Valence States Of Co Research Articles

Magnetic and electrical transport properties of the pyrochlore iridate Bi2-xCoxIr2O7

In the present paper, we have studied the magnetic order and electrical transport properties of frustrated magnet Bi2-xCoxIr2O7 (x = 0, 0.2, 0.4, 0.6) polycrystalline. The behavior of the electrical resistivity above 50 K in the composites emanate from the electron–electron scattering processes. Grain boundary effects play a dominant role in the conduction process. It is also found from M-T data that the antiferromagnetic interaction and frustration enhances with increasing content of Co. Effective magnetic moments show a possibility of mixed valence state of Co (Co3+ and Co4+). The M-H data of doped samples taken at 2 K show hysteresis loops, which suggests the existence of ferromagnetic interaction originated from canted antiferromagnetic state. The magnetic behavior results from the competition between ferromagnetic and antiferromagnetic interaction at each magnetic site.

Structural Distortion in MnO₂ Nanosheets and Its Suppression by Cobalt Substitution.

Co–Mn oxide nanosheets with the chemical composition H0.23Co0.23Mn0.77O2 (C23M77NS) and MnO2 nanosheets (M100NS) were prepared by exfoliation of layer-structured oxides via chemical processing in an aqueous medium. The optical properties of C23M77NS and M100NS were compared using UV-Vis spectroscopy, and the valence states of Mn and Co and local structures around them were examined using X-ray absorption spectroscopy. M100NS with an average Mn valence of 3.6 exhibits large structural distortion, whereas C23M77NS with an average Mn valence of 4.0 does not exhibit structural distortion. Spontaneous oxidization of Mn occurs during ion-exchange and/or exfoliation into nanosheets. These results have originated the hypothesis that structural distortion determines the valence state of Mn in compounds with CdI2-type-structured MnO2 layers.

Crystal Structure Analysis of Electrochemically Magnesiated MgxLi0.1Mn0.54Ni0.13Co0.13O2

INTRODUCTION Recently, the Mg secondary battery has been studied for the purpose of the development of the rechargeable battery with larger energy density than the Li-ion battery. In this study, we focused attention on the Li1.2Mn0.54Ni0.13Co0.13O2, which is a positive electrode material of the high capacity lithium ion battery, to examine the use for the cathode material capable of inserting Mg ions. We have reported the crystal and electronic structures and thermodynamic stability of the chemically delithiated Li1.2-xMn0.54Ni0.13Co0.13O2 (x = 0.3, 0.6, 0.9)1). Although there is no knowledge about the insertion and desorption of Mg for the delithiated material, the high capacity such as shown in the Li ion battery is expected for Mg secondary battery. The purpose of the present study was to evaluate the electrode property of delithiated Li1.2-xMn0.54Ni0.13Co0.13O2 in the Mg secondary battery, to analyze the crystal and electronic structure changes before and after Mg insertion, to reveal the valence change of transition metals, and furthermore to examine the charge/discharge mechanism. EXPERIMENTAL Li1.2Mn0.54Ni0.13Co0.13O2 was synthesized by using a solid-state reaction via co-precipitated precursors. Precipitation was obtained by dissolving each metal nitrate in distilled water and adding it dropwise to an aqueous solution of lithium hydroxide. Based on the predetermined metal ratio of the precipitate, we added lithium hydroxide to the precipitate again, and dried and heated at 650 ºC for 15h in air. After that, the powder was calcined at 900 ºC for 8h in air to obtain a sample. The obtained samples were identified by powder XRD. The metal composition was determined by the ICP-AES. The chemically delithiation treatment was pursued as the previously reported using NO2BF4 in an argon filled glovebox1). The electrochemical measurements were subjected to a charge-discharge cycle test (at 3.0 – 0.5 V vs. Mg/Mg2 +, the negative electrode: AZ31, electrolyte: 0.5 mol/L Mg(N(SO2CF3)2)2 in acetonitrile, separator: polypropylene) at 60 ºC by using the HS cell (Hosen Ltd.). The XAFS (BL14B2, SPring-8) and synchrotron XRD measurements (BL19B2, SPring-8) were performed on the Li1.2-xMn0.54Ni0.13Co0.13O2, i.e. before Mg insertion, and the MgxLi1.2-xMn0.54Ni0.13Co0.13O2 electrode after Mg insertion to evaluate the valence change of transition metals and to analyze the crystal structures by Rietveld analysis (Rietan-FP). RESULTS AND DISCUSSION Chemically delithiated Li0.13Mn0.54Ni0.13Co0.13O2 was synthesized using NO2BF4 for Li1.2Mn0.54Ni0.13Co0.13O2. In order to refine the crystal structure of this sample, synchrotron X-ray diffraction measurement was performed and Rietveld analysis was performed. As a result of the analysis, it was revealed that the 97 % of the pristine monoclinic (C2/m) structure converted to the trigonal (R m) structure after delithiation. As a result of discharging test of this sample with the counter electrode of Mg alloy (AZ31), the discharge capacity was 287 mAh g-1 (Fig.1) which corresponded to 0.52 apfu as the insertion amount of Mg and corresponded to approximately half of chemically eliminated Li. In order to refine the crystal structure of the electrode after electrochemical Mg insertion, synchrotron XRD measurement was performed and Rietveld analysis was performed. As a result of the analysis, it was revealed that the 91% of the delithiated phase remained as trigonal R m structure. In addition, it was revealed that unit cell volume expanded by 2.8% at C2/m and by 2.7% at R m after Mg insertion. From the results of XANES, it was also revealed that the valence states of Mn and Co decreased by about 1 valence after Mg insertion, respectively. Therefore, it is concluded that the discharge mechanism was explained by the reduction of transition metals and the intercalation of Mg, which is supported that the lattice volume expanded due to the reduction of the transition metal by increasing the ionic radius. References 1) N. Ishida, N. Tamura, N. Kitamura, and Y. Idemoto., J. Power Sources, 319, 255-261 (2016) . Figure 1

