Valence State Of Manganese Research Articles

Hydrogen Peroxide Decomposition over Nanosized La1–XCaXMnO3 (0 ≤ X ≤ 0.6) Perovskite Oxides

AbstractLa1–XCaXMnO3 (X = 0.0, 0.2, 0.4, and 0.6) perovskite oxides were synthesized using the citrate gel method. Thermogravimetry and differential thermal analyses, x‐ray diffraction, scanning electron microscopy, and specific surface area results were used for characterization of the perovskite powders obtained. Also, the average manganese valence state and δ‐oxygen non‐stoichiometry in series compounds were determined by the chemical analysis. Catalytic activities of these perovskites were examined for the peroxide decomposition reaction. It was observed that the catalytic activity was improved with increasing calcium content in the catalyst and it is suggested that factors such as δ‐oxygen non‐stoichiometry and average manganese valence state were responsible for the catalytic activity of these materials.

Determination of manganese valence states in (Mn3+, Mn4+) minerals by electron energy-loss spectroscopy

Various manganese valence quantification methods using manganese L2,3 and oxygen K electron energy loss near-edge spectra (ELNES) were applied to determine the relative portions of individual valence state in a mixed (Mn3+, Mn4+) valence system. Multiple linear least-squares (MLLS) fitting of Mn L2,3 ELNES using reference spectra and Gaussian peak fitting of Mn L3 edge are newly developed and the feasibility of these methods was tested on a set of cryptomelane minerals with different valence states. The selection of appropriate standards is crucial to the success of the MLLS method. The O K-edge structures for manganese oxides can provide valuable guidance in the selection of appropriate reference spectra for quantitative determination of Mn valence state. Gaussian peak fitting, however, failed to determine the Mn valence for (Mn3+, Mn4+) minerals due to the small separation between the primary L3 peaks from Mn4+ and Mn3+ valence. As to the methods based on calibration curves, the energy difference between Mn L3 and oxygen K, i.e., ΔE (L3-O K) vs. valence, is the most valence-sensitive method in the range of Mn3+ and Mn4+ and yields good agreement with the actual values.

Synthesis and electrochemical performance of LiNi0.475Mn0.475Al0.05O2 as cathode material for lithium-ion battery from Ni–Mn–Al–O precursor

LiNi0.475Mn0.475Al0.05O2 cathode material was prepared by solid-state reaction using Ni–Mn–Al–O solid solution as precursor. The solid solution is of spinel structure, in which nickel, manganese, and aluminum are sufficiently mixed at atomic level. Rietveld refinement of X-ray diffraction data revealed that Al doping in LiNi0.5Mn0.5O2 was significantly effective to decrease the degree of Li/Ni cation mixing. XPS analysis showed that the valence states of nickel and manganese were mainly +2 and +4, respectively. LiNi0.475Mn0.475Al0.05O2 delivered a stable capacity of about 206 mAh g−1 with high reversibility. High-rate capability test was also performed.

Depth profile analysis of a cycled lithium ion manganese oxide battery electrode via the valence state of manganese, with soft X-ray emission spectroscopy

A 50-μm thick lithium manganese oxide (parent material LiMn 2O 4) battery electrode (positive electrode; cathode) was charged, slightly discharged and then sliced with a scotch tape test-type method. A selected number of slices was then subject to synchrotron soft X-ray emission spectroscopy near the Mn L α,β emission lines in order to determine changes in the oxidation state of the manganese as a function of sampling depth. The emission spectra showed a minute yet noticeable and systematic chemical shift of up to 0.25 eV between the layer near the current collector and the layer near the electrolyte separator. The average manganese oxidation state near the separator was smaller than the average oxidation state in the interior of the electrode, or near the current collector. Since the data provide an oxidation state depth profile of the cathode, a Li + depth profile can be inferred. This method provides information on the spatial chemical inhomogeneity of electrodes prior to and after electrochemical cycling, and thus can aid in degradation studies.

