Spin-state Transition Research Articles

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.

Theoretical analysis of resonant inelastic x-ray scattering spectra of Ca2RuO4

We calculate the resonant inelastic x-ray scattering (RIXS) spectra of a $4d$-electron Mott insulator ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ that exhibits spin-orbit entangled magnetic ordering. Considering the Ru ${L}_{3}$-edge RIXS, we identify four different types of excitations: magnetic dipolar transitions, quadrupolar transitions changing total angular momentum by $\mathrm{\ensuremath{\Delta}}J=2$, spin-state transitions with $\mathrm{\ensuremath{\Delta}}S=1$, and ${t}_{2g}\ensuremath{\rightarrow}{e}_{g}$ orbital transitions across the crystal-field and Hund's coupling derived levels. We provide analytical expressions for the magnetic transitions and the $\mathrm{\ensuremath{\Delta}}J\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}2$ transitions in the tetragonal compression limit, which are generally applicable to other compounds with ${t}_{2g}^{4}$ configurations in a square lattice. Intriguingly, we find that upon tetragonal compression the dipole-forbidden $\mathrm{\ensuremath{\Delta}}J=2$ transitions acquire large spectral weights with pronounced polarization dependencies, which are consistent with the experimental observations. Our numerical simulations show that this is due to a constructive interference between dipolar and quadrupolar scattering channels.

Room-temperature valence transition in a strain-tuned perovskite oxide

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

Tailoring the Optical and Electronic Properties of 2D Hybrid Dion–Jacobson Copper Chloride Perovskites

The upsurge of low-dimensional Dion–Jacobson (DJ) phase perovskites has brought significant interest in view of their appealing stability against harsh environmental conditions as well as their promising performance in optoelectronic applications. Few reports to date have concentrated on the fundamental relationship of fine-tuning the control of diamine-based perovskite single crystals toward their electronic properties and optical behaviors. Here, we demonstrate that cationic control is proposed to regulate the role of hydrogen bonding of organic ligands with the edge-sharing [CuCl6]4– octahedral layers, leading to strong differences in the material excitonic profile and tunability of their electronic properties. Interestingly, we observe a significant reduction of photoluminescence intensity upon controlling the Cu2+/Cu+ proportion in this hybrid system. According to the photoemission measurements, variation in the oxidation states of Cu cations plays a crucial role in stabilizing the diammonium-based perovskite geometric structure. Interestingly, we find that the electronic signatures of the singlet spin-state and high-energy region transition are not influenced by the thermal effect, as probed by temperature-dependent X-ray absorption spectroscopy (XAS) at elevated temperature. Density functional calculations suggest that such an electronic difference originates from the hydrogen bonding reduction that altered the magnitude of the octahedral distortion within the DJ layered structure. As a result, the 3+NHC4H9NH3+ conformation produces a non-negligible interaction toward tuning the optical and electronic properties of DJ copper-based perovskites.

Continuous Modulation of Electrocatalytic Oxygen Reduction Activities of Single-Atom Catalysts through p-n Junction Rectification.

Fine-tuning single-atom catalysts (SACs) to surpass their activity limit remains challenging at their atomic scale. Herein, we exploit p-type semiconducting character of SACs having a metal center coordinated to nitrogen donors (MeNx ) and rectify their local charge density by an n-type semiconductor support. With iron phthalocyanine (FePc) as a model SAC, introducing an n-type gallium monosulfide that features a low work function generates a space-charged region across the junction interface, and causes distortion of the FeN4 moiety and spin-state transition in the FeII center. This catalyst shows an over two-fold higher specific oxygen-reduction activity than that of pristine FePc. We further employ three other n-type metal chalcogenides of varying work function as supports, and discover a linear correlation between the activities of the supported FeN4 and the rectification degrees, which clearly indicates that SACs can be continuously tuned by this rectification strategy.

