoXO6(X=Co, Rh, Ir) studied by x-ray photoemission spectroscopy

We have obtained Co $2p$, Rh $3d$, Ir $4f$, and valence-band x-ray photoemission spectra of ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$, ${\mathrm{Ca}}_{3}\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_{6}$, ${\mathrm{Ca}}_{3}\mathrm{Co}\mathrm{Ir}{\mathrm{O}}_{6}$, and ${\mathrm{Sr}}_{3}\mathrm{Zn}\mathrm{Ir}{\mathrm{O}}_{6}$ in order to determine the valence states of Co. The spectral features are consistent with the picture that Co is in the trivalent state at both trigonal prismatic and octahedral sites with high- and low-spin states, respectively, in ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$, whereas Co is in the high-spin ${\mathrm{Co}}^{2+}$ state for ${\mathrm{Ca}}_{3}\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_{6}$ and ${\mathrm{Ca}}_{3}\mathrm{Co}\mathrm{Ir}{\mathrm{O}}_{6}$. This conclusion resolves a recent controversy regarding the valence and spin states of Co in ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{Ca}}_{3}\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_{6}$. This is the first x-ray photoemission spectroscopic study of this family of oxides.

Ab initio study of the effects of Mg doping on electronic structure of Li(Co , Al)O2

Electronic structures of Li(Co, Al)O2 and Mg-doped Li(Co, Al)O2 were studied theoretically via ab initio calculation based on density-functional theory. Studies of band structures and density of states show that hole states appear in the valence bands of the Mg-doped material as well as an increase in bandwidth, which are in charge of the increase in the observed electronic conductivity. It was found by analysis of Co3d density of states that the divalence Mg doping induces a higher cobalt valence state between Co3+ and Co4+ in Li(Co, Al, Mg)O2, which is in good agreement with experimental results. Using the band theory, we have calculated the average valence states of Co and O.

Cobalt valence states and origins of ferromagnetism in Co doped TiO2 rutile thin films

Co doped rutile thin films were fabricated on α-Al2O3 (10-12) substrates by laser molecular beam epitaxy. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy indicated that the rutile thin films are (101) oriented and have smooth surfaces with no impurity phases. Co K-edge x-ray absorption near-edge structure, extended x-ray absorption fine structure, and x-ray photoelectron spectroscopy revealed the coexistence of different valence states of Co in the film. Magnetic circular dichroism studies suggest that the observed ferromagnetism is uniform and is related to the electron band structure of TiO2 rutile. Contribution of oxidized (Co2+) and metallic (Co0) cobalt to the ferromagnetism is discussed.

Electronic Structure of Li(Co, Mg)O2 Studied by Electron Energy-Loss Spectrometry and First-Principles Calculation

Electronic structures of Mg-doped LiCoO2 were studied both experimentally via electron energy-loss spectroscopy (EELS) and theoretically via ab initio calculations based on density-functional theory. The Co L2,3 edge and O K edge were employed to probe effects of Mg doping on the electronic structure of Co 3d and O 2p states. According to the EELS results, Co tends to be a mixed-valence state, and O becomes more closed-shell characteristic with Mg doping. Quantum-mechanical calculations are performed using a total-energy pseudopotential code, CASTEP. Density of states and partial density of states are calculated, which are consistent with EELS spectra. Both experimental and theoretical evidence of the change in the valence state of both Co and O are revealed. A charge balance mechanism in Mg-doped LiCoO2 is suggested.

Anomalous barium filling fraction and n-type thermoelectric performance of BayCo4Sb12

Barium-filled skutterudites BayCo4Sb12 with an anomalously large filling fraction of up to y=0.44 have been synthesized. The lattice parameters increase linearly with Ba content. Magnetic susceptibility data show that Ba0.44Co4Sb12 is paramagnetic, which implies that some of the Co atoms in BayCo4Sb12 have acquired a magnetic moment. The presence of the two different valence states of Co (Co3+ and Co2+) leads to the anomalously large barium filling fraction even without extra charge compensation. All samples show n-type conduction. The electrical conductivity increases with increasing the Ba filling fraction. The lattice thermal conductivity of BayCo4Sb12 is significantly depressed as compared to unfilled Co4Sb12. The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature reaching a maximum value of 1.1 for Ba0.24Co4Sb12 at 850 K.

