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The development of high-performance, sustainable sodium-ion batteries requires a mechanistic understanding of cathode redox processes and structural stability. Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a promising cathode material due to its non-toxicity, high average working voltage, and excellent structural and thermal stability. However, its practical application is hindered by impurity phase formation and low intrinsic electronic conductivity. To address these challenges, Na₄Fe_3-xM_x(PO₄)₂(P₂O₇) (M: Mn, Ni) composites are synthesized via a sol-gel method. A comprehensive characterization approach combining X-ray diffraction (XRD), X-ray absorption spectroscopy (XANES/EXAFS, soft XAS), and resonant inelastic X-ray scattering (RIXS) revealed that low-level substitution of Mn²⁺ and Ni²⁺ into Fe sites suppresses the formation of electrochemically inactive maricite NaFePO₄ and modifies the Fe-O coordination environment. These effects may result in lower ion migration energy barriers and better electrochemical reversibility. Among the doped samples, Mn-NFPP exhibited the best electrochemical performance, delivering a discharge capacity of ∼92 mAh g⁻¹ at 0.1 C and ∼80 mAh g⁻¹ at 2 C, with 99.5 % capacity retention after 100 cycles at 0.1 C. This work provides fundamental insights into the redox mechanism and atomic-scale structure–property relationship of NFPP, guiding the design of high-performance polyanionic cathodes for sodium-ion batteries.

The nonstoichiometric Fe₂P-type FeMn_(1-x)V_x(P_0.5Si_0.5)_1-x alloys (x = 0, 0.01, 0.02, and 0.03) have been investigated as potential candidates for magnetic refrigeration near room temperature. The magnetic ordering temperature decreases with increasing FeV concentration x, which can be ascribed to decreased ferromagnetic coupling strength between the magnetic atoms. The strong magnetoelastic coupling in these alloys results in large values of the isothermal entropy change (ΔS_M); 15.7 J/(kg K), at 2 T magnetic field for the x = 0 alloy. ΔS_M decreases with increasing x. Results from Mössbauer spectroscopy reveal that the average hyperfine field (in the ferromagnetic state) and average center shift (in the paramagnetic state) have the same decreasing trend as ΔS_M. The thermal hysteresis (ΔT_hyst) of the magnetic phase transition decreases with increasing x, while the mechanical stability of the alloys improves due to the reduced lattice volume change across the magnetoelastic phase transition. The adiabatic temperature change ΔT_ad, which highly depends on ΔT_hyst, is 1.7 K at 1.9 T applied field for the x = 0.02 alloy.

The design of iron complexes with long-lived charge transfer states suitable for applications as photosensitizers remains a formidable challenge. Here, we investigated the effect of an extended ligand pi-system on the ground- and excited-state properties of iron(II) complexes with N-heterocyclic carbene (NHC) ligands. For this purpose, three iron complexes based on the established [Fe(II)(pbmi)2]2+ motif (pbmi = (1,1 '-(pyridine-2,6-diyl)bis(3-methylimidazole-2-ylidene))) have been modified with phenylethynyl moieties attached to the pyridine part of the ligand. In general, the introduction of the phenylethynyl units served to red shift the main absorption band, as well as to increase the extinction coefficient of the same, compared to the parent complex. The lowered MLCT energies are in line with the electrochemical data that revealed substantially easier reduction of the phenylethynyl-modified ligands, while the potentials of the Fe(III/II) couple are only moderately increased. Only minor modifications of the electronic effect intrinsic to the phenylethynyl moieties could be implemented with bromide and dimethylamino substituents on the phenylene units. As a result, all three complexes experience similar stabilization of their 3MLCT states, about 0.3 eV compared to the parent complex, and feature transient absorption data in line with ES dynamics that are dominated by a moderately long-lived (similar to 17 ps) 3MLCT state. These values exceed the 3MLCT lifetimes reported for the parent complex (up to 9 ps) and resemble the results for carboxylic acid and imidazolinium derivatives with comparable 3MLCT energies and lifetimes.

Layered Na₃Fe₃(PO₄)₄ can function as a positive electrode for both Li- and Na-ion batteries and may hold advantages from both classical layered and phosphate-based electrode materials. Using a combination of ex-situ and operando synchrotron radiation powder X-ray diffraction, void space analysis, and Mössbauer spectroscopy, we herein investigate the structural evolution of the Na₃Fe₃(PO₄)₄ framework during Li- and Na-ion intercalation. We show that during discharge, Li- and Na-intercalation into Na₃Fe₃(PO₄)₄ occurs via a solid solution reaction wherein Na-ions appear to be preferentially intercalated into the intralayer sites. The intercalation causes an expansion of the unit cell volume, however at open circuit conditions after ion-intercalation (i.e., after battery discharge), Na_3+xFe₃(PO₄)₄ and Li_xNa₃Fe₃(PO₄)₄ undergo a structural relaxation, wherein the unit volume contracts below that of the pristine material. Rietveld refinement suggests that the ions intercalated into the intra-layer sites diffuse to the sites in the inter-layer space during the relaxation. This behavior brings new perspectives to understanding structural relaxation and deviations between structural evolution observed under dynamic and static conditions.