Directly synthesized V-containing BEA zeolite: Acid-oxidation bifunctional catalyst enhancing C-alkylation selectivity in liquid-phase methylation of phenol

V-containing BEA zeolite (Vβ) with the V/Si molar ratio of 1‰ was hydrothermally synthesized and fully characterized. It delivered the tetrahedral, highly isolated atomically dispersed V species owning the intermediate covalence state between +4 and +5 (closer to +5) within zeolite matrix, endowing oxidation capability without compromising acidity. In the low-temperature liquid-phase methylation of phenol with methanol, Vβ showed high yield (turnover number up to 5946), good reusability and preferential chemoselectivity towards the C-alkylation products (o- and p-cresols). In situ FTIR spectra suggested an acid-oxidation bifunctional mechanism involving the oxidative activation of methanol via formaldehyde. Moreover, alkylation of phenol with various other alcohols was also efficiently catalyzed by Vβ, wherein the large-sized alkylation agency showed special regioselectivity. This work not only achieves selective production of cresols through methylation of phenol at low temperature, but highlights the potential of designing high performance heterogeneous catalyst with highly dispersed active sites via controlling the chemical composition of heteroatom zeolite.

Effect of oxygen intercalation into oxygen-deficient SrFe0.8Co0.2O3−δ thin films

Controlling oxygen contents is an easy way to tune physical properties in the transition metal oxides. In particular, Fe and/or Co containing oxides are often found to show different electronic states with different valence states by the method. In this report, we synthesized oxygen-deficient SrFe0.8Co0.2O3−δ thin films on (001) (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 substrates by pulsed laser deposition and observed the modification of its electronic state by facile low temperature oxidative annealing. As a result of simple oxidative annealing, we achieved a huge decrease of the lattice constant, reduction in electric resistivity, an increase of the valence states of both Co and Fe, and an increase of hybridization between oxygen and transition metal ions without losing the crystallinity.

Mechanochemical Process Enhanced Cobalt and Lithium Recycling from Wasted Lithium-Ion Batteries

Cobalt (Co) and lithium (Li), rare and valuable elements, are mainly used to prepare lithium cobalt oxide (LiCoO2) for applications in lithium-ion batteries (LIBs). Developing an effective method to recover Co and Li from the waste LIBs is of great significance. In the present study, Co and Li were extracted from pure LiCoO2 powders and the extracted cathode materials powders from the waste LIBs after acid dissolution via a mechanochemical reduction process with iron powders. For pure LiCoO2 powders, the effects of Fe to LiCoO2 mass ratio, rotation speed, and mechanochemical reduction time were examined. These parameters influenced positively the extraction of Co, while they showed negligible effects on the leaching of Li. The X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses indicated a promoted extraction of Li arising from the reduction of particle sizes, magnification of specific surface area, and change of the crystal structure of particles. For high-efficiency leaching of Co by the mechanochemical reduction process with iron powders, X-ray photoelectron spectroscopy (XPS) analysis indicated the changes in the valence state of Co. The actual cathode materials disassembled from the wasted LIBs pretreated by this novel mechanochemical reduction process were also explored. The results indicated that the leaching ratios of Li, Co, Mn, and Ni could reach 77.15%, 91.25%, 100%, and 99.9%, respectively. This novel mechanochemical process would be of great importance for the recovery of valuable metals from waste LIBs.