Quantification of Manganese Valence States Using Mn L2,3 Electron Energy-Loss Near-Edge Spectra

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Unusual Optical Properties of Mn‐doped ZnO: The Search for a New Red Pigment—A Combined Experimental and Theoretical Study

A pigment of your imagination: A range of polycrystalline solid solutions of a zinc-rich Zn(x-1)Mn(x)O system (see figure) have been prepared and studied in terms of their colour, diffuse reflectance spectra, Mn valence state and electronic structure. The intense optical absorption arises from Mn(2+) doping and is thought to be due to forbidden or partially forbidden transitions between the valence and the conduction band.We report an investigation of zinc-rich polycrystalline solid solutions of the Zn(1-x)Mn(x)O system concerning the colour, the diffuse reflectance spectra, the valence state of manganese and the electronic structure. Samples were prepared by a chemical-vapour-transport-assisted route and optimized with respect to colour strength. In agreement with previous experimental results, EPR studies showed that manganese is in the divalent charge state. The nature of the very intense optical absorption, which is caused by Mn(2+) doping and determines the colour of the material, is discussed. It is argued that the Mn(2+)-induced optical absorption is due to forbidden or partially forbidden transitions between the valence and the conduction band that involve Mn admixed states. This assignment is also confirmed by quantum chemical calculations using the semiempirical molecular orbital method MSINDO.

Valence states of 3d ions in lanthanum manganites-cobaltites, determined by X-ray spectroscopy

The valence states of manganese and cobalt ions in lanthanum manganites-cobaltites La1 − xCaxMn0.5Co0.5O3 have been determined by 2p X-ray absorption and 3s X-ray photoelectron spectroscopies. The nanostructured P1 manganite-cobaltite phase with the concentration x = 0, obtained by the citrate method at a temperature of 700°C, is characterized by a valence state of cobalt ions close to 2+ and a valence state of Mn ions of about 3.5+. The P2 phase at the same calcium concentration, synthesized by the high-temperature solid-phase method at a temperature of 1300°C, is characterized by a manganese valence of 3.5+ and a cobalt valence of about 2.3+. An increase in the dopant concentration leads to an increase in the average valence of 3d ions.

Factors influencing the structure of electrochemically prepared α-MnO2 and γ-MnO2 phases

Abstract The α- and γ-phases of MnO2 prepared by electrolysis of MnSO4 and MxSO4 (where M = Li+, Na+, K+, Rb+, Cs+ or Mg2+) in aqueous solutions at various pH and voltage Ev values under ambient conditions have been systematically studied. The structures of powdery MnO2 produced are found to depend on the radius of the Mz+ counter cation in addition to the pH and Ev conditions. In order to achieve the α-phase for MnO2 formation under neutral pH condition, the radius of counter cation must be equal to or greater than 1.41 A, the size of the K+ cation. The relative concentration ratio of [MnO4−]transient/[Mn2+], which is related to the pH–Ev conditions, also affects the structure of MnO2 produced with counter ions smaller than K+. For samples prepared in acidified solution with the counter ions of Li+, Na+ or Mg2+ at 2.2 V, the electrolysis products display the γ-MnO2 phase while those prepared at 2.8 V electrolysis produce a mixture of γ-MnO2 and α-MnO2 phases. Single phase of α-MnO2 is identified in the 5 V electrolysis products. Furthermore, the valence state of manganese was found to decrease as the applied voltage was reduced from 5.0 to 2.2 V. This implies that the lower [MnO4−]transient/[Mn2+] ratio or the less oxidative condition is responsible for the non-stoichiometric MnO2 structure with oxygen deficiency.