Recently, polyanionic compounds have received great interest as alternative cathode materials to conventional oxides due to their different advantages in cost, safety, structural stability, as well as being environmentally friendly. However, the vast majority of polyanionic materials reported so far rely exclusively upon the redox reaction of the transition metal for lithium/sodium transfer.

The development of multielectron redox-active cathode materials is a top priority for achieving high energy density with long cycle life in the next-generation secondary battery applications. Triggering anion redox activity is a promising strategy to enhance the energy density of polyanionic cathode materials for Li/Na-ion batteries. In addition to transition metal redox activity, the oxalate group also shows redox behavior enabling reversible charge/discharge and high capacity without gas evolution.

Herein, we report NaLiFe(C₂O₄)₂ as a new positive electrode and use different characterization techniques such as Raman spectroscopy and Mössbauer analyses to characterise this dual-ion redox process experimentally. First-principles calculations also help to understand the interactions between the transition metal and the oxalate group as the main factor that modulates the cationic and polyanionic redox couples in these materials.

Magnetic hyperthermia holds significant therapeutic potential, yet its clinical adoption faces challenges. One obstacle is the large-scale synthesis of high-quality superparamagnetic iron oxide nanoparticles (SPIONs) required for inducing hyperthermia. Robust and scalable manufacturing would ensure control over the key quality attributes of SPIONs, and facilitate clinical translation and regulatory approval. Therefore, we implemented a risk-based pharmaceutical quality by design (QbD) approach for SPION production using flame spray pyrolysis (FSP), a scalable technique with excellent batch-to-batch consistency. A design of experiments method enabled precise size control during manufacturing. Subsequent modeling linked the SPION size (6–30 nm) and composition to intrinsic loss power (ILP), a measure of hyperthermia performance. FSP successfully fine-tuned the SPION composition with dopants (Zn, Mn, Mg), at various concentrations. Hyperthermia performance showed a strong nonlinear relationship with SPION size and composition. Moreover, the ILP demonstrated a stronger correlation to coercivity and remanence than to the saturation magnetization of SPIONs. The optimal operating space identified the midsized (15–18 nm) Mn_0.25Fe_2.75O₄ as the most promising nanoparticle for hyperthermia. The production of these nanoparticles on a pilot scale showed the feasibility of large-scale manufacturing, and cytotoxicity investigations in multiple cell lines confirmed their biocompatibility. In vitro hyperthermia studies with Caco-2 cells revealed that Mn_0.25Fe_2.75O₄ nanoparticles induced 80% greater cell death than undoped SPIONs. The systematic QbD approach developed here incorporates process robustness, scalability, and predictability, thus, supporting the clinical translation of high-performance SPIONs for magnetic hyperthermia.

Na4Fe3(PO4)(2)(P2O7) (NFPP) as a promising cathode material for sodium-ion batteries possesses excellent structural stability, minimal volume change, low cost, and non-toxicity. However, its practical application is hindered by the formation of impurity phases and its intrinsically low electronic conductivity. Herein, crystalline high purity carbon-coated NFPP (NFPP/CC) is synthesized by performing a green and scalable combustion method to enhance its overall electrochemical performance. The effects of pre-treatment and the calcination atmosphere on the structure and purity of NFPP are systematically investigated for a variety of synthesis parameters. The electrochemical performance of NFPP cathodes is evaluated in both half-cells with the sodium metal anode and full-cells with the hard-carbon anode via galvanostatic charge-discharge cycling measurements. The "combustion" synthesized NFPP/CC cathode delivers a reversible discharge capacity of similar to 102 mA h g(-1) at 0.1C in an operating voltage window of 1.8-3.8 V (vs. Na/Na+) retaining 99.7% of its initial capacity over 100 cycles. Furthermore, it demonstrates enhanced rate capability in comparison to the NFPP/CC cathode synthesized via the conventional calcination route. This study sheds light on using the combustion method as a facile and effective strategy to simultaneously mitigate the formation of impurity phases, reduce the carbon content, enhance the quality of carbon coating, improve the homogeneity of nanoparticles and pores within the structure, and enhance the electronic conductivity and physical stability of NFPP cathodes, paving the way for their practical application in high-performance sodium-ion batteries.