Search for Spinel Type Cathode Materials of Magnesium Secondary Battery By First-Principles Calculation and the Battery Properties, Crystal and Electronic Structure of Cathode Material

INTRODUCTION Currently, the cathode materials of the high-capacity lithium ion battery have been developed. However it is difficult to be tremendous capacity improvement. Therefore divalent magnesium ions have been noted. Due to approximately the same ionic radius of Mg2+ as the Li+, the theoretical capacity of magnesium per volume is higher than lithium, so that magnesium secondary batteries are expected as a next-generation battery. The charge-discharge test of the positive electrode material, such as MgCo2O4, was performed. Although the stable crystal structure and diffusion mechanism of Mg ions is not clear. In this study, the electrochemical evaluation and the crystal structure analysis of the MgCo2O4 and Li2-xCo2O4 were performed and the crystal structure determined by Rietveld analysis. Moreover, the first-principles calculation for MgCo2O4 evaluated the electronic structure especially in the M-O bond. The purpose of this study was to resolve the relationship between the phase stability and Mg/Co mixing in the crystal structure. EXPERIMENTAL MgCo2O4(MCO) was synthesized by reverse co-precipitation method2). Li2Co2O4 was synthesized by solid-state reaction which was chemically delithiated by using the Na2S2O8. The resulting materials were identified by XRD and determined by ICP for the composition of the metals. The charge/discharge test (HS cells : 3.5~0 V vs. Mg/Mg2+; three electrode cells : 0.345 - -0.855V vs. Ag/Ag+, the negative electrode metal Mg or Mg alloy (AZ31), reference electrode : Ag, separator : polypropylene or glass fiber filter, electrolyte : 0.5M-Mg(N(SO2CF3)2)2/CH3CN or Triglyme). These samples were performed by synchrotron X-ray diffraction (BL02B2, SPring-8). It was also investigated for the valence state of Co by XAFS (BL01B1, SPring-8). In addition, the first-principles calculations were performed for the three types of structure models that extends the unit cell of the MCO(VASP-code in 2 × 2 × 1 cell) and were evaluated for binding and the structural stability. RESULTS AND DISCUSSION The XRD for MCO and Li2Co2O4 showed that all of the diffraction peaks were attributed to the spinel structure (S.G.Fd-3m). The Rietveld analysis for the synchrotron X-ray diffraction data showed that Mg and Co in MCO were almost disorderly distributed and cation mixing. Chemically delithitated Li2Co2O4 sample, i.e. Li2-xCo2O4, was synthesized. From the results of charge-discharge test of MCO and Li2-xCo2O4 (counter Mg), the overvoltage of Li2-xCo2O4 was lower than that of MCO, since diffusion path of Mg could be secured by the regular arrangement of Mg/Co. As a result of the charge / discharge test of MCO using three-electrodes cell, we found a plateau of about 2.6 V (Mg/Mg2+) at the discharge (Fig.1). We also investigated the phase stability, Mg/Co mixing, and electronic structure of MCO by first-principles calculations. It was found that the model where the Mg occupied at only 8a site and Co occupied 16d site was stable arrangement. The electron densities between Co-O for all models exhibited higher than those of Mg-O and it means high covalency of Co-O. Therefore, the Mg of the 8a site became weak bond to oxygen to deintercalate. By Mg/Co mixing, the diffusion path of Mg would be inhibited by strong covalent bonds of Co-O. Then, MCO showed larger overvoltage than less mixing of Li2-xCo2O4. It was performed by help of ALCA-SPRING and shows thanks to the members concerned. References 1) N. Kamioka, T. Ichitubo, T. Uda, S. Imashuku, Y. Taninuchi and E. Matsubara, Materials Transactions, 49, 4, pp.824-828 (2008). 2) S.Yagi et al., Jpn. J. Appl. Phys., 52, 025501(2013). Figure 1