A study on mechanism of charging/discharging at amorphous manganese oxide electrode in 0.1 M Na 2SO 4 solution

In the present work, the mechanism of charging/discharging at the amorphous manganese oxide electrode was investigated in 0.1 M Na 2SO 4 solution with respect to amount of hydrates and valence (oxidation) states of manganese using a.c.-impedance spectroscopy, anodic current transient technique and cyclic voltammetry. For this purpose, first the amorphous manganese oxide film was potentiostatically electrodeposited, followed by heat-treatment at 25–400 °C to prepare the electrode specimen with different amounts of hydrates and oxidation states of manganese. For as-electrodeposited electrode with the most hydrates, the anodic current transient clearly exhibited a linear relationship between the logarithm of current density and the logarithm of time, with a slope of −0.5, indicating that the charging/discharging is purely limited by Na +/H + ion diffusion. From the analyses of the impedance spectra combined with anodic current transients measured on the hydrated electrode heat-treated at 25–150 °C, it was found that as the amount of hydrates decreases, the depth of cation diffusion in the electrode becomes shallower and the ratio of charge-transfer resistance to diffusion resistance also increases. This suggests that a transition occurs of pure diffusion control to a mixed diffusion and charge-transfer reaction control. For the dehydrated electrode heat-treated at 200–400 °C, the charging/discharging purely proceeds by the charge-transfer reaction. The reversibility of the redox reaction increases with increasing amount of hydrates and oxidation states of manganese, which provides us the higher power density. On the other hand, the pseudocapacitance decreases in value with increasing heat-treatment temperature, thus causing the lower energy density.

Effect of MnO2, Bi2O3, and ZnO additions on the electrical properties of lead zirconate titanate piezoceramics

The effect of manganese dioxide additions on the electrical properties of lead zirconate titanate (PZT) piezoceramics has been investigated. The results demonstrate that, taken alone, manganese dioxide does not ensure the formation of hard PZT. The valence state of manganese in the piezoceramics is shown to be 4+ if no other dopants are present and 3+ if manganese is introduced in combination with Bi and Zn. Microstructural examination indicates that the grain size of the singly doped ceramics is 5–15 µm, while that of the codoped ceramics is 1–3 µm. The polarization current curves of the piezoceramics containing manganese, bismuth, and zinc oxides have extra maxima, which points to significant internal fields. The manganese is shown to reside at grain boundaries. The conclusion is made that it is the composition of Mn-containing intergranular phases, rather than the presence of manganese ions, that plays a key role in the formation of hard piezoceramics.

X-ray Photoemission Spectroscopy and Magnetic Properties of Bi Doped LaMnO3

Perovskite-type La1−xBixMnO3 (LBMO), which are prepared by doping the Bismuth (Bi) ion in LaMnO3, were shown to exhibit the colossal magnetoresistance (CMR) effect. Samples were produced using a sintering process in an air atmosphere as a function of the Bi composition ratio x. X-ray diffraction measurements showed that the LBMO crystal structure varied among an orthorhombic (or rhombohedral) type with 0.0<x<0.3, to a cubic type with 0.3<x<0.5, and to a monoclinic type with for 0.5<x. The CMR effect of LBMO for x=0.2 was approximately 400% at 140 K. In addition, the magnetoresistance of the LBMO system was found to be proportional to the square of the magnetization. It was interpreted that, from XPS analysis, the valence states of manganese and bismuth atoms in LBMO contain [Mn3+,Mn4+] and [Bi3+, Bi2+ (or pseudo-divalent Bismuth)] ionic states.

Effects of Zr/Ti ratio on structural, dielectric and piezoelectric properties of Mn- and (Mn, F)-doped lead zirconate titanate ceramics

The influence of the Zr/Ti ratio on the piezoelectric properties of (Mn and 1 at.% F)-doped PZT and Mn-doped PZT ceramics is investigated. Lead zirconate titanate ceramics (PZT) are prepared from chemical route based on co-precipitation of oxalates and hydroxides. Structural analysis (XRD) and measurement of electrical properties were carried out. Scanning electron microscopy is used to determine the grain size of the materials. The valency states of Mn in fluorinated PZT ceramics are studied by Electron Spin Resonance (ESR). From the experimental results, the F O substitution increases the piezoelectric activity at the morphotropic phase boundary (MPB) and the Q m in the rhombohedral region. In the (Mn, F)-co-doping case, the high-piezoelectric activity found at the MPB is due to an electron transfer mechanism that involved the three valency states of manganese whereas the improvement of Q m can be explained by the valence state of manganese and the dipole defect concept.