Investigation of the permeation behavior and stability of a Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ oxygen membrane

SrCo 0.8Fe 0.2O 3− δ (SCFO) and Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCFO) oxides were successfully prepared, using a combined citrate–EDTA complexing method. The results of O 2-TPD and XRD showed that the introduction of barium into SCFO could effectively suppress the oxidation of Co 3+and Fe 3+ to higher valence states of Co 4+ and Fe 4+ in the lattice, and stabilize the perovskite structure under lower oxygen partial pressures. Oxygen permeation experiment showed that BSCFO membrane also had higher oxygen permeation flux than that of SCFO under air/He oxygen partial pressure gradient. At 950°C, the permeation flux through 1.80 mm BSCFO membrane exposed to flowing predried air ( P O 2 ′=0.21 atm) and helium ( P O 2 ″=0.037 atm) is ca. 1.4 ml/cm 2 min and the activation energy for oxygen transportation is 40.9 kJ/mol within the temperature range of 775–950°C. The permeation flux of BSCFO was less sensitive to minor amounts of CO 2 and water vapor presented in the air than that of SCFO. Long-term oxygen permeation study of more than 1000 h at 850°C indicated that the BSCFO membrane could operate stably as an oxygen generator at that temperature. A very slow exponential decay in the measured oxygen permeation flux occurred at temperatures lower than 825°C, which was caused by a phase transition. The phase transition was found to be reversible at higher temperatures, but a long time was needed for the equilibration.

LCAC‐SW quantum scattering dynamic study on the ion‐pair formation process

AbstractLCAC‐SW method has been extended to study the reaction dynamics for ion‐pair formation processes. M+ X2→Mt + X2 reaction system involves two potential energy surfaces, i.e., the covalence state (M + X2) and the ionic state (M + X2) and their crossing effect. The working equations for calculating state‐to‐state probability have been derived based on the above two‐state model, Satisfied results d collinear state‐to‐state probabilities for K+ I2 → K+ + I2 ion‐pair formation system have been obtained.

An extended LCAC–SW quantum scattering study on the ion-pair formation reactions

In this Letter the LCAC–SW method has been extended to study the reaction dynamics for ion-pair formation processes on an M+X 2→M ++X 2 − reaction system involving two potential energy surfaces, i.e., the covalence state (M+X 2) and the ionic state (M ++X 2 −), and their crossing effect. The working equations for calculating the state-to-state probability have been derived based on the above two-state model, and the collinear state-selected probabilities for M+I 2→M ++I 2 − (M=Na, K, Cs) ion-pair formation systems have been obtained.

Mapping the Valence States of Transition-Metal Elements Using Energy-Filtered Transmission Electron Microscopy

The properties of transition-metal oxides are related to the presence of elements with mixed valences, such as Mn and Co. Spatial mapping of the valence-state distribution of transition-metal elements is a challenge to existing microscopy techniques. In this letter, using the valence-state information provided by the white lines observed in electron energy-loss spectroscopy in a transmission electron microscope (TEM), an experimental approach is demonstrated to map the valence-state distributions of Mn and Co using the energyfiltered TEM. An optimum spatial resolution of 2 nm has been achieved for two-phase Co oxides with sharp boundaries. This provides a new technique for quantifying the valence states of cations in magnetic oxides. Many physical and chemical properties of inorganic materials are determined by the elements with mixed Valences in the structural unit, 1 by which we mean that an element has two or more different valences while forming a compound. Transitionand rare-earth-metal elements with mixed valences are unique for initiation of electronic, structural, and/or chemical evolutions. We have demonstrated previously the valence states of Mn and Co in their oxides by applying electron energy-loss spectroscopy (EELS) for quantitative determination. 2-5 In EELS, the L ionization edges of transition-metal, rare-earth, and actinide compounds usually display sharp peaks at the near-edge region. These threshold peaks are known as white lines. The unoccupied 3d states form a narrow energy band, the transition of a 2pstate electron to the 3d levels, leading to the formation of white lines observed experimentally. EELS experiments have shown that a change in the valence states of cations introduces significant variation in the intensity ratio of the white lines, giving the possibility of identifying the occupation number of 3d or 4d electrons (or cation valence states) using the measured white line intensities. 6,7 The information obtained using EELS is an integration over the spatial region illuminated by the incident electron beam. In this paper, we introduce a new experimental approach which can give the distribution maps of cation valences in real space, allowing a direct identification of cations with different valence states. This is useful for studying nanocomposite magnetic oxide materials with mixed valences.

Cobalt valence and crystal structure of La0.5Sr0.5CoO2.25

Abstract Oxygen vacancies play a key role in determining the functionality and performance of many oxide smart materials. In this paper, the crystal structure of an anion-deficient perovskite-type orthorhombic (with a ≈ b) phase La0.5Sr0.5CoO2.25 (or La8Sr8Co16)36) has been determined for the first time with the combined use of energy-dispersive X-ray spectroscopy, electron-energy-loss spectroscopy, high-resolution transmission electron microscopy and electron diffraction. The unit cell is made of two types of fundamental module, and it is composed of a total of eight modules. Each module is a c-axis stacking of the anion-deficient SrCoO3-z and LaCoO3-δ basic perovskite cells. The unit cell preserves the characteristics of the perovskite framework, and it is a superstructure induced by O vacancies. This study illustrates the correlation of anion deficiency with the valence state of Co, and it also proves that the O atom positions can be determined using the combination of transmission electron microscopy and associated techniques. This is likely to be an effective method for probing the microstructure of perovskite-type functional materials.