Synthesis, Crystal Structure and Electrochemical Properties of Rock-Salt Type (Mg,Ni,Co)O2 As Cathode Materials for Mg Secondary Battery

INTRODUCTION Recently, research has been conducted on high-performance next-generation batteries to replace secondary batteries in practical use because of the limitations of their capacity and safety. One of those currently studied is a secondary battery that uses divalent Mg2+ as a movable ion. The Mg secondary battery has been expected to be a realization of high energy density storage battery. However, the problem to be solved is slow diffusion of Mg2+ within the crystal structure since strong electrostatic interaction between Mg2+ and O2- ions. Thus, the practical application of magnesium secondary battery has not yet. Ideally, layered rock-salt type (MgNiyM1-y)O2 was expected to be synthesized as a cathode electrode material because layered oxides LiNi0.8Co0.2O2 and LiNi1/3Co1/3Mn1/3O2 have already been put to practical use as the positive electrode material of lithium ion battery. Since high electrochemical characteristics of Ni, Co and Mn showed, we have attempted to synthesized the new rock-salt MgxNiyCo1-yO2, to analyze the crystal structure and to evaluate electrochemical characteristics. EXPERIMENTAL The samples were synthesized by the reverse co-precipitation1). After mixing the metal nitrate solution at a predetermined ratio, was added dropwise to aqueous sodium hydrogen carbonate solution, the precipitate was filtered, washed, dried, and calcined. The samples were characterized by XRD and ICP measurements. Particle morphology, particle size and lattice image were observed by SEM and TEM. These samples were performed by synchrotron X-ray diffraction (BL02B2, SPring-8). The data was refined by the Rietveld technique using Rietan-FP program. By maximum entropy method (MEM) using the Dysnomia program, the electron densities were analyzed using synchrotron X-ray diffraction. We also examined the valence state of Ni and Co by XAFS measurement (BL01B1, SPring-8). The electrochemical measurements were subjected to a charge-discharge cycle test (3.5 - 0 V vs. Mg/Mg2 +, 0.345 - -1.955V vs. Ag/Ag +, 4.5 - 1.5V vs. Li/Li+, the negative electrode: AZ31 or metal Li, reference electrode: Ag, electrolytic solution: 1.0 M-Mg(N (SO2CF3)2)2/triglyme or 1M-LiPF6/EC: DMC (1: 2), separator: glass fiber or polypropylene) at 60 ºC or 25 ºC by using the HS cell and 3-electrode flat cell. Some of the samples were ground in a planetary ball-mill, we tried to improve the cathode performance by reducing the grain size. RESULTS AND DISCUSSION From powder X-ray diffractions, obtained samples were found that a product can be attributed to a single phase of the rock-salt structure with space group of Fm-3m. The ICP analysis indicated that Mg composition was increased by increasing the feed Mg amount. As a result of refinement for the site occupancy with Rietveld analysis, it was suggested that the cation defect slightly exists. The TEM images corresponding to the (111) and (200) plane were observed. Moreover, STEM-EDS indicated that Ni and Co are uniformly dissolved in a local region of the product (Fig.1). According to the above, we have succeeded in the synthesis of new rock-salt type MgxNiyCo1-yO2 of the new composition. From the discharge-charge test results, the discharge characteristics vary depending upon the Mg composition and Ni/Co ratio. The XANES spectra showed that the electrode after the initial discharge was somewhat lower valence than the original powder sample. Therefore, it suggested the insertion of Mg into the MgxNiyCo1-yO2 by the electrochemical discharge. It was performed by help of ALCA-SPRING and shows thanks to the members concerned. References (1) S. Yagi et al., Jpn. J. Appl. Phys., 52, 025501 (2013). Figure 1

Effect of Sintering Temperature on Interfacial Structure and Interfacial Resistance for All-Solid-State Batteries

