Spin-state Transition Of Co Research Articles

Influence of oxygen stoichiometry on [formula omitted] spin state transitions and magnetic properties in [formula omitted] cobaltites

The layered EuBaCo2O5+δ (δ= 0.47) (EBCO) cobaltite shows various magnetic phases at high temperature and magnetic glassy state at low temperature. Further, the various magnetic phases of the EBCO are coupled with the spin state transition of Co3+ octahedra. In the present study, we report the structural and magnetization properties of the EBCO by changing the oxygen stoichiometry. The various oxygen compositions were obtained after post annealing the as-prepared EBCO cobaltite under argon and oxygen atmospheres and the final stoichiometry was detected from the iodometric titrations. The δ= 0.13 and 0.67 samples show the tetragonal symmetry (space group, P4/mmm), whereas the δ= 0.41, 0.47 and 0.58 samples show the orthorhombic symmetry (space group, Pmmm). The magnetization measurements on the δ= 0.13 sample shows the paramagnetic to antiferromagnetic ordering at TN= 288 K and the electrical resistivity analysis shows the Co2+ and Co3+ charge ordering at TCO= 180 K. Further, the δ= 0.13 sample show the absence of the ferromagnetic phase and Co3+ spin state transitions. The δ= 0.58 and 0.67 samples show the coexistence of ferromagnetic and antiferromagnetic phases, however, the features of Co3+ intermediate spin (IS) to low spin (LS) state transition is suppressed, which is observed in δ= 0.47 sample at 151 K. Our results indicate a tuning of the crystal structure along with the magnetic properties and also the Co3+ spin state transitions by oxygen stoichiometry variation in EuBaCo2O5+δ cobaltites.

Proposal of spin crossover as a reversible switch of catalytic activity for the oxygen evolution reaction in two dimensional metal-organic frameworks.

The development of oxygen evolution reaction (OER) catalysts with high activity and controllability is crucial for clean energy conversion and storage but remains a challenge. Here, based on first-principles calculations, we propose to utilize spin crossover (SCO) in two-dimensional (2D) metal-organic frameworks (MOFs) to achieve reversible control of OER catalytic activity. The theoretical design of a 2D square lattice MOF with Co as nodes and tetrakis-substituted cyanimino squaric acid (TCSA) as ligands, which transforms between the high spin (HS) and the low spin (LS) state by applying an external strain (∼2%), confirms our proposal. In particular, the HS-LS spin state transition of Co(TCSA) considerably regulates the adsorption strength of the key intermediate HO* in the OER process, resulting in a significant reduction of the overpotential from 0.62 V in the HS state to 0.32 V in the LS state, thus realizing a reversible switch for the activity of the OER. Moreover, the high activity of the LS state is confirmed by microkinetic and constant potential method simulations.

Tailoring Spin State of Perovskite Oxides by Fluorine Atom Doping for Efficient Oxygen Electrocatalysis.

Promoting the initially deficient but economical catalysts to high-performing competitors is important for developing superior catalysts. Unlike traditional nano-morphology construction methods, this work focuses on intrinsic catalytic activity enhancement via heteroatom doping strategies to induce lattice distortion and optimize spin-dependent orbital interaction to alter charge transfer between catalysts and reactants. Experimentally, a series of different concentrations of fluorine-doped lanthanum cobaltate (Fx -LaCoO3 ) exhibiting excellent electrocatalytic activity is synthesized, including a low overpotential of 390mV at j= 10mA cm-2 for OER and a large half-wave potential of 0.68V for ORR. Meanwhile, the assembled rechargeable Zn-air batteries deliver an excellent performance with a large specific capacity of 811 mAh/gZn under 10mA cm-2 and stability of charge/recharge (120h). Theoretically, taking advantage of density functional theory calculations, it is found that the prominent OER/ORR performance arises from the spin state transition of Co3+ (Low spin state (LS, t2g 6 eg 0 ) → Intermediate spin state (IS, t2g 5 eg 1 ) and the mediated d-band center upshift by F atom incorporation. This work establishes a novel avenue for designing superior electrocatalysts in perovskite-based oxides by regulating spin states.

