Low-spin Fe2 Research Articles

Unlocking the electrochemical ammonium storage performance of copper intercalated hexacyanoferrate

The aqueous NH4+ batteries (AAIBs) have fascinated significant attention because of their excellent safety, small cost, fast diffusion kinetics, and environmental friendliness. It is essential to develop high-performance electrode materials that can stably exist during the rapid ammoniation/deammoniation processes. Iron-based Prussian blue (PBA) possesses significant application potential in AAIBs due to its considerable theoretical capacity and the abundant availability of iron ore resources. Nevertheless, the capacity contribution from the low-spin Fe2+/Fe3+ coupling is relatively modest, and the presence of Fe(CN)6 vacancies and crystalline water significantly diminishes the active sites for NH4+, adversely impacting the electrochemical performance of AAIBs. The substitution of Cu can partially stabilize the crystalline structure of the material and help mitigate side reactions. Herein, a CuFe Prussian blue analog (CuFe-PBA) nanomaterial with a grain size of ∼100 nm prepared by an easy coprecipitation pathway is described as a high-performance cathode for AAIBs. The electrochemically inactive Cu2+ raises the reaction potential, enhancing the stability of the battery. The material exhibits exceptional cycling stability, maintaining 74.5 % of its initial capacity after stable cycling for 20,000 cycles at 10 A/g, which corresponds to a low-capacity attenuation of 0.0013 % per cycle. These results provide new insights for the development of low-cost, long-life cathode for AAIBs.

Low-Spin Fe3+ Evoked by Multiple Defects with Optimal Intermediate Adsorption Attaining Unparalleled Performance in Water Oxidation.

Electrocatalytic water splitting is long constrained by the sluggish kinetics of anodic oxygen evolution reaction (OER), and rational spin-state manipulation holds great promise to break through this bottleneck. Low-spin Fe3+ (LS, t2g 5eg 0) species are identified as highly active sites for OER in theory, whereas it is still a formidable challenge to construct experimentally. Herein, a new strategy is demonstrated for the effective construction of LS Fe3+ in NiFe-layered double hydroxide (NiFe-LDH) by introducing multiple defects, which induce coordination unsaturation over Fe sites and thus enlarge their d orbital splitting energy. The as-obtained catalyst exhibits extraordinary OER performance with an ultra-low overpotential of 244 mV at the industrially required current density of 500 mA cm-2, which is 110 mV lower than that of the conventional NiFe-LDH with high-spin Fe3+ (HS, t2g 3eg 2) and superior to most previously reported NiFe-based catalysts. Comprehensive experimental and theoretical studies reveal that LS Fe3+ configuration effectively reduces the adsorption strength of the O* intermediate compared with that of the HS case, thereby altering the rate-determining step from (O* → OOH*) to (OH* → O*) of OER and lowering its reaction energy barrier. This work paves a new avenue for developing efficient spin-dependent electrocatalysts for OER and beyond.

Dehydration Achieving the Iron Spin State Regulation of Prussian Blue for Boosted Sodium-Ion Storage Performance.

Prussian blue analogs (PBAs) show promise as cathodes for sodium-ion batteries due to their notable cycle stability, cost-effectiveness, and eco-friendly nature, yet the presence of interstitial water limits the specific capacity and obstructs Na+ mobility within the material. Although considerable experimental efforts are focused on dehydrating water for capacity enhancement, there is still a deficiency of a comprehensive understanding of the low capacity of low-spin Fe resulting from interstitial water, which holds significance in Na+ storage. This study introduces a novel gas-assisted heat treatment method to efficiently remove interstitial water from Fe-based PBA (NaFeHCF) electrodes and combines experiments and theoretical calculations to reveal the iron spin state regulation that is related to the capacity enhancement mechanism. This dehydration strategy significantly enhances battery capacity, especially the portion at higher voltages (3.4-4.0V). The increase in capacity is attributed to the following factors: an enhanced proportion of Fe2+, reduced water content which facilitates faster charge transfer, and the activation of low spin Fe2+. The optimized NaFeHCF demonstrated impressive half-cell performance of retaining 87.3% capacity after 2000 cycles at a 5C rate and achieving 100mAhg-1 capacity over 200 cycles when being paired with hard carbon, exhibiting its practical potential.

