Intermediate Spin Research Articles

Electrochemical capacitance and cubic-rhombohedral phase transition of sodium intercalated ferrimagnetic manganese hexacyanoferrate based open framework material

We report a synthesis of a highly crystalline, sodium intercalated manganese hexacyanoferrate open framework structure based on Prussian blue analogues with chemical stiochiometry of Na2xMn[Fe(CN)6]2(x+1)/3·zH2O (x = 0, 0.5, and 1) using controlled co-precipitation method. The effect of Na intercalation on structural properties, electrochemical performance along with magnetic ordering are prersnted. The formation of uniformly cube shaped sodium‑manganese hexacyanoferrate is evident from field-emission scanning electron microscopy. Infra-red spectroscopy shows presence of characteristic absorption peaks of MnII-C ≡ N-FeIII bonds, confirm the formation of manganese hexacyanoferrate. The Rietveld refinement of x-ray data reveals that the compounds are crystallized in Fm3m cubic symmetry up to x = 0.5, whereas, excess Na intercalation (x = 1) leads to a R-3 mH, rhombohedral symmetry. The dc magnetization measurements show ferrimagnetic nature of these compounds. The magnetic transition temperature increases with Na intercalation and was found to be ∼11, 12.5, and 13.4 K for x = 0, 0.5, and 1. The Mössbauer spectroscopy study confirms low and intermediate spin states of both Fe+2 and Fe+3 ions in the studied compounds, consistence with their magnetic properties. The electrochemical performance of the compounds with Na intercalation in terms of charging-discharging are studied using cyclic voltammetry. The compounds show capacitance values of 182.6 F/g at 10 A/g, 117.5 F/g at 6A/g, and 110.1 F/g at 2.8 A/g for x = 0, 0.5, and 1, respectively, suggesting their suitability as electrochemical capacitors.

Population transfer under local dephasing

Stimulated Raman adiabatic passage is a well-known technique for quantum population transfer due to its robustness again various sources of noises. Here we consider quantum population transfer from one spin to another via an intermediate spin which is under dephasing noise. We obtain an analytic expression for the transfer efficiency under a specific driving protocol, showing that dephasing could reduce the transfer efficiency, but the effect of dephasing could also be suppressed with a stronger laser coupling or a longer laser duration. We also consider another commonly used driving protocol, which shows that this analytic picture is still qualitatively correct.

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.

Quantum spin liquid with emergent chiral order in the triangular-lattice Hubbard model

The interplay between spin frustration and charge fluctuation gives rise to an exotic quantum state in the intermediate-interaction regime of the half-filled triangular-lattice Hubbard model, while the nature of the state is under debate. Using the density matrix renormalization group with SU(2)$_{\rm{spin}} \otimes $U(1)$_{\rm{charge}}$ symmetries implemented, we study the triangular-lattice Hubbard model defined on the long cylinder geometry up to circumference $W=6$. A gapped quantum spin liquid, with on-site interaction $9 \lesssim U / t \lesssim 10.75$, is identified between the metallic and the antiferromagnetic Mott insulating phases. In particular, we find that this spin liquid develops a robust long-range spin scalar-chiral correlation as the system length $L$ increases, which unambiguously unveils the spontaneous time-reversal symmetry breaking. In addition, the degeneracy of the entanglement spectrum supports symmetry fractionalization and spinon edge modes in the obtained ground state. The possible origin of chiral order in this intermediate spin liquid and its relation to the rotonlike excitations have also been discussed.

Anion Modified Spin Crossover in [Fe(qsal-4-F)]+ Complexes with a 4-Position Substituted Qsal Ligand.