Mn-doped Cu2O thin films grown by rf magnetron sputtering

Cu 2 O has excellent optical properties and could be a good host semiconductor for dilute magnetic semiconductors because doped Cu2O is a p type and direct wide-band-gap semiconductor. In this work, Mn-doped Cu2O thin films were deposited on thermally oxidized silicon substrates by radio-frequency magnetron sputtering. X-ray diffraction results show that highly (200) oriented (Mn0.06Cu0.94)2O films can be achieved at elevated substrate temperature. X-ray photoelectronic spectroscopy confirms that the valence of copper in the films is only +1, and no other valence states. The Mn-doped Cu2O films show primary paramagnetic behaviors above 25K. Very weak ferromagnetic property interspersed with paramagnetic phase appears near 5K. The high magnetic moment of 5.3μB per Mn ion and high resistivity suggest the valence state of manganese is mainly +1.

Low-field microwave response and electron paramagnetic resonance identification of valence states of manganese including octahedral Mn5+ in YAlO3 and CaYAlO4

Microwave response near zero magnetic field has been observed in YAlO3 and CaYAlO4 crystals dilutely doped with Mn in concentration ranging from 0.005 to 0.2 at. %. The response is due to magnetic-field-dependent nonresonant microwave losses, which co-exist with normal electron paramagnetic resonance (EPR) absorption due to Mn2+, Mn4+, and Mn5+ ions in YAlO3 and CaYAlO4. The valence states of manganese have been identified using a dual mode EPR cavity that allowed spectrum recording with polarization of the magnetic vector B1 of the microwave perpendicular or parallel to the static magnetic field B0. The Mn2+ ion, electron spin S=52, and Mn4+, S=32 have been identified using normal B1⊥B0 polarization, while Mn5+, S=1, in unusual octahedral position, has been identified using B1∥B0 polarization. The low-field response did not depend on mutual orientation of the vectors B1 and B0 and had the opposite phase with respect to the paramagnetic absorption. This shows that Mn-doped YAlO3 and CaYAlO4 exhibit magnetically induced microwave absorption, which has a minimum at zero magnetic field and increases with the applied magnetic field. The response in Mn-doped YAlO3 increases with the introduction of Ca2+ ions. The existence of manganese in different valence states, particularly metastable Mn5+ in octahedral position, and the effect of calcium co-doping suggest the formation of relatively free charge carriers that can migrate in the vicinity of paramagnetic manganese ions. The spin-dependent mechanism of the charge migration is discussed to explain magnetoinduced microwave losses.

Pro- or anti-oxidant manganese: a suggested mechanism for reconciliation

The neurodegeneration induced by manganese has been attributed to its ability to undergo redox cycling, and catalysis of reactive oxygen species (ROS) formation, as with other transition metals. However, the characterization of manganese as a pro-oxidant is confounded by increasing evidence that the metal may scavenge superoxide anions and protect cells from oxidative damage. The current study was designed to address conflicting reports pertaining to the oxidative capacity of manganese. We found that the metal has distinctive redox dynamics in which the divalent reduced form, unlike iron, possessed no intrinsic oxidative capacity. The apparent ability of Mn 2+ to promote the formation of ROS within a cortical mitochondrial-synaptosomal fraction was quenched by the depletion of contaminating nanomolar concentrations of trivalent metals. The addition of manganic ions at trace concentrations dose-dependently restored the oxidative capacity attributed to divalent manganese, whereas the presence of the ferric ion retarded the rate of ROS generation. This result was paralleled by the spectrophotometric demonstration that the kinetics of iron oxidation is accelerated by trivalent but not divalent manganese. The markedly different capacities of the lower and higher valence states of manganese to promote free-radical formation in cortical fractions and to modulate the process of iron oxidation may account for earlier contradictory reports of anti- and pro-oxidant properties of manganese.