High Capacity and Fast Kinetics Enabled by Metal-Doping in Prussian Blue Analogue Cathodes for Sodium-Ion Batteries.

Prussian blue analogues (PBAs) have gained tremendous attention as promising low-cost electrochemically-tunable electrode materials, which can accommodate large Na+ ions with attractive specific capacity and charge-discharge kinetics. However, poor cycling stability caused by lattice strain and volume change remains to be improved. Herein, metal-doping strategy has been demonstrated in FeNiHCF, Na1.40Fe0.90Ni0.10[Fe(CN)6]0.85 ⋅ 1.3H2O, delivering a capacity as high as 148 mAh g-1 at 10 mA g-1. At an exceptionally high rate of 25.6 A g-1, a reversible capacity of ~55 mAh g-1 still can be obtained with a very small capacity decay rate of 0.02 % per cycle for 1000 cycles, considered one of the best among all metal-doped PBAs. This exhibits the stabilizing effect of Ni doping which enhances structural stability and long-term cyclability. In situ synchrotron X-ray diffraction reveals an extremely small (~1 %) change in unit cell parameters. The Ni substitution is found to increase the electronic conductivity and redox activity, especially at the low-spin (LS) Fe center due to inductive effect. This larger capacity contribution from LS Fe2+C6/Fe3+C6 redox couple is responsible for stable high-rate capability of FeNiHCF. The insight gained in this work may pave the way for the design of other high-performance electrode materials for sustainable sodium-ion batteries.

Magnetic suppression for a possible Fe-poor organic–inorganic hybrid superconductor Fe14Se16(tepa)0.8 (tepa = tetraethylenepentamine) with a superconducting transition at ∼42 K

Composition/structure-dependent superconductivity for FeSe-based superconductors attracted great attention not only due to their high superconducting transition temperatures (TC), but also for understanding the origin of iron-based superconductivity. Here, we report a new Fe-poor organic-inorganic hybrid material Fe14Se16(tepa)0.8 with a paramagnetic-diamagnetic transition at ∼42 K grown by a high-temperature organic-solution-phase method with soluble iron/selenium sources in a tepa solution, alternative to previous intercalation strategies. The Fe14Se16(tepa)0.8 phase is in a tetragonal layered hybrid structure with a nanoplate shape. Composition analyses reveal a Fe-poor characteristic of the hybrid in contrast to previous FeSe-intercalated superconductor, and selected area electron diffraction pattern is featured by Fe3Se4 superstructures with a √2 × √2 of Fe vacancy order. Ab initio density functional calculations show that minus Fe3Se4 ions are stable in the hybrid and ∼0.25e−/Fe0.75Se is obviously larger than the reported values of approximately 0.2e−/FeSe in other FeSe-intercalated superconductors. Typical hysteresis loops and temperature dependence of dc/ac susceptibilities of the Fe14Se16(tepa)0.8 measured below ∼42 K suggest a presence of the Meissner effect in this material. Effects of synthesis conditions on structures and magnetic properties of the hybrids show a magnetic evolution from a long-range ferrimagnetic (FIM) order of Fe14Se16(tepa) to a coexistence of FIM and superconducting (SC) orders of Fe14Se16(tepa)0.9 and an SC order of Fe14Se16(tepa)0.8. X-ray absorption spectrum (XAS) confirms the presence of ferric/ferrous irons. Mössbauer studies reveal that the high-TC superconductivity originates from a suppression of the FIM order through tuning the spin states of irons from high-spin Fe3+ (S = 5/2) and Fe2+ (S = 2) in the Fe14Se16(tepa) to low-spin Fe3+ (S = 1/2) and Fe2+ (S = 0) in the Fe14Se16(tepa)0.8. Although no zero resistance is detected even at a temperature of 2 K, the resistivity at 2 K decreases by more than 1600 times compared to that in a normal state calculated by a variable range hopping (VRH) model, suggesting that the high-TC superconductivity of Fe14Se16(tepa)0.8 is possible.

