Mixed-valence Manganese Research Articles

The paradigm of magnetic molecule in quantum matter: Slow molecular spin relaxation

The quantum nature of single-ion magnets, single-molecule magnets, and single-chain magnets has been manifested among other phenomena by magnetic hysteresis due to slow spin relaxation, competing with fast quantum tunneling at low temperatures. Slow spin relaxation, described by Arrhenius-type law with the effective barrier energies Ueff = 50 cm–1, was discovered 3 decades ago in paramagnetic Mn12-acetate complex of oxy-bridged mixed-valence manganese ions, below the blocking temperature TB = 3 K. In contrast to common magnetic materials, it is governed primarily by magnetic anisotropy, set by zero-splitting of spin states of a magnetic ion in a field of ligands, and spin-lattice coupling. The emerging studies on the border of coordination chemistry, physics of spin systems with reduced dimensionality, and nanotechnologies, were performed in search of routes for enhancement of Ueff and TB characteristics, in line with increase of operation temperature and quantum correlation time, mandatory for quantum applications. The best results with TB ∼ 80 K and Ueff ∼ 1261 cm–1, were obtained for DyIII single-ion magnet, so far. Numerous excellent research and review articles address particular activities behind this achievement. It follows, that present challenges are dictated by the rational development of novel, smart magnetic molecules, featured by butterfly cores, cyano-bridges, 2D metal-organic frameworks, and metal-free graphene nanoclusters, as well as stable free radicals, magnetized by spare electrons. These species are briefly considered here with respect to the unique experience of international collaborative activity, established by Prof. Juan Bartolomé.

Tuning the Composition, Crystal Structure and Morphology of Manganese(III/IV) Oxide for High-Power Storage Applications

If inherently intermittent renewable energy technologies are to offer a practical alternative to conventional heat engines, it is critically important that efficient, cost-effective and sustainable energy storage solutions are developed to decouple supply and demand profiles. A promising strategy is to pair the competitive energy densities of batteries with the superior cycling lifetimes and power densities of supercapacitors,[1] employing an appropriate maximum power point tracking (MPPT) system to optimise energy exchange between these complementary devices.[2] Mixed-valence manganese(III/IV) oxides (MnOz, where z ranges between 1.5 and 2.0) have emerged as foremost candidates for the electroactive material in supercapacitor cathodes due to their low toxicity, minimal cost,[3] ready availability (manganese appears at an average concentration of ca. 1,000 ppm in the Earth’s crust[4]), wide electrochemical stability window (ca. 1.0 V in aqueous electrolytes[5]), and high maximum theoretical specific capacitance (ca. 1,370 F g-1, based on the reduction of Mn4+ ions by a single electron[6]). Charge storage in MnOz is predominantly mediated by the pseudocapacitive intercalation of cations from the surrounding electrolyte in each discharge phase, followed by the release of these guest ions during charging.Of the various crystallographic phases of MnOz, cryptomelane (α-MnOz) and birnessite (δ-MnOz) polymorphs are deemed two of the most promising candidates for storage applications because they possess characteristically wide channels for intercalation/de-intercalation:[8,9] α-MnOz consists of ca. 4.6 Å diameter one-dimensional tunnels enclosed between corner-sharing MnO6 octahedra, whereas δ-MnOz comprises two-dimensional layers of MnO6 octahedra separated by ca. 7.0 Å. Herein, a scalable, low-cost synthesis procedure is discussed that enables the production of highly pseudocapacitive birnessite and cryptomelane electrodes with tuneable material characteristics, including particle morphology, crystallographic structure, average Mn oxidation state, and the level of pre-intercalation by Na+ and K+ ions.[10] In addition to comparing the electrochemical properties of these NaxKyMnOz products in aqueous conditions, their application within an asymmetric supercapacitor is showcased: by optimising the relative masses of a NaxKyMnOz cathode and activated carbon anode in a 100 cm2 aqueous full-cell configuration, discharge capacitances in the range 10-30 F g-1 can be achieved over a voltage span of 1.8 V at current densities as high as 10 A g-1, with Coulombic efficiency exceeding 80% up to ca. 20 A g-1. With these competitive performance characteristics, the developed NaxKyMnOz materials are a promising option for high-power supercapacitor cathodes, helping to pave the way towards more efficient and sustainable energy storage.