Spin State and Spin‐State Transition of Co3+ Ion in BiCoO3

Confusion has arisen about the Co spin‐state transition under high pressure for multiferroic BiCoO3. The crystal structure analysis indicates a high‐spin (HS) to low‐spin (LS) state transition with increasing pressure, while Co Kβ X‐ray emission spectra present a high‐spin to intermediate‐spin (IS) state transition. Herein, the spin state of Co3+ in BiCoO3 is investigated utilizing soft X‐ray absorption spectroscopy (XAS) at the Co‐L2,3 and O‐K edges at room temperature under ambient pressure. Furthermore, the spin state transition of Co3+ under high pressure is investigated by Co Kβ X‐ray emission spectra. A pure HS‐Co3+ ground state is found in the BiCoO3 sample prepared from the precursor and oxidizer mixed method—and the existence of a small part of LS‐Co3+ in the BiCoO3 sample prepared from the precursor and oxidizer sandwiched method. A complete HS‐Co3+ to LS‐Co3+ transition is observed under high pressure up to 32.3 GPa. The results clarify the debate on the spin‐state transition of BiCoO3 under high pressure and will deepen our understanding of the intrinsic physical properties on it.

Optimized Conductivity and Spin States in N-Doped LaCoO3 for Oxygen Electrocatalysis.

The spin state of antibonding orbital (eg) occupancy in LaCoO3 is recognized as a descriptor for its oxygen electrocatalysis. However, the Co(III) cation in typical LaCoO3 (LCO) favors low spin state, which is mediocre for absorbing oxygen-containing groups involved in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), thus hindering its further development in electrocatalysis. Herein, both experimental and theoretical results reveal the enhancement of bifunctional electrocatalytic activity in LaCoO3 by N doping. More specifically, electron energy loss spectroscopy and superconducting quantum interference devices magnetic analysis demonstrate that the Co(III) cation in N-doped LaCoO3 (LCON) achieves a moderate eg occupancy (≈1) compared with its low spin state in LaCO3. First-principle calculation results reveal that N dopants play a bifunctional role of tuning the spin-state transition of Co(III) cations and increasing the electrical conductivity of LCO. Thus, the optimized LCON exhibits an OER overpotential of 1.69 V at the current density of 50 mA/cm2 (1.94 V for pristine LCO) and yields an ORR limiting current density of 5.78 mA/cm2 (4.01 mA/cm2 for pristine LCO), which offers a new strategy to simultaneously modulate the magnetic and electronic structures of LCO to further enhance its electrocatalytic activity.

Valence-state and spin-state transition of Co in LaCo0.5Rh0.5O3

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Low-spin Co3+ make great contributions to the magnetism of BiFeO3

Various spin states of Co3+ were tested in Co-doped BiFeO3 (BFO) through first-principle method, aimed to reveal the role of spin state transition of Co3+ in developing ferrimagnetism. It is found that low-spin (LS) Co3+ contributed much more to resultant magnetic moment than high-spin (HS) Co3+ (~ 3.914 µB). To explore the electronic origins of the enhanced magnetism, a more detailed analysis is carried out including electronic configuration of HS and LS Co3+ as well as Co–O hybridization. It is demonstrated that the LS Co3+ which is nonmagnetic could destroy the cycloid–dal spin structure of BFO. Moreover, LS Co3+ has vacant 3d orbitals so that the p–d covalency is enhanced and the local ferrimagnetism is improved. The structural origin of the Co3+ spin-state transition is ascribed to the distortion of CoO6 octahedra. To explore the influence of Co doping experimentally, the 8% Co-doped and un-doped BFO powders were prepared. The Hysteresis loops indicated that the substitution of Co led to a slight increase of magnetization, i.e. HS Co3+ made limited contribution to the magnetism of BFO.

Relation between the Co-O bond lengths and the spin state of Co in layered Cobaltates: a high-pressure study

The pressure-response of the Co-O bond lengths and the spin state of Co ions in a hybrid 3d-5d solid-state oxide Sr2Co0.5Ir0.5O4 with a layered K2NiF4-type structure was studied by using hard X-ray absorption and emission spectroscopies. The Co-K and the Ir-L3 X-ray absorption spectra demonstrate that the Ir5+ and the Co3+ valence states at ambient conditions are not affected by pressure. The Co Kβ emission spectra, on the other hand, revealed a gradual spin state transition of Co3+ ions from a high-spin (S = 2) state at ambient pressure to a complete low-spin state (S = 0) at 40 GPa without crossing the intermediate spin state (S = 1). This can be well understood from our calculated phase diagram in which we consider the energies of the low spin, intermediate spin and high spin states of Co3+ ions as a function of the anisotropic distortion of the octahedral local coordination in the layered oxide. We infer that a short in-plane Co-O bond length (<1.90 Å) as well as a very large ratio of Co-Oapex/Co-Oin-plane is needed to stabilize the IS Co3+, a situation which is rarely met in reality.