Hydrogen-bonded assemblies of iron(II) spin crossover complexes.

The paper reports on the synthesis, crystal structure, thermal and magnetic properties of spin crossover (SCO) salts containing the [Fe(bpp)2]2+ cation (bpp = 2,6-bis(pyrazol-3-yl)pyridine) and different rigid polycarboxylate anions, such as anthracene-9,10-dicarboxylate (ADC), benzene-1,3,5-tricarboxylate (BTC) and biphenyl-4,4'-dicarboxylate (BPDC). Compound [Fe(bpp)2](ADC)·9H2O (1) shows a porous hydrogen-bonded structure with water molecules sitting in the channels. It contains low-spin (LS) Fe2+ cations that undergo crossover to the high-spin (HS) state upon dehydration. Anhydrous 1 remains HS on cooling at low temperatures. A similar magnetic behaviour is obtained for the partially protonated BTC salt [Fe(bpp)2](HBTC)·5H2O (2), showing a spin change concomitant with dehydration to a HS phase that undergoes gradual and partial SCO on cooling, affecting 25% of the Fe2+ cations. Instead, the BPDC salt [Fe(bpp)2](BPDC)·5H2O (3) has a ground HS state in its fully hydrated form.

Characterizing the Biosynthesis of the [Fe(II)(CN)(CO)2(cysteinate)]- Organometallic Product of the Radical-SAM Enzyme HydG by EPR and Mössbauer Spectroscopy.

[FeFe]-hydrogenases employ a catalytic H-cluster, consisting of a [4Fe-4S]H cluster linked to a [2Fe]H subcluster with CO, CN- ligands, and an azadithiolate bridge, which mediates the rapid redox interconversion of H+ and H2. In the biosynthesis of this H-cluster active site, the radical S-adenosyl-l-methionine (radical SAM, RS) enzyme HydG plays the crucial role of generating an organometallic [Fe(II)(CN)(CO)2(cysteinate)]- product that is en route to forming the H-cluster. Here, we report direct observation of this diamagnetic organometallic Fe(II) complex through Mössbauer spectroscopy, revealing an isomer shift of δ = 0.10 mm s-1 and quadrupole splitting of ΔEQ = 0.66 mm s-1. These Mössbauer values are a change from the starting values of δ = 1.15 mm s-1 and ΔEQ = 3.23 mm s-1 for the ferrous "dangler" Fe in HydG. These values of the observed product complex B are in good agreement with Mössbauer parameters for the low-spin Fe2+ ions in synthetic analogues, such as 57Fe Syn-B, which we report here. These results highlight the essential role that HydG plays in converting a resting-state high-spin Fe(II) to a low-spin organometallic Fe(II) product that can be transferred to the downstream maturase enzymes.

Crystal structure, magnetism, and thermoelectric properties of Nd1−x Sr x FeO3−δ (0.1 ≤ x ≤ 0.9)

Polycrystalline Nd1−x Sr x FeO3−δ (0.1 ≤ x ≤ 0.9) samples exhibit a single-phase perovskite-type crystal structure and G-type antiferromagnetism with small ferromagnetic order. Assuming that the Fe site is in the mixed-valence state, the spin state of Fe changes from a low-spin (LS) or intermediate spin Fe3+-dominant state at x ≤ 0.5 to a LS Fe4+ dominant state at x ≥ 0.6. This strongly suggests that the charge carriers change from holes to electrons. In fact, at temperatures less than 500 K, these samples show a p-type Seebeck coefficient for 0.1 ≤ x ≤ 0.5 and an n-type Seebeck coefficient for 0.6 ≤ x ≤ 0.9. However, for 0.6 ≤ x ≤ 0.9, Fe4+ is reduced to Fe3+ because of an oxygen deficiency in the 500–600 K temperature range. In particular, Nd0.7Sr0.3FeO2.99(1) shows the largest p-type ZT = 0.025 at 765 K, whereas compositions with high n-type ZT could not be identified.