Colossal magnetoresistance effect of the electron-doped manganeseoxide La1−xSbxMnO3 (x = 0.05, 0.1)

La–Sb–Mn–O compounds have been prepared by the solid-state reaction of La2O3, Sb2O3 andMnCO3.The materials have a perovskite structure and exhibit the colossalmagnetoresistance (CMR) effect, and the maximum magnetoresistanceratio could reach as high as 60% (at 225 K under 5 T) for La0.9Sb0.1MnO3.The valence states of manganese and antimony have been identified as Mn3+, Mn2+ and Sb5+.The temperature dependences of electrical transport and magneticproperties have been studied. The results suggest that the mechanismof CMR for La1−xSbxMnO3is probably due to the double exchange interaction between Mn3+ and Mn2+ions.

NMR probe of phase separation in lightly doped manganites

The 55Mn NMR spectra in an ordered state in a local field and the static magnetic susceptibility were measured for the polycrystalline (LaMn) 1–2 x O 3 ( x=0, 0.03) samples with different concentrations of vacancies in the cation sublattice. It is shown that microscopic regions with a different magnetic structure and manganese valence state (4+) are formed near vacancies in the antiferromagnetic matrix corresponding to the stoichiometric LaMnO 3. The relative volume of these regions and their average size (∼40 Å) were estimated from an analysis of the 55Mn NMR spectra.

TEM studies of an incommensurate charge ordering modulation in La 0.5Ca 0.5MnO 3

Electron energy-loss spectroscopy in the transmission electron microscope has been used to determine the valence state of manganese in La 0.5Ca 0.5MnO 3 at room temperature and at 93 K. An incommensurate charge ordering modulation in La 0.5Ca 0.5MnO 3 was detected by transmission electron microscopy. A structural model, based on the selected area electron diffraction and high-resolution electron microscopy observation, is proposed for the incommensurate modulation.

Study of the Low Field Microwave Response in Yttrium Aluminates Dilutely Doped with Manganese

ABSTRACTMicrowave response near zero magnetic field was observed in YAlO3 and CaYAlO4 crystals dilutely doped with Mn in concentration ranging from 0.05 to 2 atomic %. The response is due to non-resonant microwave absorption, which co-exists with normal electron paramagnetic resonance (EPR) absorption due to different paramagnetic valence states of manganese. Mn2+ and Mn4+ charge states were identified in Mn-doped YAlO3, and Mn2+, Mn4+ and Mn5+ in Mn-doped CaYAlO4. The low field response has the opposite phase with respect to the paramagnetic absorption. This shows that Mn-doped YAlO3 and CaYAlO4 exhibit magnetically induced microwave absorption, which has a minimum at zero magnetic field and increases with the applied magnetic field. This effect is similar to microwave magneto-resistance effects observed in manganite perovskites, where spin-dependent electron tunneling occurs between ferromagnetically coupled manganese ions in different valence states. We show, however, that in the present case of diluted paramagnetic systems, magneto-induced microwave losses are due to intramolecular spin-dependent tunneling, where central paramagnetic ion does not change its charge state and spin-dependent charge migration occurs in the first coordination sphere of paramagnetic ion. Evidences are presented that this ion is Mn2+ exhibiting the highest electron spin S = 5/2.

Enthalpies of Formation of La1−xAxMnO3±δ (A=Ca and Sr) Measured by High-Temperature Solution Calorimetry

High-temperature reaction calorimetry using molten lead borate as a solvent has been used to study the thermochemistry of La1−xAxMnO3 (A=Ca and Sr, 0≤x≤1). The structural properties and the oxygen stoichiometry of the calcined powders were characterized by powder X-ray diffraction, thermogravimetry, and iodometry. The enthalpy of formation of the materials from their binary constituents has been calculated from measured enthalpy of solutions. The enthalpy of formation of La1−xCaxMnO3 from the component oxides becomes more exothermic with increasing substitution level x due to the increasing valence state of Mn, and demonstrates a nearly linear dependence of the average valence state of manganese. La1−xSrxMnO3 has a significantly more exothermic enthalpy of formation from the binary constituents than the Ca-substituted materials, and demonstrates a negative deviation from ideal behavior. The present thermodynamic data are discussed in relation to the Goldschmidt tolerance factor for the perovskite structure. Finally, the phase stability of Sr- and Ca-substituted LaMnO3 is addressed in relation to the application of these materials in solid oxide fuel cells.