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+.

Effect of oxygen content on transport and magnetic properties of PrBaCo2O5.50+δ

Samples of PrBaCo2O5.50+δ (δ=−0.15–0.14), synthesized by solid-state reactions, were investigated to ascertain oxygen compositional effects on transport/magnetic properties. Resistivity decreases with increasing oxygen content, indicative of p-type conduction. A metal-insulator transition was observed at 330K only for sample PrBaCo2O5.52, coinciding with phase transition and spin-state transition of Co3+. When δ deviates from zero, samples show insulator–insulator transitions, although for sample δ=0.14, no transition occurs but only semi-conductive behavior appears. Electronic transport is governed by the hopping mechanism at lower temperatures and thermal activation at higher temperatures. All samples underwent paramagnetic–ferromagnetic–antiferromagnetic transitions, except PrBaCo2O5.64, which only went through a paramagnetic–ferromagnetic transition. The ferromagnetic state for δ<0 originates with the Co3+/Co2+ super-exchange interaction; for δ>0, it stems from the Co3+/Co4+ double exchange interaction.

An enhancement ZT and spin state transition of Ca3Co4O9 with Pb doping

We reported the structural, electrical, thermal and magnetic properties of Pb-doped Ca3Co4O9 in the range of 300–5K. DTA analysis showed that the stability of the Ca3Co4O9 was increased with Pb doping. According to XRD analysis, it is found that Pb ions were successfully doped in the Ca3Co4O9 structure. The temperature of resistivity minima, Tmin, increased by increasing the Pb doping level and it is seen that incommensurate spin density wave state becomes more stable with Pb doping. The enhancement of thermopower was explained that Pb doping in Ca sites caused a decrease of Co4+ fraction such that Co4+ ions transformed into Co3+ or Co2+. The room temperature ZT value of the polycrystalline sample reaches about 16 times larger value than that of the un-doped polycrystalline sample which is the promising candidate for high temperatures in the thin film applications.According to magnetic susceptibility measurement, the increase of effective magnetic moment by Pb concentration was explained by spin state transition of Co3+ from low spin to intermediate spin and high spin state together with some orbital angular momentum contribution which comes from 5T2 term due to a decrease of the ligand field splitting energy.

Spin states and glassy magnetism in LaCo1−xNixO3 (0 ≤ x ≤ 0.5)

We investigated the effect of Ni substitution on electronic structure and magnetic properties of perovskite LaCoO3 in the substitution range 0 ≤ x ≤ 0.5. A homovalent +3 state and spin state transition of Co+3 has been observed upon Ni substitution in x-ray absorption measurements at the Co and the Ni K-edges. Thermally driven spin state transition has been found to disappear with Ni substitution. A change in nature of magnetic interactions from antiferromagnetic to ferromagnetic and spin glass behavior with substitution is observed in dc and ac magnetization measurements. Ni substitution has been found to lower the average effective magnetic moment which has been ascribed to the decrease in Co/Ni ratio. Changes in fine structure and magnetic properties due to Ni substitution have been explained through the stabilization of intermediate spin state of Co+3 by the lattice expansion induced changes in crystal field. The Jahn-Teller distortion is assumed to be suppressed in the expanded lattice and possibility of antiferro-orbital ordering has been proposed for the ferromagnetic super-exchange interactions Co+3(IS)–O–Co+3(IS). The present work provides possible explanation for the ambiguity in the origin of FM in Co and Ni based perovskite cobaltites as well as supports the idea of lattice expansion induced ferromagnetism.