Unveiling anomalous lattice shrinkage induced by Pi-backbonding in Prussian blue analogues

Transition-metal (TM)-based Prussian blue and its analogues (TM-PBAs) have attracted considerable attention as cathode materials owing to their versatile ion storage capability with tunable working voltages. TM-PBAs with different crystal structures, morphologies, and TM combinations can exhibit excellent electrochemical properties because of their unique and robust host frameworks with well-defined <100> ionic diffusion channels. Nonetheless, there is still a lack of understanding regarding the performance dependence of TM-PBAs on structural changes during charging/discharging processes. In this study, in situ X-ray diffraction and X-ray absorption fine structure analyses elucidate the TM-dependent structural changes in a series of TM-PBAs during the charging and discharging processes. During the discharging process, the lattice volume of Fe-PBA increased while those of Ni- and Cu-PBAs decreased. This discrepancy is attributed to the extent of size reduction of the cyanometallate complex ([Fe(CN)6]) via pi-backbonding from Fe to C due to redox flips of the low-spin Fe3+/2+ ion. This study presents a comprehensive understanding of how TM selection affects capacity acquisition and phase transition in TM-PBAs, a promising class of cathode materials.

In Situ Exploring of the Origin of the Enhanced Oxygen Evolution Reaction Efficiency of Metal(Co/Fe)–Organic Framework Catalysts Via Postprocessing

The oxygen evolution reaction (OER) is of vital importance for electrochemical energy conversion technologies. The use of metal–organic framework (MOF) catalysts after postprocessing is an important method to optimize catalytic activity; however, little attention has been paid to the transformation of the electronic structure and coordinated ions under operando conditions. Here, we focus on a Prussian blue analogue (PBA) with the formula Na2Co2+[Fe2+(CN)6]·nH2O after postprocessing at a temperature of 400 °C. The catalysts exhibit a significant improvement in the OER activity with an overpotential of 254 mV against 320 mV for the initial PBA at 10 mA cm–2 in 1 M KOH. Our ex situ soft X-ray absorption spectroscopy (XAS) results demonstrate that some of the high-spin Co2+ and low-spin Fe2+ ions of PBA exhibit exchange with N and C ligands. Our fast operando hard XAS study revealed a dynamic evolution process of the transformation of ligands from N(C) in the initial Na2Co[Fe(CN)6]·nH2O to oxygen and the formation of pure low-spin Co3+ oxide and high-spin Fe3+ oxide after OER. The amorphous (Co,Fe)OOH chemical state was identified as the catalytically active state based on our operando X-ray spectroscopy results. The results of this study provide insights into a dynamic evolution process of MOFs after postprocessing and the formation of real active sites during electrocatalysis.

Structural Evolution, Redox Mechanism, and Ionic Diffusion in Rhombohedral Na2FeFe(CN)6 for Sodium-Ion Batteries: First-Principles Calculations

Prussian blue analogs (Na2FeFe(CN)6) have been regarded as potential cathode materials for sodium-ion batteries (SIBs) due to their low-cost iron resources and open framework. Herein, the detailed first-principles calculations have been performed to investigate the electrochemical properties of Na x FeFe(CN)6 during Na ion extraction. The material undergoes a phase transition from a dense rhombohedral to open cubic structure upon half-desodiation, which is resulted from competition of the Na−N Coulomb attraction and d−π covalent bonding of Fe−N. The analyses on the density of states, magnetic moments and Bader charges of Na x FeFe(CN)6 reveal that there involve in the successive redox reactions of high-spin Fe2+/Fe3+ and low-spin Fe2+/Fe3+ couples during desodiation. Moreover, the facile three-dimensional diffusion channels for Na+ ions exhibit low diffusion barriers of 0.4 eV ∼ 0.44 eV, which ensures a rapid Na+ transport in the Na x FeFe(CN)6 framework, contributing to high rate performance of the battery. This study gives a deeper understanding of the electrochemical mechanisms of Na x FeFe(CN)6 during Na+ extraction, which is beneficial for the rational design of superior PBA cathodes for SIBs.