Absolute differential cross sections for electron-impact excitation of CO near threshold: I. The valence states of CO

Absolute differential cross sections for the electronic excitation of valence states in CO have been measured in the energy range from threshold to 3.7 eV. In particular, a newly developed experimental technique to control the analyser's efficiency allowed us to measure absolute angular dependences of those states for scattering angles between and . The transitions from the ground state to the and states are forbidden by spin selection rules. It is found that the shape resonance located at 1.8 eV partly decays into these valence states, which are located several eV above the resonance. The decay occurs by ejection of a core electron whose spin is antiparallel to that of the incident electron causing a sharp rise of the cross sections just at threshold. Three new core-excited shape resonances were found, one for each of , and symmetry. By comparison with recently calculated cross sections their electron configurations have been classified. Resonant excitation via these three resonances or via the low-lying resonance dominates the cross sections of the two spin forbidden transitions in the detected energy range. The excitation of the optically allowed transition, however, is dominated by non-resonant processes.

Vibrational-state-dependent decay of the CO C(1s) excitation.

Molecular ionization and ionic fragmentation of core excited CO valence and Rydberg states are studied by partial-ion-yield spectroscopy at vibrational resolution. The vibrational intensities of the Rydberg excitations differ significantly depending on the ionization and/or fragmentation channel. The observed intensity variations are explained by nondissociative excited final ionic states of the subsequent molecular Auger decay. The vibrational levels of these final states become selectively depopulated due to interaction with dissociative decay channels leading to ionic fragmentation.

The k3Π Valence State of CO

The high-resolution VUV k3Π(v = 2, 3, 5) ← X1Σ+(v = 0) "forbidden" absorption bands of CO have been observed and rotationally analyzed in the wavenumber range 90 000-98 000 cm−1, confirming the recent 3Π assignment for the upper valence state given by Baker et al. (Chem. Phys.178, 569-579 (1993)). It is also shown that the k3Π2(v = 3) spin-orbit component is responsible for the indirect predissociation at J = 31 in the E1Π(v = 0) state, while the k3π0(v = 5) spin-orbit component is responsible for the indirect predissociation at J = 7 in the E1Π(v = 1) state.

K XANES studies of 3d transition metal dopants in YBa2(Cu1-xMx)3O7- delta ; x

In continuation of an earlier report on the EXAFS studies (see ibid., vol.4, p.6865 (1992)) the authors present the results of K XANES measurements on the 3d dopants, Mn to Ni and Zn, in the 1-2-3 lattice. It is found, perhaps for the first time, that the Mn dopant in a 1-2-3 lattice is in a divalent state. It is inferred from the K XANES features that Mn occupies the Cu(1) chain site in the 1-2-3 lattice. The results of consolidated K XANES measurements on the valence states of Fe, Co, Ni and Zn dopants are consistent with the earlier reports on these heavy 3d ions in 1-2-3 compounds.

High-Tc superconductors studied by57Co,57Fe,119Sn, and151Eu m�ssbauer spectroscopy

151Eu,119Sn,57Fe, and57Co Mossbauer spectroscopy was used to study YBa2Cu3O7−δ high-Tc superconductors in which sites were replaced by various Mossbauer nuclides in order to get information about site preference and structural changes. By decomposition of the Mossbauer spectra, the Y site substituted by Eu and the Cu sites occupied by Sn, Fe, and Co substituents were identified. It was found that Fe and Co prefer the Cu(1) site while Sn prefers the Cu(2) site in the orthorhombic perovskite lattice of these superconductors. The site preference is enhanced in the presence of additional substituents. A sequence of thermal heat treatments permits reversible cycling of57Co between the two copper sites in the YBa2[Cu(57Co)]3O7−δ perovskite. In all cases Eu(III) and Sn(IV) states are determined, but the valence state of Fe and Co can be both three and four. BelowT c, low-temperature phase transformation and phonon softening were shown from the anomalous temperature dependence of isomer shifts and the area fractions of151Eu and119Sn Mossbauer spectra of EuBa2(Cu1-xSnx)3O7-δ cuprates.

Valence control of Fe and CO ions in LPE garnet films by substitution of Ca or Ge

Results are presented for the valence control of Fe and Co ions in magnetic garnets for Ca- or Ge-substituted YIG. The measurement of spectral dependence of Faraday rotation was found to be a very sensitive method for observation of the valence state of Co and Fe ions in garnet films. It is observed that the values of the distribution coefficients for uncompensated Ca or Ge are three times lower than those obtained in the compensated state (simultaneous Ca and Ge substitution).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electron energy-loss spectroscopy of carbon monoxide using a new position-sensitive multidetector spectrometer. I. The energy region 6-10.9 eV

An electron impact spectrometer incorporating a novel position-sensitive multidetector has been used to study the excited electronic states of carbon monoxide. This paper presents results for the energy region from 6 to 10.9 eV. Many new lines are observed, some of which correspond to high vibrational levels of known electronic states and some of which have been provisionally assigned to new valence states of CO. The resolution with which the measurements are made is sufficient for the effects of rotational excitation to be observed.