Four iron(III) complexes, [Fe(qsal-4-F)2]Y·sol (Hqsal-4-F = 4-fluoro-N-(8-quinolyl)salicylaldimine; Y = NO3-, sol = 0.91MeOH·0.57H2O (1NO3); Y = PF6- (2PF6); Y = BF4- (3BF4); Y = OTf-, sol =1.5MeOH (4OTf)), with a new 4-position substituted qsal type ligand Hqsal-4-F have been synthesized and structurally and magnetically characterized. Complexes 1NO3-3BF4 consist of 1D chains formed by the [Fe(qsal-4-F)2]+ cations connected by π-π and C-H···O interactions, which are further linked by more weak interactions to form 2D layers and 3D networks. On the other hand, complex 4OTf has a structure of nearly isolated 1D column where the [Fe(qsal-4-F)2]+ cations are connected by π-π, C-H···π, and C-F···π interactions. Magnetic studies revealed the occurrence of two-step symmetry-breaking SCO in 1NO3 and two-step gradual SCO in 2PF6. Complex 3BF4 undergoes a gradual SCO, whereas 4OTf remains almost high-spin. The smaller anions tend to stabilize the low-spin state, while larger anions tend to stabilize the high-spin state. In addition, the intermediate spin state of 1NO3 could be thermally trapped by quenching from the high temperature, thereby kinetically suppressing the spin transition to the full low-spin state. This work represents a good example that the position of the substituent and the anions plays critical roles in the preparation of SCO materials with tunable properties.

Origin of the Unusual Ground-State Spin S = 9 in a Cr10 Single-Molecule Magnet.

The molecular wheel [Cr10(OMe)20(O2CCMe3)10], abbreviated {Cr10}, with an unusual intermediate total spin S = 9 and non-negligible cluster anisotropy, D/kB = -0.045(2) K, is a rare case among wheels based on an even number of 3d-metals, which usually present an antiferromagnetic (AF) ground state (S = 0). Herein, we unveil the origin of such a behavior. Angular magnetometry measurements performed on a single crystal confirmed the axial anisotropic behavior of {Cr10}. For powder samples, the temperature dependence of the susceptibility plotted as χT(T) showed an overall ferromagnetic (FM) behavior down to 1.8 K, whereas the magnetization curve M(H) did not saturate at the expected 30 μB/fu for 10 FM coupled 3/2 spin Cr3+ ions, but to a much lower value, corresponding to S = 9. In addition, the X-ray magnetic circular dichroism (XMCD) measured at high magnetic field (170 kOe) and 7.5 K showed the polarization of the cluster moment up to 23 μB/fu. The magnetic results can be rationalized within a model, including the cluster anisotropy, in which the {Cr10} wheel is formed by two semiwheels, each with four Cr3+ spins FM coupled (JFM/kB = 2.0 K), separated by two Cr3+ ions AF coupled asymmetrically (J23/kB = J78/kB = -2.0 K; J34/kB = J89/kB = -0.25 K). Inelastic neutron scattering and heat capacity allowed us to confirm this model leading to the S = 9 ground state and first excited S = 8. Single-molecule magnet behavior with an activation energy of U/kB = 4.0(5) K in the absence of applied field was observed through ac susceptibility measurements down to 0.1 K. The intriguing magnetic behavior of {Cr10} arises from the detailed asymmetry in the molecule interactions produced by small-angle distortions in the angles of the Cr-O-Cr alkoxy bridges coupling the Cr3+ ions, as demonstrated by ab initio and density functional theory calculations, while the cluster anisotropy can be correlated to the single-ion anisotropies calculated for each Cr3+ ion in the wheel.

Metal-Organic Frameworks with Assembled Bifunctional Microreactor for Charge Modulation and Strain Generation toward Enhanced Oxygen Electrocatalysis.

Two-dimensional metal-organic frameworks (MOFs) have served as favorable prototypes for electrocatalytic oxygen evolution reaction (OER). Despite promising catalytic activity, their OER reaction kinetics are still limited by the sluggish four-electron transfer process. Herein, we develop a ferrocene carboxylic acid (FcCA) partially substituted cobalt-terephthalic acid (CoBDC) catalyst with a bifunctional microreactor composed of two species of Co active sites and ligand FcCA (CoBDC FcCA). Benefiting from the ultrathin nanosheet structure, CoBDC FcCA catalyst exhibits an excellent OER performance with a low overpotential of 280 mV to reach 10 mA cm-2 and a small Tafel slope of 53 mV dec-1. Structure characterization together with theoretical calculations directly unravel the coordination for two species of Co active moieties with FcCA forming a microreactor of tensile strain, leading to a conversion of the Co spin from a high spin state (t2g5eg2) to an intermediate spin state (t2g6eg1) to regulate antibonding states of Co 3d and O 2p orbital. In situ spectroscopic measurements for mechanistic understanding reveal that this CoBDC FcCA catalyst possesses an optimal OH* adsorption energy for propitious formation of O-O bonds in the OOH* intermediate, thus effectively decreasing the thermodynamic Gibbs free energy of the rate-determining step (O* → OOH*) to accelerate reaction kinetics for the whole OER process. When loaded on an integrated BiVO4 photoanode as a cocatalyst, CoBDC FcCA enables highly active solar-driven oxygen production from water splitting.