Grain size effect on the critical behavior of the ferromagnetic transition in electronically phase separated La0.5Ca0.5MnO3

Mixed valence manganese oxides (manganites) are known to exhibit intrinsic inhomogeneous magnetic states, which are influenced by a great variety of factors, including oxygen stoichiometry, nanostructuration and ceramic grain size. In this context, the prototypical half-doped compound La0.5Ca0.5MnO3was extensively studied. Here we reloaded the phase separated scenario, focusing on the thermodynamical critical behavior of this system at the paramagnetic - ferromagnetic transition and its correlation with the evolution of its ordering capabilities controlled through the ceramic grain size.Our findings establish a clear connection between the correlation length and the grain size, shedding light on the role of this parameter in the control of inhomogeneous magnetic ground states in manganites. Therefore it is possible to tailor fundamental physical properties as the scaling laws of the system just tuning the ceramic grain size.

A comparative study of structural, magnetic and catalytic properties of half doped nanocrystalline La0.5A0.5Mn0.5Fe0.5O3 (A = Ca, Sr) perovskite oxides: Highly efficient and versatile candidates for enhanced oxidative degradation of hazardous organic dyes at ambient conditions

This work presents a systematic study of half doped nanocrystalline La0.5A0.5Mn0.5Fe0.5O3 (A = Ca, Sr) perovskite oxides to examine the effect of A-site cation radius on structural, magnetic and catalytic properties via different characterization techniques. Rietveld refinements of powder X-ray diffraction data confirmed the orthorhombic symmetry for both the samples with Pbnm space group. The results show an increase in lattice parameters and unit cell volume with doping of larger A cation. The BET surface area for Ca and Sr-doped samples has been found to be 32.3 m2/g and 33.8 m2/g respectively. The magnetic properties of nano structured samples show magnetic inhomogeneity due to coexistence of ferromagnetic and antiferromagnetic interactions. The magnetization has enhanced in Sr-doped sample which may be associated with increase in double exchange interactions between mixed valence manganese ions through oxygen. Moreover, the synthesized nano samples have been employed to catalytic degradation of rhodamine B dye as model pollutant under normal atmospheric conditions and it has been found that Sr-doped sample has shown better catalytic dye degradation than Ca-doped (99% degradation in 25 min) due to the presence of greater number of Mn3+/Mn4+ redox couples as determined by iodometric titration. Additionally, methylene blue and methyl orange dyes were also tested for catalytic degradation by more active Sr-doped catalyst in order to check its versability.

Unprecedented pseudo-zeroth-order kinetics of the catecholase-like activity of mixed-valence MnIIMn2III complexes

The pseudo-zero-order kinetics of the catecholase-like activity of biologically active linear trinuclear mixed-valence manganese complexes have been explored; the formation of singlet oxygen is thought to be the rate-limiting step.

Two Magnetic Orderings and a Spin-Flop Transition in Mixed Valence Compound Mn3O(SeO3)3.

A mixed-valence manganese selenite, Mn3O(SeO3)3, was successfully synthesized using a conventional hydrothermal method. The three-dimensional framework of this compound is composed of an MnO6 octahedra and an SeO3 trigonal pyramid. The magnetic topological arrangement of manganese ions shows a three-dimensional framework formed by the intersection of octa-kagomé spin sublattices and staircase-kagomé spin sublattices. Susceptibility, magnetization and heat capacity measurements confirm that Mn3O(SeO3)3 exhibits two successive long-range antiferromagnetic orderings with TN1~4.5 K and TN2~45 K and a field-induced spin–flop transition at a critical field of 4.5 T at low temperature.