Preparation of Cathode Material for Co-Sintering with Electrolyte at High Temperature

Ti-doped La-Sr-Co-Fe-based oxides (LSTCFs) were prepared as cathode material for co-sintering with electrolyte at high temperatures. LSTCFs have relatively high solid solubility limits of Ti up to 80mol%. Before reaching the solubility limits, LSTCFs were well densified and single phase with a rhombohedral structure (R-3c). Meanwhile, after exceeding the solubility limits, a Ti-rich secondary phase appeared and microstructures tend to be porous. Electrical conductivity and thermal expansion coefficient (TEC) for LSTCF decreased with increasing Ti content. Ti4+appears to work as a donor and suppress electronic spin state transition of Co3+ ions and formation of oxygen vacancies. The composition, such as La0.6Sr0.4Ti0.4(Co0.2Fe0.8)0.6O3-δ (LSTCF28) with 40 mol%Ti and La0.6Sr0.4Ti0.6(Co0.8Fe0.2)0.4O3-δ (LSTCF82) with 60 mol%Ti, showed low sinterability and sufficient thermomechanical compatibility with GDC electrolyte. Those, however, possess more than two order of magnitude lower electrical conductivity than those of typical cathode materials.

Electronic Structures of LaCoO3-Based Oxides Studied by Soft X-ray Absorption Spectroscopy under Controlled Temperatures and Oxygen Partial Pressures

In situ soft X-ray absorption spectroscopic measurements were performed to evaluate electronic structures of LaCoO3. X-ray absorption spectra at Co L II- and L III-edges were measured under p(O2) = 10-4 bar at room temperature to 873 K. Depending on temperature, the shape and the peak energy of the absorption spectra were varied, indicating thermally induced spin state transition of Co3+ ion. The results demonstrated that our in situ soft X-ray absorption technique is an useful tool to investigate the electronic structures of components for solid oxide fuel cells under operating conditions.

Effect of Mn on the electrochemical properties of a layered perovskite NdBa0.5Sr0.5Co2 − xMnxO5 + δ (x = 0, 0.25, and 0.5) for intermediate-temperature solid oxide fuel cells

NdBa0.5Sr0.5Co2−xMnxO5+δ (x=0, 0.25, and 0.5) specimens are synthesized and investigated as cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The interaction between two different transition metals, Mn and Co, may affect the electronic conducting properties and electrochemical performance. The substitution of Mn for Co lowers the thermal expansion coefficient (TEC) of Co-based materials by suppression of the spin state transition of Co3+ which might consequently improve the mechanical compatibility with the electrolyte. The electrochemical performance, however, could be slightly degraded by trapping of the effective mobile charge with increasing Mn content. Such a substitution of Co by Mn would provide a tradeoff relationship between TEC and electrochemical performance in IT-SOFCs. In this paper, the tradeoff between electrochemical and thermal expansion is explored.

Evolution of the spin-state transition with doping in La1−xSrxCoO3

The thermally induced spin-state transition of Co${}^{3+}$ ions in the cobaltite LaCoO${}_{3}$, found at temperatures in the range 40 to 120 K, has been the subject of extensive experimental and theoretical investigation. Much less is known about what happens to the spin-state transition in hole-doped La${}_{1\ensuremath{-}x}$Sr${}_{x}$CoO${}_{3}$ (LSCO). The present ${}^{139}$La NMR experiments show that spin-state transitions persist in nanoscale hole-poor regions of the inhomogeneous doped material. In fact, thermally induced spin-state transitions remain important for doping levels close to the metal-insulator critical concentration of ${x}_{C}=0.17$. This finding suggests that the unusual glassy behavior seen in doped LSCO for $x&lt;0.18$ results from the interplay of spin-state transitions in hole-poor regions and ferromagnetism in hole-rich regions.

Tunable strain effect and ferroelectric field effect on the electronic transport properties of La0.5Sr0.5CoO3 thin films

Tensiled La0.5Sr0.5CoO3 (LSCO) thin films were epitaxially grown on piezoelectric 0.67Pb (Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-PT) single-crystal substrates. Due to the epitaxial nature of the interface, the lattice strain induced by ferroelectric poling or the converse piezoelectric effect in the PMN-PT substrate is effectively transferred to the LSCO film and thus reduces the tensile strain of the film, giving rise to a decrease in the resistivity of the LSCO film. We discuss these strain effects within the framework of the spin state transition of Co3+ ions and modification of the electronic bandwidth that is relevant to the induced strain. By simultaneously measuring the strain and the resistivity, quantitative relationship between the resistivity and the strain was established for the LSCO film. Both theoretical calculation and experimental results demonstrate that the ferroelectric field effect at room temperature in the LSCO/PMN-PT field-effect transistor is minor and could be neglected. Nevertheless, with decreasing temperature, the ferroelectric field effect competes with the strain effect and plays a more and more important role in influencing the electronic transport properties of the LSCO film, which we interpreted as due to the localization of charge carriers at low temperature.