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay.

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories. The NACs are further processed using a reduced density matrix approach with the Redfield formalism. The computational results are presented for electronic density of states, absorption spectra, charge carrier dynamics, and photoluminescence (PL) by comparing various spin multiplicities. Results on spin α and spin β components are independent and quite different because of the partial occupation of Fe 3d states. Overall, HS is the most stable with the largest Fe-O distances. One finds different nonradiative relaxation pathways in space and on the time scale for electrons and holes. The Redfield PL reveals obvious Fe 3d-3d transitions for LS and IS.

Intermediate spin state and the B1−B2 transition in ferropericlase

Ferropericlase (fp), (Mg$_{1-x}$Fe$_x$)O, the second most abundant mineral in the Earth's lower mantle, is expected to be an essential component of super-Earths' mantles. Here we present an ab initio investigation of the structure and magnetic ground state of fp up to $\sim$ 3 TPa with iron concentrations (x$_{Fe}$) varying from 0.03 to 0.12. Calculations were performed using LDA+U$_{sc}$ and PBE exchange-correlation functionals to elucidate the pressure range for which the Hubbard U (U$_{sc}$) is required. Similar to the end-members FeO and MgO, fp also undergoes a B1 to B2 phase transition that should be essential for modeling the structure and dynamics of super-Earths' mantles. This structural transition involves a simultaneous change in magnetic state from a low spin (LS) B1 phase with iron total spin S=0 to an intermediate spin (IS) B2 phase with S=1. This is a rare form of pressure/strain-induced magnetism produced by local cation coordination changes. Phonon calculations confirm the dynamical stability of the iron B2-IS state. Free energy calculations are then carried out including vibrational effects and electronic and magnetic entropy contributions. The phase diagram is then obtained for low concentration fp using a quasi-ideal solid solution model. For x$_{Fe}$ > 0.12 this approach is no longer valid. At ultra-high pressures, there is an IS to LS spin state change in Fe in the B2 phase, but the transition pressure depends sensitively on thermal electronic excitations and on x$_{Fe}$.

Octahedral rotation induced spin state and metal–insulator transition in LaCoO[formula omitted] films

The discussions over the origins of the magnetic and insulating properties in LaCoO3 films has continued unabated even now. We investigate the energy of natural low spin, homogeneous intermediate spin, and mixed spin configurations using first principle calculations, taking into account various distortions in bulk and films. The spin state transition can be controlled by rhombohedral distortion and octahedral rotations in films bulk. The octahedral rotation reserved in the films bulk, and it can also induce the breathing distortion or the Jahn–Teller distortion when increase the mismatch ratio. The breathing distortion or the Jahn–Teller distortion change the electronic structure from a metal to semiconductor. The octahedral rotations can be a efficient regulatory to control electronic structure and spin states. The constrain of mixed spin configuration (IS:LS=1:2) potentially cause a charge disproportionation, resulting in a quasi-breathing distortion similar to magnetoelastic effect.