Introduction Bulk-type all-solid-state rechargeable lithium batteries using oxide solid electrolytes (Ox-SSBs) require low-resistive electrode-solid electrolyte composite. Both electrode and solid electrolyte are fragile and hard ceramics and then they need a densification process to enlarge charge transfer reaction sites at the interface. Those composites have been developed by heating processes such as co-sintering [1], spark plasma sintering [2, 3], fusion bonding using low-melting solid electrolyte [4], etc. However, one of the crucial problems to apply such heating process is a formation of mutual diffusion layer at the interface. Formation of mutual diffusion layer can work as a driving force of solid-solid adhesion, but at the same time, that can retard charge transfer reaction at the interface, resulting in the reducing of the battery power density. Therefore, it is important to examine interfacial structure and interfacial resistance by a formation of mutual diffusion layer in detail. This work focuses on LiNi1/3Co1/3Mn1/3O2 (NMC: cathode) / LATP (solid electrolyte) interface. The NMC shows smaller volume change (less than 1%) during charge-discharge reactions among the various positive electrode materials [5]. Also, LATP is one of the highest Li+ conductive solid electrolytes (~10-4 S cm-1) [6]. Here, the NMC films were deposited by pulsed laser deposition (PLD) on mirror-polished LATP sheets (150 μm in thickness, Ohara Inc.) at different temperature as a model system of sintered LATP/NMC interface. Experimental NMC films were deposited on one side of LATP sheets by PLD at 700 °C or 900 °C, which will be denoted as NMC-700 and NMC-900, respectively. Bare side of the LATP sheet was immersed in a liquid electrolyte, where both reference and counter electrodes of lithium were immersed. Electrochemical properties of those NMC/LATP half-cells were measured by cyclic voltammetry (CV) and AC impedance spectroscopy. Structure analyses of NMC/LATP interface were carried out by XRD, TEM, EDX, and EELS. Results and discussion Thin films of NMC prepared at 700 °C or 900 °C on Pt/Ti-coated SiO2 substrates showed almost same redox reactions at around 3.6-3.8 V in liquid electrolyte, and the charge transfer resistance (R ct) at 4.0 V was 200-350 Ω cm2 in both cases. However, they showed quite different reactivity on the LATP sheets. Fig. 1(a) shows CVs of the NMC-700 and NMC-900 measured at 1 mV s-1. The NMC-700 appeared redox peaks at around 3.6-3.8 V as with the film in liquid electrolyte, while the NMC-900 did not show any redox peaks. The R ct of NMC-700 at 4.0 V was 315 Ω cm2, while that of NMC-900 was 114000 Ω cm2. These results suggest that the R ct of NMC/LATP strongly depend on the sintering temperature. Fig. 1(b) shows the EDX line profile of O, Ti, P, Ni, Co, and Mn around the interface of NMC-900. Also, integrated intensity ratio of L edge peaks, L3-edge/L2-edge, in Co (L3/L2-Co) and Mn (L3/L2-Mn) are shown below, which corresponds to the valence state of Co and Mn [7]. A mutual diffusion layer was observed around the interface, but its thickness of 30 nm was same with the case of NMC-700. XRD analysis detected impurity peaks in case of the NMC-900 while not NMC-700/LATP, indicating that mutual diffusion layer is crystallized. Also, both EDX and EELS analyses suggested that Co in the NMC films looks to diffuse toward the LATP as a reduced valence at higher sintering temperature. Based on these results, we will conclude that both structural disorder in NMC around the interface and crystallization of mutual diffusion layer will be key issues to increase the R ct of NMC/LATP interface at higher sintering temperature. Acknowledgement This work was supported by JST-ALCA.

Preparation of SrCoOx thin films on LaAlO3 substrate and their reversible redox process at moderate temperatures

Using magnetron sputtering and annealing techniques, we have prepared SrCoOx films on LaAlO3 and SrTiO3 substrates. Distinctly different structures of the films have been found on the two substrates. It is suggested that positive lattice mismatch between film and substrate promotes SrCoO2.5 films with an orthorhombic structure grown on SrTiO3 substrate, whereas negative lattice mismatch from LaAlO3 substrate is in favor of increasing the valence state of Co and thus the growth of oxygen-rich SrCoOx with a tetragonal structure. In addition to the structural characterization, magnetic and electrical measurements confirm that the oxygen content x is between 2.75 and 2.88 for the latter. Reversibility of the topotactic phase transformation between SrCoO2.5 and the oxygen-rich SrCoOx films has also been studied by changing the oxygen pressure during annealing process. Even in the presence of a negative lattice mismatch, the results reveal that the tetragonal SrCoOx films on LaAlO3 substrate retain high oxygen mobility identified before in cubic SrCoOx films.

Understanding the Origin of Enhanced Performances in Core-Shell and Concentration-Gradient Layered Oxide Cathode Materials.