Understanding the Effect of Nickel Doping in Cobalt Spinel Oxides on Regulating Spin State to Promote the Performance of the Oxygen Reduction Reaction and Zinc–Air Batteries

Metal-doped cobalt spinel oxides are considered to be effective materials to improve the oxygen reduction reaction (ORR) performance. However, the doping sites of nickel in Co3O4 spinel (octahedral sites or tetrahedral sites) remain controversial, leading to a poor understanding of the effects of nickel. Herein, directionally introducing nickel into the octahedral sites of Co3O4 (NiOh-Co3O4) is proposed by exploiting the different redox capabilities of different nickel sources, which yields an encouraging ORR and rechargeable Zn–air battery performance. Magnetic measurements and theoretical calculations reveal that the nickel occupied at the octahedral site can effectively promote the spin-state transition from low spin to higher spin states for cobalt ions and the hybridization of Co 3d–O 2p electrons, resulting in the optimization of the adsorption strength for the oxygen intermediates, thus improving the oxygen catalytic activity of NiOh-Co3O4.

Preparation of [Fe(paptH)2] (CF3SO3)2 and [Fe(paptH)2] [Fe(CN)5NO] (paptH = bis(2-(pyridin-2-ylamino)-4-(pyridin-2-yl)thiazole: Magnetic, Mössbauer, and Electronic Spectral Properties

Complexes of [Fe(paptH)2][X]2.nH2O, (paptH = 2-(pyridin-2-ylamino)-4-(pyridin-2-yl)- thiazole, X = CF3SO3 and n = 1.5, and X2 = [Fe(CN)5NO] and n = 3) have been characterized in magnetic, Mössbauer, and electronic spectral properties. The studies reveal that the two complexes are predominantly quintet state at room temperature, but singlet at low temperature (below 91K). The change is assigned as a thermally spin state transition, singlet 1A1g (LS, low-spin) ↔ quintet 5T2g (HS, high-spin), in iron(II), being gradual, continuous, and incomplete at the experimental temperature range, and without hysteresis in these instances. The two complexes were yellowish brown at room temperature (~ 298K) but darkening to reddish brown at the lower temperatures.

Quantitative decorating Ni-sites for water-oxidation with the synergy of electronegative sites and high-density spin state

Oxygen evolution reaction (OER) is the rate-limiting step in water-splitting. The spin-state transition of H2O/OH- to O2 is the critical contributor to slow kinetics, but has not yet received insufficient attention, and model catalysts with explicit active sites are scarce. Herein, we regulated the electronic structure and spin state density of Ni-sites by introducing transition metals into Ni-MOF (NiM-MOF, M = Cr, Mn, Fe, Co, and Cu). Theoretical calculations and experimental results indicated that OER activity has a volcanic relationship with the d‐band center, where NiFe-MOF was at the volcano summit, showing an ultra-low overpotential of 172 mV at 10 mA cm−2, which was currently one of the highest catalytic activities. Benefiting from the synergistic effect of electronegativity sites and high-density spin state, balanced the adsorption/desorption of intermediates, and minimized the energy barrier for OER. This work provides a new reference for understanding the catalytic mechanism and designing spin electrocatalysts.

Tuning the spin state of Fe single atoms by Pd nanoclusters enables robust oxygen reduction with dissociative pathway

<h2>Summary</h2> The development of Fe single-atom catalysts is greatly impeded by their lower oxygen reduction reaction (ORR) performance in acid electrolytes when compared with Pt/C catalyst. Herein, we report an unprecedented ORR catalyst consisting of Pd nanoclusters (Pd_NC) and Fe single atoms (Fe–N–C/Pd_NC). Experimental investigations and theoretical calculations indicate that the enhanced ORR activity results from the Pd_NC-induced spin-state transition of Fe(II) from low spin to intermediate spin, activating O–O bond through the side-on overlapping and achieving the pathway transition from associative to dissociative process. The over-binding of OH∗ can be facilely released to suppress the site-blocking effect and accelerate the removal of OH∗. In acid conditions, Fe–N–C/Pd_NC exhibits excellent ORR performance with impressive half-wave potential (0.87 V), insignificant decay after 30,000 cycles, and a high maximum power density of 920 mW/cm² in fuel cell, superior to Pt/C catalyst. This work provides an efficient pathway to design non-Pt catalysts through the spin-regulation strategy.