Redox Chemistry of tris(β-diketonate)cobalt(III) Complexes: A Molecular View

Reduction of low spin S = 0 tris(β-diketonate)cobalt(III) complexes leads to an intermediate spin S = 3/2 anion. This electron rearrangement upon reduction could be the reason why a large peak potential difference between the reduction and re-oxidation peaks was observed during electrochemical analysis, using cyclic voltammetry. It is shown that experimental redox potentials obtained from the following methods, namely (i) polarography, (ii) cyclic voltammetry on a hanging Hg electrode, as well as (iii) cyclic voltammetry on a carbon fibre surface, related linearly to various empirical (Gordy scale group electronegativities, Hammett meta-substituent constants, pK a and the Lever electronic parameters) and density functional theory (DFT) calculated (LUMO energy, adiabatic electron affinity, chemical potential, global electrophilicity index and molecular electrostatic potential on Co) reactivity parameters. Deviations from these linear trends were due to a resonance effect of aromatic substituent groups. The linear relationships obtained for complexes containing aliphatic substituent groups can be used to predict redox potential and the electrophilic reactivity of related tris(β-diketonate)cobalt(III) complexes.

Optimizing the electronic spin state and delocalized electron of NiCo2(OH)x/MXene composite by interface engineering and plasma boosting oxygen evolution reaction

The electrocatalytic activity of transition-metal-based compounds is closely related to the electronic configuration. However, optimizing the surface electron spin state of catalysts remains a challenge. Here, we developed a spin-state and delocalized electron regulation method to optimize oxygen evolution reaction (OER) performance by in-situ growth of NiCo2(OH)x using Oswald ripening and coordinating etching process on MXene and plasma treatment. X-ray absorption spectroscopy, magnetic tests and electron paramagnetic resonance reveal that the coupling of NiCo2(OH)x and MXene can induce remarkable spin-state transition of Co3+ and transition metal ions electron delocalization, plasma treatment further optimizes the 3d orbital structure and delocalized electron density. The unique Jahn-Teller phenomenon can be brought by the intermediate spin state (t2g5eg1) of Co3+, which benefits from the partial electron occupied eg orbitals. This distinct electron configuration (t2g5eg1) with unpaired electrons leads to orbital degeneracy, that the adsorption free energy of intermediate species and conductivity were further optimized. The optimized electrocatalyst exhibits excellent OER activity with an overpotential of 268 mV at 10 mA cm−2. DFT calculations show that plasma treatment can effectively regulate the d-band center of TMs to optimize the adsorption and improve the OER activity. This approach could guide the rational design and discovery of electrocatalysts with ideal electron configurations in the future.

Weak antilocalization, spin\u2013orbit interaction, and phase coherence length of a Dirac semimetal Bi0.97Sb0.03

The present study develops a general framework for weak antilocalization (WAL) in a three-dimensional (3D) system, which can be applied for a consistent description of longitudinal resistivity rho_{xx} left( B right) and Hall resistivity rho_{xy} left( B right) over a wide temperature (T) range. Compared to the previous approach Vu et al. (Phys Rev B 100:125162, 2019), which assumes infinite phase coherence length (lϕ) and a zero spin–orbit scattering length (lSO), the present framework is more general, covering high T and the intermediate spin–orbit coupling strength. Based on the new approach, the rho_{xx} left( B right) and rho_{xy} left( B right) of the Dirac semimetal Bi0.97Sb0.03 was analyzed over a wide T range from 1.7 to 300 K. The present framework not only explains the main features of the experimental data but also enables one to estimate lϕ and lSO at different temperatures. The lϕ has a power-law T dependence at high T and saturates at low T. In contrast, the lSO shows negligible T dependence. Because of the different T dependence, a crossover occurs from the lSO-dominant low-T to the lϕ-dominant high-T regions. Accordingly, the hallmark features of weak antilocalization (WAL) in rho_{xx} left( B right) and rho_{xy} left( B right) are gradually suppressed across the crossover with increasing T. The present theory describes both low-T and high-T regions successfully, which is impossible in the previous approximate approach. This work offers insights for understanding quantum electrical transport phenomena and their underlying physics, particularly when multiple WAL length scales are competing.

Stabilizing Layered Structure in Aqueous Electrolyte via Dynamic Water Intercalation/Deintercalation.