Core-shell and concentration-gradient layered oxide cathode materials deliver superior electrochemical properties such as long cycle life and outstanding thermal stability. However, the origin of enhanced performance is not clear and seldom investigated until now. Here, a specific structured layered oxide (LiNi0.5Co0.2Mn0.3O2) consisting of concentration-gradient core, transition layer, and stable outer shell, is designed and achieved from double-shelled precursors to overcome the great challenge by comparison with the normal layered LiNi0.5Co0.2Mn0.3O2. As expected, the specific structured layered oxide displays excellent cycle life and thermal stability. After numerous cycles, the valence state of Ni and Co at normal layered oxide surface tends to a higher oxidation state than that of the specific structured oxide, and the spinel phase is observed on particle surface of normal layered oxide. Also, the deficient spinel/layered mixed phases lead to high surface film and charge-transfer resistance for normal layered oxide, whereas the specific structured one still remains a layered structure. Those results first illustrate the origin of improved electrochemical performance of layered core-shell and concentration-gradient cathode materials for lithium-ion batteries.

(Invited) Capabilities of Analytical Transmission Electron Microscopy for the Analysis of Structural, Chemical and Electronic Properties Exemplified by the Study of Y-Doped (Ba,Sr)(Co,Fe)O3-δ

Transmission electron microscopy (TEM) is a standard technique for microstructure studies with a resolution that meanwhile has reached values better than 0.1 nm. Essential is the capability of obtaining images and diffraction patterns from the same small specimen region which facilitates crystal structure determination of small precipitates. Particularly unique is the possibility to perform atomic-scale chemical analyses by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS). Information on the electronic material properties like, e.g., valence states or bond configurations between neighbor atoms is obtained by the electron near edge structure of ionization edges in EELS spectra. In addition to these well-known capabilities, many more imaging modes and evaluation techniques are available which yield additional material properties and render electron microscopy an extremely powerful technique. We will illustrate the capabilities of analytical TEM for the example of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (BSCF) which is a promising and highly complex material for oxygen separation membranes. BSCF suffers from degradation of the oxygen diffusivity at temperatures below 840 oC which is attributed to the decomposition of the cubic phase into several secondary phases [1,2]. Composition and crystal structure analyses of the different phases combined with Co-valence state determination sheds light on the origin of the instability of the cubic phase. Indispensable is the determination Co-valence state in the cubic and hexagonal BSCF phases which required the introduction of a new evaluation technique due to the overlap of the Co-L2,3ionization edges with ionization edges of other elements in BSCF [3]. Doping with Yttrium has been used to stabilize the cubic Sr1−x Y x Co1−y Y y O3−δ phase [4] which also appears to be a promising strategy for BSCF. Substitution of B-site Co- and Fe-cations in BSCF by Y3+-cations with a fixed valence state should lead to the stabilization of the Co-valence and, hence, the stabilization of the cubic BSCF phase. The volume fraction of secondary phases is indeed substantially reduced in Y-doped BSCF. Only the hexagonal BSCF phase is observed which is confined to grain boundaries (Fig. 1). In contrast, a large volume fraction of different secondary phases is found in undoped BSCF (Fig. 2). A closer inspection suggests, however, that the suppression of secondary phases in Y-doped samples could be partially related to the lack of nucleation sites rather than the stabilizing effect of Y-doping. To achieve an improved understanding of the effect of Y-doping, the occupation sites of Y-cations were analyzed by atom location by channeling enhanced microanalysis (ALCHEMI) as proposed by Spence and Taftø [5]. ALCHEMI analyses demonstrate that Y-cations occupy A- and B-sites. Despite substitution of B-site cations, 25 % to 40 % of the Y-cations occupy A-sites depending on the doping level. This indicates the complex effect of Y-doping in BSCF which will be discussed.

First-principles study of structural, electronic and magnetic properties of double perovskite oxides Ba2CoMO6 (M=Mo and W)

Ba2CoMO6 (M=Mo and W) ordered double perovskite oxides have been investigated by using the first-principles of full potential linearized muffin-tin orbital (FP-LMTO) computational method. The structural, electronic and magnetic properties of Ba2CoMO6 were calculated using both the local spin density approximation (LSDA) and generalized gradient approximation (GGA) methods. The room temperature crystal structures of Ba2CoMO6 are face-centered cubic (space group Fm3¯m and tilt system a0a0a0) with lattice constants of (a=8.011Å) and (a=8.031Å) for (M=Mo) and (M=W), respectively. The crystals of Ba2CoMO6 contain alternating CoO6 and MO6 octahedra, almost fully ordered in the basal ab planes. It was shown that the obtained lattice constants agree well with the experimental data. The influence of M-cation on structural, electronic and magnetic properties of Ba2CoMO6 compounds is analyzed. The total and partial densities of states (DOSs) and partial and total spin magnetic moments are calculated, and the valence states of Co and M ions are examined. The magnetic and electronic properties and bond valence sums are consistent with the electronic configuration Co2+ (3d7)–M6+ (4d/5d)0 with Co2+ in high spin states.

