Transition Of Co3 Research Articles

Adjusting the Covalency of Metal-Oxygen Bonds in LaCoO3 by Sr and Fe Cation Codoping to Achieve Highly Efficient Electrocatalysts for Aprotic Lithium-Oxygen Batteries.

Developing high-efficiency dual-functional catalysts to promote oxygen electrode reactions is critical for achieving high-performance aprotic lithium-oxygen (Li-O2) batteries. Herein, Sr and Fe cation-codoped LaCoO3 perovskite (La0.8Sr0.2Co0.8Fe0.2O3-σ, LSCFO) porous nanoparticles are fabricated as promising electrocatalysts for Li-O2 cells. The results demonstrate that the LSCFO-based Li-O2 batteries exhibit an extremely low overpotential of 0.32 V, ultrahigh specific capacity of 26 833 mA h g-1, and superior long-term cycling stability (200 cycles at 300 mA g-1). These prominent performances can be partially attributed to the existence of abundant coordination unsaturated sites caused by oxygen vacancies in LSCFO. Most importantly, density functional theory (DFT) calculations reveal that codoping of Sr and Fe cations in LaCoO3 results in the increased covalency of Co 3d-O 2p bonds and the transition of Co3+ from an ordinary low-spin state to an intermediate-spin state, eventually resulting in the transformation from nonconductor LCO to metallic LSCFO. In addition, based on the theoretical calculations, it is found that the inherent adsorption capability of LSCFO toward the LiO2 intermediate is reduced due to the increased covalency of Co 3d-O 2p bonds, leading to the formation of large granule-like Li2O2, which can be effectively decomposed on the LSCFO surface during the charging process. Notably, this work demonstrates a unique insight into the design of advanced perovskite oxide catalysts via adjusting the covalency of transition-metal-oxygen bonds for high-performance metal-air batteries.

Magnetooptical Properties of Chiral [Co2Ln] Clusters.

Two pairs of enantiomeric 3d-4f metal clusters, [Co2Ln[(R)/(S)-L]4]·Cl5·(H2O)2·CH3OH·CH3CH2OH [Co2Ln; Ln = Gd (1S and 1R), Dy (2S and 2R)], were synthesized by the reaction of chiral Schiff-base ligand (R)/(S)-3-[(2-hydroxybenzylidene) amino]propane-1,2-diol [(R)/(S)-HL] with CoCl2·6H2O and LnCl3·6H2O. The circular dichroism spectra of (S)/(R)-Co2Ln display a mirror-symmetry effect with seven peaks at 210-800 nm, which can be ascribed to π-π* transitions, exciton coupling, charge-transfer transitions between ligands and Co3+, and characteristic d-d transitions of Co3+ ions. Interestingly, the chiral Co2Ln metal clusters display strong magnetic circular dichroism signals at room temperature. This work suggests that the chiromagnetic metal cluster is expected to show a strong magnetooptical response.

Anomalous Thermal Behaviour of Mixed Cobaltites-Ferrites and Cobaltites-Chromites

Series of the mixed cobaltites-ferrites RCo1−xFexO3 (x = 0.1–0.9) and cobaltites-chromites RCo0.5Cr0.5O3 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er) with the orthorhombic perovskite structure (space group Pbnm) have been prepared from stoichiometric amounts of rare earth oxides, Co3O4, Fe2O3 and Cr2O3 by solid-state reaction in air at 1473–1573 K. In situ high-temperature X-ray synchrotron powder diffraction revealed anomalous lattice expansion of both series, which is reflected in a sigmoidal dependence of the unit cell dimensions and in abnormal anisotropic increase of the thermal expansion coefficients (TEC). Depending on the composition, several broad maxima in the temperature range of 500−1000 K are observed, which are evidently connected with the transitions of Co3+ cations to the higher spin states. The observed anomalies in the mixed cobaltites-ferrites and cobaltites-chromites are less pronounced compared with the corresponding “pure” cobaltites.