Insulator–metal transition driven by pressure and B‐site disorder in double perovskite La2CoMnO6

The ground state of double perovskite oxide La2CoMnO6 (LCMO) and how it is influenced by external pressure and antisite disorder are investigated systematically by first-principles calculations. We find, on the consideration of both the electron correlation and spin-orbital coupling effect, that the LCMO takes on insulating nature, yet is transformed to half metallicity once the external pressure is introduced. Such tuning is accompanied by a spin-state transition of Co(2+) from the high-spin state (t(5)2ge(2)g) to low-spin state (t(6)2ge(1)g) because of the enhancement of crystal-field splitting under pressure. Using mean-field approximation theory, Curie temperature of LCMO with Co(2+) being in low-spin state is predicted to be higher than that in high-spin state, which is attributed to the enhanced ferromagnetic double exchange interaction arising from the shrinkage of Co-O and Mn-O bonds as well as to the increase in bond angle of Co-OMn under pressure. We also find that antisite disorder in LCMO enables such transition from insulating to half-metallic state as well, which is associated with the spin-state transition of antisite Co from high to low state. It is proposed that the substitution of La(3+) for the rare-earth (RE) ions with smaller ionic radii could open up an avenue to induce a spin-state transition of Co, rendering thereby the RE2CoMnO6 a promising half-metallic material.

Spin-state transition in Ba2Co9O14

Ba${}_{2}$Co${}_{9}$O${}_{14}$ is a charge-ordered Co${}^{2+/3+}$ cobaltite that consists of building blocks of CdI${}_{2}$-type Co${}^{2+/3+}$O${}_{6/3}$ layers, face-shared Co${}^{3+}$${}_{3}$O${}_{12}$ octahedral trimers, and corner-sharing Co${}^{2+}$O${}_{4}$ tetrahedra. The Co-${L}_{2,3}$ x-ray absorption spectroscopy spectrum at room temperature indicates a high and a low spin state for Co${}^{2+}$ and Co${}^{3+}$ ions, respectively. Measurements of high-temperature resistivity and thermoelectric power revealed an insulator-to-insulator phase transition at ${T}_{t}$ $=$ 570 K, above which the inverse magnetic susceptibility $\ensuremath{\chi}$${}^{\ensuremath{-}1}$($T$) deviates from the Curie-Weiss law. By using a combination of soft x-ray absorption spectroscopy at the O-K edge and high-resolution synchrotron x-ray powder diffraction measurements on crossing ${T}_{t}$, we have successfully determined the origin of this phase transition as a low-to-higher spin-state transition of Co${}^{3+}$ ions within the face-shared Co${}_{3}$O${}_{12}$ octahedral trimers in Ba${}_{2}$Co${}_{9}$O${}_{14}$.

Influence of Gd Doping on Magnetic Behavior in La0.67Sr0.33CoO3, La0.67Sr0.33MnO3

Abstract M-T curves, and M-H curves of La 0.67 -x Gd x Sr 0.33 CoO 3, La 0.67 -x Gd x Sr 0.33 MnO 3 ( x =0.00, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.67) systems were measured. The results show that with increasing of doping Gd, magnetic structure of the La 0.67 -x Gd x Sr 0.33 CoO 3 system exhibits cluster glass state, and the M-T curves of samples with x ≥0.10 exhibit a peculiar phenomenon that M value rises sharply in a low temperature range; magnetic structure of La 0.67 -x Gd x Sr 0.33 MnO 3 system changes from long-rang ferromagnetic order to cluster glass state, anti-ferromagnetic state, and the M-T curves of samples with x ≥0.50 decline sharply in a low temperature range. The different phenomena shown by the two systems come from different coupling function between Gd and Co, Mn and from spin state transition of Co.