In situ synthesis of Tree-branch-like Copper-manganese oxides nanoarrays supported on copper foam as a superior efficiency Fenton-like catalyst for enhanced degradation of 4-chlorophenol

Recently, considerable attention has been paid to develop novel Fenton-like oxidation systems for environmental remediation. The activation of periodate (NaIO4) is a desirable oxidation process that drive redox reactions to produce abundant reactive oxygen species (ROS) by a suitable activator. This study reports a simple hydrothermal-calcination route for forming novel tree-branch-like copper-manganese oxides (CuMnOx@MnOx, CMM) nanoarrays, and the production of radical species from NaIO4 induced by CMM for the removal of rhodamine B and 4-chlorophenol from wastewater. The synergistic effects of Cu oxides and Mn oxides composite significantly elevated the activation of periodate, exhibited excellent degradation performance in the CMM/NaIO4. Furthermore, reduced copper species and mixed-valence manganese species play a major role in reaction via XPS analysis. Additionally, the underlying degradation mechanism of this work was systematically researched by radical quenching tests and EPR analysis. Superoxide anion radical (O2−) was the major free radicals in this system, simultaneously the production of non-radical singlet oxygen (1O2) via the electron transfer, which are contributed to remove organic pollutants. This work provides a facile way for fabricating monolithic multi-component metal oxides, and new insights into understand activation mechanism of manganese-based periodate activator.

Mixed-valence manganese oxide/reduced graphene oxide composites with enhanced pseudocapacitive performance

Mixed-valence manganese oxide/reduced graphene oxide composites with enhanced pseudocapacitive performance

Syntheses and Characterization of the Mixed-Valent Manganese(II/III) Fluorides Mn2F5 and Mn3F8

We obtained single crystals of the binary mixed-valent fluorides Mn2F5 and Mn3F8 using a high-pressure/high-temperature approach. Mn2F5 crystallizes isotypic to CaCrF5 in the monoclinic space group C2/c (No. 15), with a = 8.7078(8) Å, b = 6.1473(6) Å, c = 7.7817(7) Å, β = 117.41(1)°, V = 369.80(6) Å3, Z = 4, and mC28 at T = 173 K. Mn3F8 crystallizes in the monoclinic space group P21 (No. 4) with a = 5.5253(2) Å, b = 4.8786(2) Å, c = 9.9124(4) Å, β = 92.608(2)°, V = 266.92(2) Å3, Z = 2, and mP22 at T = 183 K and presents a new structure type. Crystal-chemical reasoning, CHARDI calculations, and quantum-chemical calculations allowed for the assignment of the oxidation states of the Mn atoms. In both bulk compounds, MnF2 was present as an impurity, as evidenced by powder X-ray diffraction and IR and Raman spectroscopy.

Identification of seven-coordinate LnIII ions in a LnIII[15-MCFe III N(shi)-5](OAc)2Cl species crystallized from methanol and pyridine.

The title metallacrown (MC) complexes LnIII[15-MCFeIIIN(shi)-5](OAc)2CI(C5H5N)6 (Ln1), where OAc- is acetate, shi3- is salicylhydroximate, and Ln = Gd and Dy, were synthesized via a self-assembly reaction in methanol and pyridine. Single crystals were grown using slow evaporation and characterized using X-ray diffraction. Seven-coordinate capped octahedron geometries were observed for the lanthanide ion in both complexes, which is uncommon for trivalent lanthanide species. The 15-MC-5 is a ruffled metallacrown archetype similar to previously reported mixed-valent manganese metallacrowns.

Resonant Stimulated X-ray Raman Spectroscopy of Mixed-Valence Manganese Complexes.

Resonant stimulated X-ray Raman spectroscopy of the bimetallic [MnIIIMnIV(μ-O)2(μ-OAC)(tacn)2]2+ manganese complex is investigated in a simulation study. Essential biological processes, including water oxidation in photosynthesis, involve charge transfer between manganese sites of different oxidation states. We study a prototypical binuclear mixed-valence transition-metal complex with two Mn atoms in different oxidation states surrounded by ligand structures and employ a pump-probe sequence of resonant X-ray Raman excitations to follow the charge transfer occurring in the molecule. This allows us to generate and monitor valence-electron wave packets at selected regions in the molecule by exploiting element-specific core-excited states. A two-color protocol is presented, with pump and probe pulses tuned to the Mn and N K-edges. A natural orbital decomposition allows the visualization of the electron dynamics underlying the signal.