Ferrimagnetic ordering and spin entropy of field-dependent intermediate spins inNa0.82CoO2

The peculiar field-dependent magnetism of Na0.82CoO2 has been investigated through an analysis of its DC and AC spin susceptibilities. To account for the easily activated narrow b2g-a1g gap of the crystal field for Co in the cobalt oxide layer, the spin-state transition of Co3+ (3d6) between the low spin (LS) state b2g^2-a1g^0 of S=0 and the intermediate spin (IS) state b2g^1-a1g^1 of S=1 is thus seen as thermally activated and exhibits a Boltzmann distribution. The IS state of Co3+ within each sqrt(13a) hexagonal superlattice formed by the S=1/2 state of the Co4+ ions appears randomly within each supercell and shows significant temperature and field dependence. The magnetic field is found to assist in pinning down the thermally activated state of Co3+ and swings the Boltzmann distribution weight toward a higher fraction of the IS state. The field dependence of the in-plane magnetic moment from the added number of S=1 spins is used to explain the origin of A-type antiferromagnetic (AF) ordering, particularly that the ferromagnetic (FM)-like behavior below TN at low field is actually a ferrimagnetic IS spin ordering of Co3+.

Structural Behaviour of EuCoO<sub>3 </sub>and Mixed Cobaltites-Ferrites EuCo<sub>1</sub><sub>−</sub><sub>x</sub>Fe<sub>x</sub>O<sub>3</sub>

Crystal structure of the mixed cobaltites-ferrites EuCo1−xFexO3, obtained by solid state reaction in air at 1573 K, has been studied by means of X-ray powder diffraction technique. It was found that all samples synthesized adopt orthorhombic perovskite structure. Obtained values of structural parameters indicate formation of continuous solid solution in the EuCoO3−EuFeO3 system. Thermal behaviour of the EuCoO3 structure has been investigated in the temperature range of 298–1173 K by means of in situ high-resolution powder diffraction technique applying synchrotron radiation. Strong anomalies in the lattice expansion have been detected between 580 K and 800 K, which are evidently connected with the transitions of Co3+ ions to the higher spin states and further metal-insulator transition occurred in EuCoO3 at about 600 K.

Influence of CoO bond length on the spin state of Co in cobaltite perovskites RCoO3 (R = Y, Ho)

A comparative study of the electronic structure and magnetic properties of YCoO3 and HoCoO3 has been carries out using ab initio density-functional calculations with local spin density approximation (LDSA) and LSDA + U methods. It is found that YCoO3 shows nonmagnetic (spin S=0) insulator character below 600 K and an electronic transition of Co3+ ions from a low spin (LS) state to a high-spin (HS) state above 600 K. In contrast, HoCoO3 shows nonmagnetic property with Co ions in a LS state over a wide temperature range (300-995 K). The intermediate-spin (IS) state becomes more stable than the LS state only a temperature of 1098 K in HoCoO3. We suggest that the variation of Co-O bond length induced by the unit cell expansion may play an-important role in the evolution of the spin state of Co3+ with increasing temperature in these perovskite cobalt oxides. The discrepancy of the averaged Co-O bond length in YCoO3 and HoCoO3 may be a likely reason for the difference of electronic properties between them. The decrease of the Co-O bond length could result in stabilization of the LS state. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Magnetic properties and photoinduced magnetism on Y0.25Sr0.75CoO3–δ

AbstractMagnetic properties and light‐induced magnetism on Y0.25Sr0.75CoO3–δ under different sintered temperatures have been investigated. A magnetization jump around 179 K (TJ) with a thermal hysteresis was found out in Y0.25Sr0.75CoO2.688 sintered at 1250 °C. Whereas, there is a hysteresis at the onset of 145 K without magnetization jump appears in Y0.25Sr0.75CoO2.626 sintered at 1100 °C together with the larger magnetization and lower Curie temperature. The large magnetization may come from the contribution of the surface/volume ferromagnetism in Co3+ ions. A light‐induced spin‐state transition of Co3+ ions from the nonmagnetic low‐spin state ($ {\rm t}^{6}_{\rm 2g} {\rm e}^{0}_{\rm g} $); S = 0) to magnetic intermediate‐spin state ($ {\rm t}^{5}_{\rm 2g} {\rm e}^{1}_{\rm g} $; S = 1) was observed in Y0.25Sr0.75CoO2.688, which increases the magnetization below TJ. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic, electrical, and thermal characterization of La0.9Te0.1Mn1−xCoxO3(0 ≤x≤ 1)