Aqueous lithium-ion batteries (ALIBs) with nonflammable feature attract great attention for large-scale energy storage. However, the layered cathode materials (such as LiCoO2 ) present serious capacity decay in ALIBs. The degradation mechanism of layered cathode materials in ALIBs is still not clear and an effective strategy to improve cycling stability remains a great challenge. In this work, the authors use LiCoO2 as a typical example to investigate its structural degradation in aqueous electrolytes. It is found that H+ insertion accelerated irreversible layered-to-spinel phase transition is the main reason causing structural degradation and fast capacity fading in LiCoO2 . Subsequently, Li-excess Li1+ t Co1- t O2- t with intermediate spin Co3+ is developed to mitigate H+ influence and the adverse phase transition in aqueous electrolyte. It is interesting to discover that reversible water intercalation/deintercalation occurs in the layered structure during charge/discharge, which effectively suppresses the layered-to-spinel phase transition with cycling. Benefiting from the stabilized layered structure, the Li-excess Li1.08 Co0.92 O1.92 shows a significantly improved cycling performance in the neutral aqueous electrolyte with a large specific capacity and excellent rate capability. This work provides a promising structural regulation strategy for the layered cathode materials, enabling their potential application in ALIBs.

Spin liquid to spin glass crossover in the random quantum Heisenberg magnet

We study quantum SU($M$) spins with all-to-all and random Heisenberg exchange interactions of root-mean-square strength $J$. The $M \rightarrow \infty$ model has a spin liquid ground state with the spinons obeying the equations of the Sachdev-Ye-Kitaev (SYK) model. Numerical studies of the SU(2) model with $S=1/2$ spins show spin glass order in the ground state, but also display SYK spin liquid behavior in the intermediate frequency spin spectrum. We employ a $1/M$ expansion to describe the crossover from fractionalized fermionic spinons to a confining spin glass state with weak spin glass order $q_{EA}$. The SYK spin liquid behavior persists down to a frequency $\omega_\ast \sim J q_{EA}$, and for $\omega < \omega_\ast$, the spectral density is linear in $\omega$, thus quenching the extensive zero temperature entropy of the spin liquid. The linear $\omega$ spectrum is qualitatively similar to that obtained earlier using bosonic spinons for large $q_{EA}$. We argue that the extensive SYK spin liquid entropy is transformed as $T \rightarrow 0$ to an extensive complexity of the spin glass state.

Semiconducting spin glass to metallic ferromagnetic state phase transition in double-substituted La1−xSrxCo1−yNiyO3−γ perovskites

A comprehensive study of the structural, magnetic, and transport properties of double-substituted ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Co}}_{1\ensuremath{-}y}{\mathrm{Ni}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\gamma}}$ ($0.1\ensuremath{\le}x\ensuremath{\le}0.5, 0.05\ensuremath{\le}y\ensuremath{\le}0.25$) perovskite systems was carried out. The average oxidation number of Co ions was found to be $3+$ for the most subsystems. The existence of several characteristic concentration regions for various degrees of substitution in the A (La, Sr) and B (Co, Ni) sublattices is shown. Low levels of Sr and Ni ion substitution subsystems demonstrate cluster spin-glass behavior with a semiconducting character of the conductivity and a diamagnetic low-spin (LS) ${\mathrm{Co}}^{3+}$ matrix. An increase of Ni substitution leads to the stabilization of intermediate-spin (IS) ${\mathrm{Co}}^{3+}$ ions in ${\mathrm{CoO}}_{6}$ octahedra, approaching the percolation threshold of ferromagnetic clusters. For strongly Sr-substituted subsystems an effect of stabilization of IS ${\mathrm{Co}}^{3+}$ is not observed, since the contribution of this effect is smaller than the effect of the dilution by antiferromagnetically interacting Ni ions. In this case, an increase of Ni content leads to a decrease in the magnetic ordering temperature ${T}_{\text{mo}}$ and magnetization values. For weakly Sr-substituted systems, on the contrary, it leads to their growth. Anomalies in the conductivity dependences for several subsystems are due to the thermally induced spin crossover of the part of ${\mathrm{Co}}^{3+}$ ions, first from the LS to the IS state, and then to the mixed IS/high-spin (HS) state, similar to the parent ${\mathrm{LaCoO}}_{3}$.