High-pressure synthesis, characterization, and equation of state of double perovskite Sr2CoFeO6**Project supported by the National Natural Science Foundation of China (Grant Nos. 51172194 and 51172091), the Program for New Century Excellent Talents in University, China (Grant No. NCET-12-0240), and Jilin Province Science and Technology Development Program, China (Grant No. 20130101023JC).

Double perovskite oxide Sr2CoFeO6 (SCFO) has been obtained using a high-pressure and high-temperature (HPHT) synthesis method. Valence states of Fe and Co and their distributions in SCFO were examined with X-ray photoelectron spectroscopy. The electric transport behavior of SCFO showed a semiconductor behavior that can be well described by Mott’s law for variable-range hopping conduction. The structural stability of SCFO was investigated at pressures up to 31 GPa with no pressure-induced phase transition found. Bulk modulus B0 was determined to be 163(2) GPa by fitting the pressure–volume data to the Birch–Murnaghan equation of state.

First-principles investigation on crystal, electronic structures and Diffusion barriers of NaNi1/3Co1/3Mn1/3O2 for advanced rechargeable Na-ion batteries

A new type of layered transition–metal mixture oxides such as NaFe1/2Mn1/2O2, NaFe1/2Co1/2O2 and NaNi1/3Fe1/3Mn1/3O2 are recently proposed for advanced cathode materials of rechargeable sodium-ion batteries. In this work, the crystal structure, electronic structures and diffusion barrier of NaNi1/3Co1/3Mn1/3O2 are studied by means of density functional theory (DFT) calculation with generalized gradient approximation (GGA) method. The theoretical calculations indicate that the NaNi1/3Co1/3Mn1/3O2 with a 3×3R30°-type superlattice in Wood’s notation based on the triangular basal net of sites in α-NaFeO2-type structure is suggested to be stable. The calculated formation energy is −122meV per a formula unit, which implies that NaNi1/3Co1/3Mn1/3O2 is energetically favorable to form homogeneous solid matrix from NaNiO2(Ni3+), NaMnO2(Mn3+) and NaCoO2 (Co3+). According to the calculation results, the formal valence states of Ni, Co and Mn in the NaNi1/3Co1/3Mn1/3O2 are +2, +3 and +4, respectively. The existence of Jahn–Teller inactive Ni2+ and Mn4+ entails the structural stability of NaNi1/3Co1/3Mn1/3O2. Furthermore, our calculations on the solid-state redox reaction on basis of a superlattice model during sodium ions deintercalation processes indicate that the reactions in the range of 2/3⩽x⩽1, 1/3⩽x⩽2/3 and 0⩽x⩽1/3 in NaxNi1/3Co1/3Mn1/3O2 are the inter-conversion of Ni2+/Ni3+, Ni3+/Ni4+ and Co3+/Co4+, respectively. Due to the lower diffusion energy barrier, the Na+ diffusion rate in NaNi1/3Co1/3Mn1/3O2 is about 10times faster than that in pristine layered NaCoO2 at room temperature.

Epitaxial growth and magnetic properties of layered cobaltite Sr1.5Pr0.5CoO4 thin film

We have studied magnetic behaviour of pulsed laser grown epitaxial thin film of layered compound Sr1.5Pr0.5CoO4 (SPCO) on LaAlO3 [001] substrate. Co-2p core level x-ray photoemission spectra mimics the mixed valence state of Co in the grown film. Arrot plots analysis implies that the magnetization of the system obeys mean field type behavior. Our study suggests that the low temperature magnetic behavior of SPCO can be classified as anisotropic ferromagnetic state, manifested through the analysis of thermo-magnetic irreversibility and cusp in ZFC magnetization (M–T) as well as absence of zero field cooled memory effect. In addition, our study confirms that there is no exchange bias effect in the material. Time dependence of the magnetization relaxation behavior reveals the existence of multiple metastable magnetic states in the system. Slow relaxation of magnetization and aging effect are found to be described by a hierarchical model.