The structure, magnetizationM, resistivity ρ, thermoelectric powerS, and thermal conductivity κ in La0.9Te0.1Mn1−xCoxO3(0 ≤x≤ 1) have been investigated systematically. The samples withx= 0 andx= 1 have a rhombohedral lattice with space groupR¯3C, while the samples withx= 0.25, 0.50, and 0.75 have an orthorhombic lattice with space groupPbnm. The samples of 0 ≤x≤ 0.75 undergo the paramagnetic–ferromagnetic (PM–FM) phase transition. Based on the temperature dependence of susceptibility, a combination of the high-spin (HS) state for Co2+and the low-spin (LS) state for Co3+can be determined. The metal–insulator transitions (MIT) observed forx= 0 sample are completely suppressed with Co-doping, and ρ(T) displays semiconducting behavior within the measured temperature region forx> 0 samples. Asx⩾ 0.25, the huge magnitude of Seebeck coefficient at low temperatures is observed, which is suggested to originate from the spin-state transition of Co3+ions from intermediate-spin (IS) state or (HS) state to (LS) state and the configurational entropy of charge carriers enhanced by their spin and orbital degeneracy between Co2+and Co3+sites. Particularly,S(T) ofx= 0.50 and 0.75 samples appears an anomalous peak, which is suggested to be related to the contribution of phonon drag. Similar toM(T) and ρ(T), all results ofS(T) are discussed according to the variations of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, based on the analysis of the temperature dependence ofS(T) and ρ(T), the transport mechanism can be determined in the different temperature region. As to thermal conduction κ(T), the changes of κ with Co-doping is suggested to come from the combined effect due to the suppression of local Mn3+O6Jahn–Teller (JT) lattice distortion because of the substitution of non JT Co3+ions with LS and HS states for JT Mn3+ions, which results in the increase of κ, and the introduction of the disorder due to Co-doping, which contributes to the decrease of κ.

Effect of cobalt substitution on magneto-transport properties of Nd0.7Sr0.3Mn1-xCoxO3 (0.0⩽x⩽1)

Magnetic and transport studies of the compounds Nd0.7Sr0.3Mn1-xCoxO3 (0.0⩽x⩽1) have been carried out. The compositions crystallize in single-phase ortho-perovskite. The conduction mechanism could be explained using the small polaron correlated hopping model. The thermal irreversibility in zero-field and field-cooled magnetization data increases with Co content, which is due to the different spin states of the Co. Transition of Co3+ from high spin∕intermediate spin to low spin state is observed at low temperatures. The nonsaturating M-H behavior is observed in cobalt containing samples due to suppression of double-exchange interaction which favors long-range ferromagnetic order.

Ultrasonic studies of the spin-state transition and the local distortion in LaCoO3

Temperature dependence of longitudinal and transverse ultrasonic velocities and attenuations of the single-phase polycrystalline LaCoO3 were presente d. A clear softening in the longitudinal ultrasonic velocity accompanied by a sharp peak in attenuation were observed near the spin-state transition temperature(≈90K), which did not appear in the transverse mode measurement. This anomalous ultraso nic behavior further demonstrates that the spin-state transition of Co3+ ions in LaCoO3 near 90K are more likely from a low spin state (LS,t62ge 0g) to an intermediate spin state (IS,t5 2ge1g ) than to a high s pin state (HS,t42ge2g ). A b ig attenuation peak accompanied by a slight softening in velocity was observed near 200K in both longitudinal and tr ansverse modes, which was probably caused by the relaxation of the local distort ions originating from Jahn-Teller effect of the randomly distributed IS state Co 3+ ions.

Photoinduced spin-state transition ofCo3+ in the layered perovskite manganite thin film

The photoinduced effect, at different temperatures, on layered perovskite manganiteLa1.2Sr1.8Mn1.8Co0.2O7 thin filmdeposited on a SrTiO3(100) substrate has been studied using a He–Ne laser (632.8 nm) with the power ofoutput 25 mW at low temperatures. The electrical transport and magneticproperties of the film are investigated systematically in the temperature regionfrom 20 to 300 K. The resistance shows a significant decrease at low temperatures(T<90 K) under laser irradiation, which can be interpreted in terms of the spin-state transition ofCo3+ ions. The magnetic field combined with the laser is also used to detect the photoinducedeffect of the spin-glass state. The time dependence of the metastable system underseveral applied fields was also discussed. The system has the ability to keep apersistent memory of the magnetic history imprinted in its zero-field resistance.

Uniaxial stress effect on the 1A1 → 1T 1 transition of Co3+ in α-Al2O 3

Uniaxial stress effect on the 1A1 → 1T 1 transition of Co3+ in α-Al2O 3