Intermediate spin pair relaxation through modulation of isotropic hyperfine interaction in frequency-swept spin-dependent recombination in 4H–SiC

Electrically detected magnetic resonance (EDMR) measurements have been extended to sub-mT magnetic fields through utilization of frequency sweeping of the oscillating magnetic field, where conventional electron paramagnetic resonance-based measurements traditionally utilize magnetic field magnitude ramping. In spin-dependent transport measurements in devices, an oftentimes pervasive near-zero field magnetoresistance effect overwhelms the sub-mT regime. This magnetoresistance effect is independent of the RF drive. Thus, by utilizing a constant DC magnetic field and a frequency sweep of the RF magnetic field, the magnetoresistance effect is not detected, leaving only the EDMR response. Interesting EDMR-based phenomena emerge at sub-mT fields when the oscillating field magnitude approaches the static field, such as multiple-photon transitions caused by the emergence of Floquet spin states and Bloch–Siegert shifts. A spectral-narrowing effect also emerges as the static field is reduced. In this work, we show that the narrowing of the frequency-swept EDMR response with static field can be modeled by changes in intermediate spin-pair relaxation through modulation of hyperfine fields caused by stochastic perturbations from the environment. We utilize recently developed theory to model the relaxation of spin pairs and show that stochastic interactions of the electron spin with the environment yield both Floquet spin states and changes in intermediate spin-pair relaxation.

Relative energies among S3 intermediates in the photosystem II revealed by DLPNO coupled cluster and hybrid DFT calculations. Possible pathways of water insertion in the S2 to S3 transition

DLPNO-CCSD(T0) calculations coupled with the exact diagonalization of spin Hamiltonian matrix elucidated two right (R)-opened hydroxide (OH–)-inserted structures with the intermediate spin (IS) (S = 3), left (L)-opened OH– and water-inserted structures with IS (S = 3) and left (L)-opened oxo-inserted structures with the high spin (HS) (S = 6) state. Multiple intermediates in the S3 state revealed by DLPNO-CCSD(T0) are compatible with the EPR and XFEL results for the S3 state of photosystem II.

Electrode reconstruction strategy for oxygen evolution reaction: maintaining Fe-CoOOH phase with intermediate-spin state during electrolysis

Computational calculations and experimental studies reveal that the CoOOH phase and the intermediate-spin (IS) state are the key factors for realizing efficient Co-based electrocatalysts for the oxygen evolution reaction (OER). However, according to thermodynamics, general cobalt oxide converts to the CoO2 phase under OER condition, retarding the OER kinetics. Herein, we demonstrate a simple and scalable strategy to fabricate electrodes with maintaining Fe-CoOOH phase and an IS state under the OER. The changes of phase and spin states were uncovered by combining in-situ/operando X-ray based absorption spectroscopy and Raman spectroscopy. Electrochemical reconstruction of chalcogenide treated Co foam affords a highly enlarged active surface that conferred excellent catalytic activity and stability in a large-scale water electrolyzer. Our findings are meaningful in that the calculated results were experimentally verified through the operando analyses. It also proposes a new strategy for electrode fabrication and confirms the importance of real active phases and spin states under a particular reaction condition.

Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes

The electronic structure and ground spin states, S, observed for mixed-valent iron–sulfur dimers (FeII-FeIII) are typically determined by the Heisenberg exchange interaction, J, that couples the magnetic interaction of the two metal centres either ferromagnetically (J > 0, S = 9/2) or antiferromagnetically (J < 0, S = 1/2). In the case of antiferromagnetically coupled iron centres, stabilization of the high-spin S = 9/2 ground state is also feasible through a Heisenberg double-exchange interaction, B, which lifts the degeneracy of the Heisenberg spin states. This theorem also predicts intermediate spin states for mixed-valent dimers, but those have so far remained elusive. Herein, we describe the structural, electron paramagnetic resonance and Mössbauer spectroscopic, and magnetic characterization of a series of mixed-valent complexes featuring [Fe2Q2]+ (Q = S2–, Se2–, Te2–), where the Se and Te complexes favour S = 3/2 spin states. The incorporation of heavier chalcogenides in this series reveals a delicate balance of antiferromagnetic coupling, Heisenberg double-exchange and vibronic coupling.