Low-spin ferric iron in primordial bridgmanite crystallized from a deep magma ocean

The crystallization of the magma ocean resulted in the present layered structure of the Earth’s mantle. An open question is the electronic spin state of iron in bridgmanite (the most abundant mineral on Earth) crystallized from a deep magma ocean, which has been neglected in the crystallization history of the entire magma ocean. Here, we performed energy-domain synchrotron Mössbauer spectroscopy measurements on two bridgmanite samples synthesized at different pressures using the same starting material (Mg0.78Fe0.13Al0.11Si0.94O3). The obtained Mössbauer spectra showed no evidence of low-spin ferric iron (Fe3+) from the bridgmanite sample synthesized at relatively low pressure of 25 gigapascals, while that directly synthesized at a higher pressure of 80 gigapascals contained a relatively large amount. This difference ought to derive from the large kinetic barrier of Fe3+ rearranging from pseudo-dodecahedral to octahedral sites with the high-spin to low-spin transition in experiments. Our results indicate a certain amount of low-spin Fe3+ in the lower mantle bridgmanite crystallized from an ancient magma ocean. We therefore conclude that primordial bridgmanite with low-spin Fe3+ dominated the deeper part of an ancient lower mantle, which would contribute to lower mantle heterogeneity preservation and call for modification of the terrestrial mantle thermal evolution scenarios.

Iron-wüstite revisited: A revised calibration accounting for variable stoichiometry and the effects of pressure

We present thermodynamic and empirical calculations for the iron-wüstite (IW) buffer applicable from 100 kPa to 100 GPa and from 1000 to 3000 K. The thermodynamic calculation self-consistently accounts for changing stoichiometry of iron-saturated wüstite as a function of temperature and pressure. In contrast to some previous models for calculating IW at high pressure, the model incorporates a thermodynamically valid representation of the free energy of stoichiometric FeO at 100 kPa. Earlier high pressure models that relied on the JANAF thermochemical tables (Chase, 1998) were compromised because JANAF has erroneous values for the properties of FeO. This resulted in predicted oxygen fugacities buffered by IW that are between 0.2 and 1.1 log units too reducing at 3000 and 1000 K, respectively. The revised thermodynamic calculations indicate that iron-saturated wüstite becomes more nearly stoichiometric with increasing pressure, but that this shift depends on temperature. Near-stoichiometric FeO (y < 0.01, for Fe1-yO) is reached close to 8 GPa at 1000 K and 17 GPa at 2000 K. An empirical function is presented that accurately reproduces the thermodynamic calculation and facilitates easy quantification of the fO2 of IW over the full range of temperatures and pressures covered by the model. Some caution is warranted in calculation of IW at high pressures, after FeO undergoes a transition from an insulator to a conductor and where low spin Fe2+ is stabilized, as the present model does not incorporate the effects of these transitions on the IW buffer.

Iron force constants of bridgmanite at high pressure: Implications for iron isotope fractionation in the deep mantle

The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its β-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.

Anomalous compressibility in (Fe,Al)-bearing bridgmanite: implications for the spin state of iron

The valence and spin states of Fe in (Fe,Al)-bearing bridgmanite (bdg) affect its physical properties, which is important for the interpretation of geophysical observations. Currently, tens of studies on the compressibility and spin states of Fe-bearing bdg have been reported. A consensus is that Fe-bearing bdg shows spin transition, which affects its elastic parameters. However, there is a conflict between reports on the compressibility and spin states of (Fe,Al)-bearing bdg in experiments using samples pre-synthesized in a multi-anvil apparatus (MA), and samples directly synthesized in a diamond anvil cell (DAC). There are no reports showing evidence of spin transition of Fe in compression experiments using (Fe,Al)-bearing bdg samples pre-synthesized in a MA, while those synthesized at relatively high pressure (at least above 45 GPa) in a DAC all exhibited the spin transition. Here, we performed synchrotron X-ray diffraction measurements on Mg0.85Fe0.09Al0.21Si0.86O3 and Mg0.85Fe0.14Al0.05Si0.96O3 bdg synthesized at relatively high pressure in a laser-heated DAC from amorphous starting material up to 47 and 56 GPa, respectively, at room temperature. The obtained pressure (P)–lattice volume (V) relations show noticeable softening at 22–30 GPa and 35–45 GPa, respectively, which is probably due to the spin transition of Fe. Combining our results and previous reports, we suggest that the lower mantle bdg is capable of containing low-spin Fe3+, which questions the general view. Such a transition changes density and may affect the physical properties of bridgmanite such as thermal conductivity and iron partitioning coefficient, thus having profound implications for mantle dynamics, and the chemical composition of the Earth.