We here report the synthesis of the homoleptic iron(II) N-heterocyclic carbene (NHC) complex [Fe(miHpbmi)(2)](PF6)(4) (miHpbmi = 4-((3-methyl-1H-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the pi-electron-withdrawing 3-methyl-1H-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)(2)](PF6)(2) (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi)(2)](PF6)(2) (cpbmi = 1,1 '-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi)(2)](PF6)(4) is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi)(2)](PF6)(4) a promising pH-insensitive analogue of [Fe(cpbmi)(2)](PF6)(2). Finally, the excited-state dynamics of the title compound [Fe(miHpbmi)(2)](PF6)(4) was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.

The Fe²⁺-dependent E. coli enzyme FucO catalyzes the reversible interconversion of short-chain (S)-lactaldehyde and (S)-1,2-propane­diol, using NADH and NAD⁺ as cofactors, respectively. Laboratory-directed evolution experiments have been carried out previously using phenyl­acetaldehyde as the substrate for screening catalytic activity with bulky substrates, which are very poorly reduced by wild-type FucO. These experiments identified the N151G/L259V double mutant (dubbed DA1472) as the most active variant with this substrate via a two-step evolutionary pathway, in which each step consisted of one point mutation. Here the crystal structures of DA1472 and its parent D93 (L259V) are reported, showing that these amino acid substitutions provide more space in the active site, though they do not cause changes in the main-chain conformation. The catalytic activity of DA1472 with the physiological substrate (S)-lactaldehyde and a series of substituted phenyl­acetaldehyde derivatives were systematically quantified and compared with that of wild-type as well as with the corresponding point-mutation variants (N151G and L259V). There is a 9000-fold increase in activity, when expressed as k_cat/K_M values, for DA1472 compared with wild-type FucO for the phenyl­acetaldehyde substrate. The crystal structure of DA1472 complexed with a non-reactive analog of this substrate (3,4-di­meth­oxy­phenyl­acetamide) suggests the mode of binding of the bulky group of the new substrate. These combined structure–function studies therefore explain the dramatic increase in catalytic activity of the DA1472 variant for bulky aldehyde substrates. The structure comparisons also suggest why the active site in which Fe²⁺ is replaced by Zn²⁺ is not able to support catalysis.

Prussian blue analogues (PBAs), A_xM[M’(CN)₆]_1–y·zH₂O, are a highly functional class of materials with use in a broad range of applications, such as energy storage, due to their porous structure and tunable composition. The porosity is particularly important for the properties and is deeply coupled to the cation, water, and [M’(CN)6]n– vacancy content. Determining internal porosity is especially challenging because the three compositional parameters are dependent on each other. In this work, we apply a new method, positron annihilation lifetime spectroscopy (PALS), which can be employed for the characterization of defects and structural changes in crystalline materials. Four samples were prepared to evaluate the method’s ability to detect changes in internal porosity as a function of the cation, water, and [M’(CN)₆]^n– vacancy content. Three of the samples have identical [M’(CN)₆]^n– vacancy content and gradually decreasing sodium and water content, while one sample has no sodium and 25% [M’(CN)₆]^n– vacancies. The samples were thoroughly characterized using inductively coupled plasma-optical emission spectroscopy (ICP-OES), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Mössbauer spectroscopy as well as applying the PALS method. Mössbauer spectroscopy, XRD, and TGA analysis revealed the sample compositions Na_1.8(2)Fe²⁺_0.64(6)Fe^2.6+_0.36(10)[Fe²⁺(CN)₆]·2.09(2)H₂O, Na_1.1(2)Fe²⁺_0.24(6)Fe^2.8+_0.76(6)[Fe^2.3+(CN)₆]·1.57(1)H₂O, Fe[Fe(CN)₆]·0.807(9)H₂O, and Fe[Fe(CN)₆]0.75·1.5H₂O, confirming the absence of vacancies in the three main samples. It was shown that the final composition of PBAs could only be unambiguously confirmed through the combination of ICP, XRD, TGA, and Mössbauer spectroscopy. Two positron lifetimes of 205 and 405 ps were observed with the 205 ps lifetime being independent of the sodium, water, and/or [Fe(CN)₆]^n– vacancy content, while the lifetime around 405 ps changes with varying sodium and water content. However, the origin and nature of the 405 ps lifetime yet remains unclear. The method shows promise for characterizing changes in the internal porosity in PBAs as a function of the composition and further development work needs to be carried out to ensure the applicability to PBAs generally.