Green Three-Step Electrodeposition of Mn+3 Rich Layered δ-Phase MnO2 Nanofibers on Silicon and Epitaxial Graphene-Silicon Carbide Heterostructures

Mixed valence manganese oxides (MnOx) have attracted significant research interest in recent years due to the easily reversible redox reactions between manganese oxidation states (Mn+2, Mn+3, and Mn+4)1 which enable applications in catalysis2, energy storage3, and gas sensing4. Of the manganese oxides manganese dioxide (MnO2) has been of particular interest due to its wide variety of synthesized structural polymorphs ((β)1x1 tunnel5, (α)1x2 tunnel5 (γ) spinel5, (δ) layered5) which allow for enhanced control over the available surface area and reactive properties of MnO2. Among these structural polymorphs, the α and δ phases exhibit mixed-valence character with Mn+3 defects being found throughout the crystal structure5. Mn+3 defects have been found to increase the catalytic activity of MnO2 making its presence desirable6. Hexagonal δ-MnO2, in particular, contains a large number of Mn+3 ions as the interlayer contains Mn+2/+3 ions to neutralize the layer charge from Mn+3 lattice defects7. However, the controlled synthesis of hexagonal δ-MnO2 has proven challenging and often relies on the use of harsh chemicals1, high temperatures1, and long reaction times1. These methods also rely on permanganate salts1 as the manganese precursor, which leads to the formation of monoclinic δ-MnO2 and significantly reduces the number of Mn+3 defects in the crystal lattice. In this work, we present the green facile synthesis of high Mn+3 content hexagonal δ-MnO2 via electrodeposition on both silicon(Si) and epitaxial graphene/silicon carbide(EG/SiC) substrates. The electrodeposition was carried out in a three-electrode electrochemical cell with an Ag/AgCl reference electrode, a Pt counter electrode, and the substrate (Si or EG/SiC) as the working electrode utilizing a 100mM manganese acetate solution. The electrodeposition process contained three steps, an initial pulse at the oxidation potential to seed the surface with oxygen, a second pulse at the reduction potential to deposit manganese on the surface, and a third pulse at the oxidation potential to oxidize the deposited manganese, with the potentials determined by cyclic voltammetry. The resulting δ-MnO2 thin films were then characterized through Raman spectroscopy, X-Ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy(AFM) to determine its crystalline phase and surface morphology. Raman spectroscopy(Fig. 1) confirmed the formation of MnO2 on both substrates due to the presence of Raman peaks between 650-640cm-1,8 575-585cm-1,8, and 490-510cm-1,8, which are associated with the Mn-O symmetric stretching bond8, the Mn-O stretching bond in the basal plane8, and Mn-O stretching bond in the [MnO6] octehedra8. The formation of the δ phase is confirmed by the presence of a weak peak at 130cm-1,2 consistent with the literature. The observed 9cm-1 redshift of the 490-510cm-1,6 between Si and SiC substrates is associated with the weakening Mn-O stretching bond on Si, which combined with the strengthening of the peak at 600-620cm-1 indicates that the growth on Si contains a higher rate of Mn+3defects2,8. XPS corroborated this result and indicated that the manganese in the δ-MnO2 deposited on EG/SiC had an average oxidation state of 3.3. The AFM images indicated that the surface was made up of nanofibrous nanoparticles, with the deposition on Si exhibiting significantly larger nanofibers (Fig 2.). SEM confirmed this (Fig. 3) and indicated that the deposition on EG/SiC was made up of microplates ~40umx40um in size, which was not observed in the deposition on Si. We believe this variation in surface morphology is due to interactions between SiC and MnO2, which are not present when δ-MnO2 is grown on Si. The reactivity of the deposited δ-MnO2 was then tested by depositing 4 Ti/Au (30nm/120nm) contacts on the δ-MnO2/EG/SiC heterostructures to form a simple gas sensor, which was tested against 5ppm NO2, 5ppm NH3, 1000ppm IPA, and 1000ppm methanol. The sensor displayed a remarkably fast response/ recovery time of 3.8s/2.2s to NH3 and 3.4/6.0s to NO2(Fig. 4), demonstrating the high reactive potential of the δ-MnO2 thin films. This reactivity demonstrates that our electrodeposited δ-MnO2 provides a good candidate for use in other applications of MnO2 such as energy storage and catalysis.