Excitonic Correlations, Spin-State Ordering, and Magnetic-Field Effects in One-Dimensional Two-Orbital Hubbard Model for Spin-Crossover Region

The electronic properties of excitonic insulators have been examined precisely in recent years. Pictures of exciton condensation may be applied to the spin-state transition observed in perovskite cobalt oxides. We examine the crystal-field and magnetic-field dependences of spatial spin structures on the basis of the density matrix renormalization group method using an effective model for the one-dimensional two-orbital Hubbard model in strong-coupling limit. We find an excitonic insulating (EI) phase and a spin-state ordering (SSO) phase in the intermediate region between low-spin and high-spin phases. In the EI phase, spin-triplet excitons are spatially fluctuating due to quantum effects, and an incommensurate spin correlation realizes. The analyses of a spin gap and degeneracy of entanglement spectra suggest the realization of the Haldane-like edge state in the EI phase. In the SSO phase, 3-fold or incommensurate SSO structures realize depending on the crystal-field splitting. These structures are stabilized as a result of the competition of exchange interactions between spin states.

Pressure Tuning of Coupled Structural and Spin State Transitions in the Molecular Complex [Fe(H2B(pz)2)2(phen)

The large volume change, which accompanies the molecular spin crossover (SCO) phenomenon in some transition metal complexes, prompts frequently the coupling of the SCO with other instabilities. Understanding the driving mechanism(s) of such coupled phase transitions is not only important for fundamental reasons but also provides scope for the development of multifunctional materials. The general theoretical expectation is that the coupling has elastic origin, and the sequence of transitions can be tuned by an externally applied pressure, but dedicated experiments remain scarce. Here, we used high-pressure and low-temperature single-crystal X-ray diffraction to investigate the high-spin (HS) to low-spin (LS) transitions in the molecular complexes [FeII(H2B(pz)2)2(bipy)] and [FeII(H2B(pz)2)2(phen)]. In the bipyridine complex, the SCO is continuous and isostructural over the whole T, P-range (100-300 K, 0-2 GPa). In the phenanthroline derivative, however, the SCO is concomitant with a symmetry-breaking transition (C2/c to P1̅). Structural analysis reveals that the coupling between the two phenomena can be tuned by external pressure from a virtually simultaneous HSC2/c-LSP1̅ transition to the sequence of HSC2/c-LSC2/c-LSP1̅ transitions. The correlation of spontaneous strain and order parameter behaviors highlights that the "separated" transitions remain still connected via strain coupling, whereas the "simultaneous" transitions are partially split.

Robust metal-insulator transition in "112″-layered double perovskites Y1−xLnxBaCo2O5.50±δ (Ln = Sm, Eu and Gd; x = 0, 0.5 and 1)

We report the synthesis, structural, magnetic, electrical transport, magnetoelectric and thermoelectric properties of ″112″-layered double perovskite Y1−xLnxCo2O5.5 (Ln = Sm, Eu and Gd; x = 0, 0.5 and 1) cobaltites. YBaCo2O5.50 exhibits simultaneous structural, magnetic and electrical transitions around 300 K. Whereas LnBaCo2O5.50 (Ln = Sm, Eu and Gd) exhibit well separated TC and TIM, respectively, below and above 300 K. It is expected from the chemical viewpoint that the substitution of Ln by Y in LnBaCo2O5.50 should result in proximity of TC and TIM. However, our investigation demonstrates that though TC shows significant upward shift, TIM exhibits robustness. This suggests that the electrical and magnetic transitions are decoupled. The large MR [(ρH-ρ0)/ρ0] values (~50%) at low temperatures are observed for Eu and Gd-phases. The low temperature electrical conduction mechanism is of variable range hopping (VRH) type and the thermoelectric properties suggest the p-type polaronic conductivity or hole like carriers in the samples. The observed results can be understood in the framework of electronic phase separation and the metal-insulator transition from viewpoint of structural transition instead of the spin state transition.

Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand.

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)2Pd(CN)4]·nG (furpy: N-(pyridin-4-yl)furan-2-carboxamide, G = H2O,EtOH (A·H2O,Et), and H2O (A·H2O)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A·H2O,Et and an abrupt single-step SCO for A·H2O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A·H2O,Et characterized by a unique FeII center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A·H2O shows a triclinic symmetry with two inequivalent FeII centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.

Combine effect of doping and strain on the thermodynamics, ferroelectric, electronic, and magnetic properties of PbTiO3

Multiferroic materials become prominent as they utilize both the charge and spin degree of electrons, which have tremendous scope in solid-state electronic-based devices. Herein, the combined effect of transition metals (TM = Fe, Co, and Ni) doping and biaxial ([110]) strain on the thermodynamics, electronic, and multiferroic properties of PbTiO3 (PTO) perovskite oxide is investigated by the inclusion of Hubbard parameter U. Formation energetics computed by utilizing the Convex Hull analysis, confirm the structural stability of the TM-doped motifs, while elastic properties affirm the mechanical regularity. In addition, positive frequencies of the phonon dispersion bands maintained the dynamical cohesion of the systems. Our results revealed that the magnitude of spontaneous polarization (P) decreases from 84.3 μCcm−2 to 70.2/67.8/63 μCcm−2 due to the reduction of the structural distortions when Fe/Co/Ni ions are doped at Ti site, which agrees well with the recent experiment [J. Phys. Chem. C 125, 12342 (2021)]. Strikingly, all the strained systems exhibit a higher P amplitude than that of the unstrained one. The most striking feature of the present study is that the Co-doped motif predicted a metallic behavior for −5% biaxial ([110]) strain. Along with this, the Ni-doped structure demonstrates a reasonable amplitude of spin-polarization with respect to strain. A high to low (2.0–0.5)/low to high (0.5–1.0) spin-state transition is observed at a critical strain of −1%/−2% in Fe/Ni, while Co lies in a low spin state with S = 0.5. Finally, spin-magnetization isosurfaces plots established that magnetism mainly arises from the admixture of 3dz2, 3dx2−y2, 3dyz + xz, and 3dxy orbitals.

A spin-crossover framework endowed with pore-adjustable behavior by slow structural dynamics

Host-guest interactions play critical roles in achieving switchable structures and functionalities in porous materials, but design and control remain challenging. Here, we report a two-dimensional porous magnetic compound, [FeII(prentrz)2PdII(CN)4] (prentrz = (1E,2E)−3-phenyl-N-(4H-1,2,4-triazol-4-yl)prop-2-en-1-imine), which exhibits an atypical pore transformation that directly entangles with a spin state transition in response to water adsorption. In this material, the adsorption-induced, non-uniform pedal motion of the axial prentrz ligands and the crumpling/unfolding of the layer structure actuate a reversible narrow quasi-discrete pore (nqp) to large channel-type pore (lcp) change that leads to a pore rearrangement associated with simultaneous pore opening and closing. The unusual pore transformation results in programmable adsorption in which the lcp structure type must be achieved first by the long-time exposure of the nqp structure type in a steam-saturated atmosphere to accomplish the gate-opening adsorption. The structural transformation is accompanied by a variation in the spin-crossover (SCO) property of FeII, i.e., two-step SCO with a large plateau for the lcp phase and two-step SCO with no plateau for the nqp phase. The unusual adsorption-induced pore rearrangement and the related SCO property offer a way to design and control the pore structure and physical properties of dynamic frameworks.

Dimerization of [FeIII(bpy)3]3+ in Aqueous Solutions: Elucidating a Mechanism Based on Historical Proposals, Electrochemical Data, and Computational Free Energy Analysis.