Study on the Conductivity of La<sub>0.4</sub>Sr<sub>1.6</sub>CoNbO<sub>6-δ</sub> as an Electrode Material for Symmetrical SOFC

The double-perovskite La0.4Sr1.6CoNbO6-δ(LSCN) powders were synthesized by the solid-state reaction method. The electrical conductivities of LSCN samples were tested in air and 5 vol%H2/Ar. The results show that the conductivity of LSCN in 5 vol%H2/Ar (8.12 Scm-1) at 850 °C was higher than that in air (7.03 Scm-1). The activation energy obtained from the Arrhenius function was 0.821 eV in air and 0.707 eV in 5 vol%H2/Ar. The analysis of XPS shows that there exit three valence states of Co (Co+2, Co+3, Co+4) and two of Nb (Nb+4, Nb+5). The loss of lattice oxygen in LSCN not only produces oxygen vacancies, but also generates excess electrons, which contributes to the electrical conductivity of the LSCN samples.

Valence states and electronic structures of Co and Mn substituted spin gapless semiconductor PbPdO2

Electronic structures of Pb(Pd0.9T0.1)O2 (T = Mn, Co) spin gapless semiconductors have been investigated by employing soft X-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). The valence states of Co and Mn ions are found to be mixed-valent (∼2.7) and tetravalent, respectively. The measured valence-band PES and O 1s XAS spectra show that both PbPdO2 and PbPd0.9Co0.1O2 are small-gap semiconductors. This finding is supported by the calculated band structures, obtained in the density functional theory with the modified Becke-Johnson potential (mBJ) scheme. This work also shows evidence for the existence of the phase separation in Mn-substituted PbPd0.9Mn0.1O2.

Fast Mapping of the Cobalt-Valence State in Ba0.5Sr0.5Co0.8Fe0.2O3-dby Electron Energy Loss Spectroscopy

A fast method for determination of the Co-valence state by electron energy loss spectroscopy in a transmission electron microscope is presented. We suggest the distance between the Co-L3 and Co-L2 white-lines as a reliable property for the determination of Co-valence states between 2+ and 3+. The determination of the Co-L2,3 white-line distance can be automated and is therefore well suited for the evaluation of large data sets that are collected for line scans and mappings. Data with a low signal-to-noise due to short acquisition times can be processed by applying principal component analysis. The new technique was applied to study the Co-valence state of Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), which is hampered by the superposition of the Ba-M4,5 white-lines on the Co-L2,3 white-lines. The Co-valence state of the cubic BSCF phase was determined to be 2.2+ (±0.2) after annealing for 100 h at 650°C, compared to an increased valence state of 2.8+ (±0.2) for the hexagonal phase. These results support models that correlate the instability of the cubic BSCF phase with an increased Co-valence state at temperatures below 840°C.

Introduction of Cobalt Ions in γ-Fe2O3Nanoparticles by Direct Coprecipitation or Postsynthesis Adsorption: Dopant Localization and Magnetic Anisotropy

The influence of cobalt doping on the magnetic anisotropy of γ-Fe2O3 nanoparticles has been investigated using two different approaches: (i) simultaneous precipitation of Fe2+, Fe3+, and Co2+ precursors in water and (ii) adsorption of Co2+ ions onto the surface of preformed iron oxide particles followed by diffusion in the solid phase upon heat treatment. The incorporation of small amounts of Co dopants, less than 1 at %, was monitored by magnetization measurements combined with X-ray absorption spectroscopy experiments at the Co K-edge. These latter measurements were carried out in fluorescence mode using a crystal analyzer spectrometer for an enhanced sensitivity. Analyses of the X-ray absorption fine structures allowed for unraveling the differences in local atomic structure and valence state of Co in the two series of samples. A thermally activated diffusion in the spinel lattice was observed in the 250–300 °C range, leading to a substantial increase in magnetocrystalline anisotropy. At higher annealing temperature, magnetic anisotropy was still found to increase due to an enhanced surface contribution associated with dehydroxylation of terminal Fe atoms. This study not only provides direct correlations between magnetic anisotropy and dopant localization in Co doped γ-Fe2O3 but also demonstrates for the first time that simultaneous coprecipitation of Fe2+, Fe3+, and Co2+ may actually lead to heterogeneous doping, with a significant part of the Co dopants adsorbed at the particle surface.