A Force Field for a Manganese-Vanadium Water Oxidation Catalyst: Redox Potentials in Solution as Showcase

We present a molecular mechanics force field in AMBER format for the mixed-valence manganese vanadium oxide cluster [Mn4V4O17(OAc)3]3−—a synthetic analogue of the oxygen-evolving complex that catalyzes the water oxidation reaction in photosystem II—with parameter sets for two different oxidation states. Most force field parameters involving metal atoms have been newly parametrized and the harmonic terms refined using hybrid quantum mechanics/molecular mechanics reference simulations, although some parameters were adapted from pre-existing force fields of vanadate cages and manganese oxo dimers. The characteristic Jahn–Teller distortions of d4 MnIII ions in octahedral environments are recovered by the force field. As an application, the developed parameters have been used to calculate the redox potential of the [MnIIIMn3IV] ⇌ [Mn4IV]+e− half-reaction in acetonitrile by means of Marcus theory.

Over-Reduction-Controlled Mixed-Valent Manganese Oxide with Tunable Mn2+/Mn3+ Ratio for High-Performance Asymmetric Supercapacitor with Enhanced Cycling Stability.

Manganese oxides composed of various valence states Mnx+ (x = 2, 3, and 4) have attracted wide attention as promising electrode materials for asymmetric supercapacitor. However, the poor electrical conductivity limited their performance and application. Appropriate regulation content of Mnx+ in mixed-valent manganese oxide can tune the electronic structure and further improve their conductivity and performance. Herein, we prepared manganese oxides with different Mn2+/Mn3+ ratios through an over-reduction (OR) strategy for tuning the internal electron structure of mixed-valent manganese, which could make these material oxides a good platform for researching the structure-property relationships. The Mn2+/Mn3+ ratio of manganese oxide could be precisely tuned from 0.6 to 1.7 by controlling the amount of reducing agent for manipulating the redox processes, where the manganese oxide electrode with the most appropriate Mn2+/Mn3+ ratio, as 1.65 (OR4) exhibits large capacitance (274 F g-1) and the assembling asymmetric supercapacitors by combining OR4 (positive) and the commercial activated carbon (as negative) achieved large 2.0 V voltage window and high energy density of 27.7 Wh kg-1 (power density of 500 W kg-1). The cycle lifespan of the OR4//AC could keep about 92.9% after 10 000-cycle tests owing to the Jahn-Teller distortion of the Mn(III)O6 octahedron, which is more competitive compared to other work. Moreover, a red-light-emitting diode (LED) can easily be lit for 15 min by two all-solid supercapacitor devices in a series.

Mixed-valent manganese oxide for catalytic oxidation of Orange II by activation of persulfate: heterojunction dependence and mechanism

The coexistence of aliovalent cations in manganese oxide catalysts with a suitable mole ratio accelerates the activation process of persulfate for the degradation of organic pollutants.

Enhanced zinc storage performance of mixed valent manganese oxide for flexible coaxial fiber zinc-ion battery by limited reduction control

While manganese-based cathodes have been intensively studied for zinc-ion batteries (ZIBs), the limited rate capability and cycle life have always been a difficult problem to be solved. Here, we report a mixed valent manganese oxide (MnOx) cathode with superior electrochemical performance, which exhibits a high specific capacity of 450 mA h/g at 0.2 C and a satisfactory specific capacity of 158.3 mA h/g at a high rate of 5 C. The mixed cathode system reduces the charge transfer resistance, and show good surface stability and adsorption properties, so it is beneficial for the storage of Zn2+. Meanwhile, coaxial fiber ZIBs (CFZIBs) with splendid flexibility are assembled utilizing the elaborately prepared cathode material. The CFZIBs achieve a reversible capacity of 255.8 mA h/g and the capacity retention rate is as high as 80 % after 1000 bending deformations. This study provides new opportunities for designing ZIBs with high performance and high flexibility.

Metal-organic frameworks as template for synthesis of Mn3+/Mn4+ mixed valence manganese cobaltites electrocatalysts for oxygen evolution reaction