AnisoDipFit: Simulation and Fitting of Pulsed EPR Dipolar Spectroscopy Data for Anisotropic Spin Centers

Pulsed electron paramagnetic resonance dipolar spectroscopy (PDS) allows to measure the distances between electron spin centers and, in favorable cases, their relative orientation. This data is frequently used in structural biology for studying biomolecular structures, following their conformational changes and localizing paramagnetic centers within them. In order to extract the inter-spin distances and the relative orientation of spin centers from the primary, time-domain PDS signals, a specialized data analysis is required. So far, the software to do such analysis was available only for isotropic S = 1/2 spin centers, such as nitroxide and trityl radicals, as well as for high-spin Gd3+ and Mn2+ ions. Here, a new data analysis program, called AnisoDipFit, was introduced for spin systems consisting of one isotropic and one anisotropic S = 1/2 spin centers. The program was successfully tested on the PDS data corresponding to the spin systems Cu2+/organic radical, low-spin Fe3+/organic radical, and high-spin Fe3+/organic radical. For all tested spin systems, AnisoDipFit allowed determining the inter-spin distance distribution with a sub-angstrom precision. In addition, the spatial orientation of the inter-spin vector with respect to the g-frame of the metal center was determined for the last two spin systems. Thus, this study expands the arsenal of the PDS data analysis programs and facilitates the PDS-based distance and angle measurements on the highly relevant class of metolloproteins.

Paramagnetic iron-containing ionic liquid crystals

Iron-containing ionic liquid crystals of N,N′-dialkylimidazolium salts ([(CnH2n+1)2IM]3[Fe(CN)6], with n = 12 (1), 14 (2), 16 (3), 18 (4), IM = imidazolium) were prepared and their structures and properties were characterized. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) as well as powder X-ray diffraction (PXRD) analysis reveal for all compounds thermotropic liquid crystalline behavior. The observed textures point to smectic A and smectic X phases. The crystal structure of the solvate [C12C12IM]3[Fe(CN)6]·2 DMSO (DMSO = dimethylsulfoxide) was examined by single crystal X-ray diffraction analysis which reveals two crystallographically independent [C12C12IM]+ cations with different conformation, U-shape and rod-shape, together with a nearly ideal octahedral [Fe(CN)6]3− anion and two DMSO solvent molecules. The crystal packing is characterized by alternating bilayers of almost linearly arranged imidazolium cations with the U-shaped cations being arranged in a head-to-head fashion and the cations' hydrophobic dodecyl chains being interdigitated. Magnetic measurements for compounds 1–4 show paramagnetic moments, μeff, of ~1.7 μB, which are close to the theoretically expected value for a d5 low-spin Fe3+ spin-only S = 1/2 system (μeffs.o. = 1.73 μB). It was possible to obtain an Fe3+ containing ILC material that combines liquid crystalline behavior over a large mesophase window (from 33 °C to 170 °C) with high magnetic susceptibility by mixing compound 3 with [C16C16IM][FeCl4] (5), which contains the d5 high-spin Fe3+ ion (S = 5/2). All compounds are highly hydrophobic and stable in the atmosphere indicating a great potential of applications as functional materials.

Dynamic Quantum Sensing of Paramagnetic Species Using Nitrogen-Vacancy Centers in Diamond.