Iron(II) tris-bipyridine, [FeII(bpy)3]2+, is a historically significant organometallic coordination complex with attractive redox and photophysical properties. With respect to energy storage, it is a low-cost, high-redox potential complex and thus attractive for use as a catholyte in aqueous redox flow batteries. Despite these favorable characteristics, its oxidized Fe(III) form undergoes dimerization to form μ-O-[FeIII(bpy)2(H2O)]24+, leading to a dramatic ∼0.7 V decrease during battery discharge. To date, the energetics and complete mechanism of this slow, sequential electrochemical-chemical (EC) process, which includes electron transfer, nucleophilic attack, ligand cleavage, μ-oxo bond formation, and spin state transition, have not been elucidated. Using cyclic voltammetry, redox flow battery data, and density functional theory calculations guided by previously proposed mechanisms, we modeled more than 100 complexes and performed more than 50 geometry scans to resolve the key steps dictating these complex chemical processes. Quantitative free energy surfaces are developed to model the mechanism of dimerization accounting for the spins and identities of any possible Fe(II), Fe(III), or Fe(IV) intermediates. Electrochemical reduction of the dimer regenerates [FeII(bpy)3]2+ in an overall reversible process. Computational electrochemistry interrogates the influence of spin state, coordination environment, and molecular conformation at the electrode-electrolyte interface through a proposed stepwise dimer reduction process. Experimentally, we show that the considerable overpotential associated with this event can be catalytically mitigated with disparate materials, including platinum, copper hexacyanoferrate, and activated carbon. The findings are of fundamental and applied significance and could elevate [FeII(bpy)3]2+ and its derivatives to play a vital role in the burgeoning renewable energy economy.

Real-Time Observation of Magnetic Domain Structure Changes with Increasing Temperature for Z-Type Hexagonal Ferrite.

Z-type hexagonal ferrites have recently received attention for their room-temperature magnetoelectric (ME), which is activated when the temperature at which the transverse-conical spin-state transitions to a ferrimagnetic state is increased. The changes in the magnetic domain structure at the transition have been well-documented; however, they are still not understood in detail. In the present study, Lorentz transmission electron microscopy (TEM) analysis combined with an in situ heating experiment was conducted to demonstrate the shift in magnetic domain structure during the transition from the transverse-conical spin arrangement to a ferrimagnetic spin order. The dynamics of the magnetic domain structure changes with the increasing temperature were acquired in real-time. At 490 K, the magnetization transition from the transverse-conical spin state to the ferromagnetic state was demonstrated. Cross-tie domain walls formed during the magnetic transition process. The increased effect of the demagnetizing field applied to the 180° magnetic domains was caused by a lower magnetocrystalline anisotropy (MCA) at the easy axis of magnetization.

Dynamics of Spin Crossover Molecular Complexes.

We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the “write” speed is an important criterion.

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

Controllable sensitivity mechanism in an energetic compound of [FeII(Rtrz)6] as a molecular switch

Transition metal iron complexes can be designed for spin crossover compounds with low spin and high spin bistable (LS and HS) states, spin transition can be triggered by external excitation, which regulates stability (sensitivity) and magnetism of complexes. [FeII(Rtrz)6] systems are bulit with the nitrogen-rich group triazole as the ligand, potentially making them high-energy compounds that can be applied in pyrotechnics and especially good candidates as green initiator for weapon systems. In this paper, two kinds of mononuclear complexes [FeIIN6-1,2,4-triazole] (1) and [FeIIN6-1,2,3-triazole] (2) are constructed, and LS ⇌ HS transition occurs by altering the bond lengths of metal ligand at room temperature; therefor, complex 1 and 2 may be spin crossover energetic materials. The ground state of both complexes is the LS state (low sensitivity), and the stable trend is 1 > 2. The spin transport mechanism is calculated by the non-equilibrium Green's function formalism combined with density functional theory. The theoretical results show that both 1 and 2 can be used as molecular switches in a small bias range, but 2 cannot be used when the bias voltage is high. In short, complex 1 is an ideal molecular switch from HS state (high-sensitivity) to LS state (low-sensitivity), acting as a reversible on ⇌ off switch. The spin state transition of complex 1 can be induced with an electric field; therefore, it is expected to control the safety of primary explosives precisely in the long term.