Cobalt-based oxides are among the most promising electrocatalysts for oxygen evolution reactions (OER). In this context, this work reports the synthesis of manganese-doped cobaltites using the Zeolitic-Imidazolate Frameworks 67 (ZIF-67) as template. The incorporation of manganese ions into ZIF-67 structure was evaluated in ethanol and methanol, in order to obtain the best synthetic route. Non-doped (ZIF-67C) and Mn-doped cobaltites (Mn/ZIF-67C(E) and Mn/ZIF-67C(M)) were obtained after thermal treatment at 350 °C. Structural and morphological properties were investigated and presence of Mn3+ and Mn4+ was confirmed by X-ray photoelectron spectroscopy (XPS) data and magnetization curves. The electrocatalytic activity in OER was investigated in alkaline medium for manganese cobaltites, and compared to the ZIF-67C. Overpotentials to generate a current of 10 mA cm−2 were 338 mV and 356 mV for Mn/ZIF-67C(E) and Mn/ZIF-67C(M), respectively. These results are superior to those found for similar materials in the literature. The material obtained in methanol (Mn/ZIF-67C(M)) presents lower overpotential, however, shows superior electrocatalytic performance for current density above 100 mA cm−2, therefore being an efficient electrode for commercial electrolysers.

A-site mixed-valence co-doping to optimize room-temperature TCR of polycrystalline La0.8K0.04Ca0.16-xSrxMnO3 ceramics

Herein, polycrystalline La0.8K0.04Ca0.16-xSrxMnO3 ceramics (A-site = La, K, Ca and Sr, 0.00 ≤ x ≤ 0.16) are successfully synthesized via the sol-gel process and the influence of Sr content on microstructural and electrical transport properties is investigated in detail. The results reveal that the replacement of Ca-ions by Sr-ions resulted in a decreased resistivity and enhanced temperature coefficient of resistance (TCR) peak temperature (Tk). The influence of A-site mixed-valence co-doping, where A-site is occupied by monovalent alkali-metal, divalent alkaline-earth element and trivalent rare-earth element, on electrical transport properties can be explained by using different conduction mechanisms in different temperature regions. At optimal doping content of 0.12, peak TCR of the resistivity was found to be 12.04% K−1 at 290.38 K, which is the best reported TCR for A-site mixed-valence co-doped perovskite manganese oxides. These results confirm that high room-temperature TCR values can be achieved by optimizing A-site mixed-valence co-doping and demonstrate the potential of polycrystalline La0.8K0.04Ca0.04Sr0.12MnO3 ceramic in uncooling infrared bolometers.

Chemical and Toxicological Analysis of Territories Affected by Radiation and Chemical Pollutants: II. Toxicity Effects of Natural Water (in vitro Studies)

The toxic properties of native water samples of surface (river) water and groundwater used to meet drinking water needs in settlements of the Zhizdrinskii and Ul’yanovskii districts of Kaluga Oblast, which were the areas most exposed to radiation after the accident at the Chernobyl nuclear power plant in 1986, are studied. The focus is on manifestations of genotoxic effects, as the most typical effects of radiation pollution of the territory. It is shown that water exhibits, in most of the analyzed samples, a reliable cytogenetic effect in relation to cells of transplanted cultures of the Chinese hamster (CHO and 237 clones). At the same time, the extractable ether-soluble organic substances contained in the analyzed water samples exhibit genotoxic activity only after they are concentrated by a factor of 100. In parallel, the mutagenic toxicity of water samples was analyzed using the Ames test. Without metabolic activation (in the absence of additions of the S9 fraction of the rat liver), most water samples from the drinking water supply sources show an acute toxicity to Salmonella strains (TA 97, TA 98, and TA 100). The results of biological tests are discussed in relation to previously obtained data on the radiation effects of the Chernobyl accident on the aquatic environment of the Dnieper reservoirs. It is concluded that the principal role in the manifestation of genotoxic properties may be played by radioactive isotopes that are hidden from traditional methods of radiation monitoring and are sources of alpha and beta radiation that lead to the formation of metal ions in the superoxidized state in the presence of environmental pollution by transition metals, in particular, to the formation of microcolloidal particles of mixed-valence manganese Mn(III, IV) that serve as carriers for the OH radical. The acute and chronic toxicity of the analyzed river and groundwater samples is established by bioassay methods using test organisms of various trophic levels (bacteria, ciliates, and Daphnia). No correlation with toxicity is detected in any of the traditionally monitored hydrochemical indices of water quality.

Aggregation of a Giant Bean-like {Mn26Dy6} Heterometallic Oxo-Hydroxo-Carboxylate Nanosized Cluster from a Hexanuclear {Mn6} Precursor

Starting from a well-known hexanuclear mixed-valent manganese cluster, we report a synthetic protocol for its extension toward very large 3d-4f clusters. The exemplary case study demonstrates that ...