Naturally occurring paramagnetic species (PS), such as free radicals and paramagnetic metalloproteins, play an essential role in a multitude of critical physiological processes including metabolism, cell signaling, and immune response. These highly dynamic species can also act as intrinsic biomarkers for a variety of disease states, while synthetic paramagnetic probes targeted to specific sites on biomolecules enable the study of functional information such as tissue oxygenation and redox status in living systems. The work presented herein describes a new sensing method that exploits the spin-dependent emission of photoluminescence (PL) from an ensemble of nitrogen-vacancy centers in diamond for rapid, nondestructive detection of PS in living systems. Uniquely this approach involves simple measurement protocols that assess PL contrast with and without the application of microwaves. The method is demonstrated to detect concentrations of paramagnetic salts in solution and the widely used magnetic resonance imaging contrast agent gadobutrol with a limit of detection of less than 10 attomol over a 100 μm × 100 μm field of view. Real-time monitoring of changes in the concentration of paramagnetic salts is demonstrated with image exposure times of 20 ms. Further, dynamic tracking of chemical reactions is demonstrated via the conversion of low-spin cyanide-coordinated Fe3+ to hexaaqua Fe3+ under acidic conditions. Finally, the capability to map paramagnetic species in model cells with subcellular resolution is demonstrated using lipid membranes containing gadolinium-labeled phospholipids under ambient conditions in the order of minutes. Overall, this work introduces a new sensing approach for the realization of fast, sensitive imaging of PS in a widefield format that is readily deployable in biomedical settings. Ultimately, this new approach to nitrogen vacancy-based quantum sensing paves the way toward minimally invasive real-time mapping and observation of free radicals in in vitro cellular environments.

Pressure-enhanced interplay between lattice, spin, and charge in the mixed perovskite La2FeMnO6

Spin crossover plays a central role in the structural instability, net magnetic moment modification, metallization, and even in superconductivity in corresponding materials. Most reports on the pressure-induced spin crossover with a large volume collapse so far focused on compounds with single transition metal. Here we report a comprehensive high-pressure investigation of a mixed Fe-Mn perovskite La2FeMnO6. Under pressure, the strong coupling between Fe and Mn leads to a combined valence/spin transition: Fe3+(S = 5/2) to Fe2+(S = 0) and Mn3+(S = 2) to Mn4+(S = 3/2), with an isostructural phase transition. The spin transitions of both Fe and Mn are offset by ~ 20 GPa of the onset pressure, and the lattice collapse occurs in between. Interestingly, Fe3+ ion shows an abnormal behavior when it reaches a lower valence state (Fe2+) accompanied by a + 0.5 eV energy shift in Fe K-absorption edge at 15 GPa. This process is associated with the charge-spin-orbital state transition from high spin Fe3+ to low spin Fe2+, caused by the significantly enhanced t2g-eg crystal field splitting in the compressed lattice under high pressure. Density Functional Theory calculations confirm the energy preference of the high-pressure state with charge redistribution accompanied by spin state transition of Fe ions. Moreover, La2FeMnO6 maintains semiconductor behaviors even when the pressure reached 144.5 GPa as evidenced by the electrical transport measurements, despite the huge resistivity decreasing 7 orders of magnitude compared with that at ambient pressure. The investigation carried out here demonstrates high flexibility of double perovskites and their good potentials for optimizing the functionality of these materials.

A Trinuclear Iron(III) Complex of a Triple Noninnocent Ligand for Spin-Structured Molecular Conductors.

N,N',N'',N''',N'''',N'''''-(Triphenylene-2,3,6,7,10,11-hexayl)hexapicolinamide (H6 hptp), a triangular ligand containing three noninnocent coordination sites, and its trinuclear iron(III) complex (PPh4 )3 [FeIII 3 (hptp)6 (CN)6 ] were prepared. In this complex, three low-spin Fe3+ ions are coordinated by the triangular hptp6- bridging ligand at its apices. Cyclic voltammetry of (PPh4 )3 [FeIII 3 (hptp)6 (CN)6 ] in PhCN solution showed one reductive wave for 3 Fe2+ /3 Fe3+ and three oxidative waves for the hptp6- ligand. The controlled-potential electronic spectra of the solution presented broad absorption bands in the near-IR (NIR) region for oxidations because a π-radical is generated on the hptp6- ligand. The EPR spectra of frozen solutions also showed large oxidization-state dependent differences caused by the strong exchange interactions between the electrons in the d orbitals of the Fe3+ ions and those of the π-radicals in the hptp6- ligand. The electronic and magnetic states of the oxidized species were investigated using density functional theory.