Oxidation State Of Mn Ions Research Articles

Effects of processing parameters on the properties of lanthanum manganite

A systematic study of the effects of processing parameters on the structure, microstructure, magnetic, and magnetotransport properties of lanthanum manganite (LMO) ceramics are presented. LMO ceramics are prepared using solid-state reaction technique by applying wide range of sintering temperatures 1100–1500 °C with varying holding times. It is observed that phase pure LMO can be formed at a sintering temperature of 1300 °C for a short duration of 2 h with measured density 4.8 g cm −3. Density does not improve with lower sintering temperature and longer duration. The grains appeared are quite homogeneous and uniform and do not differ much with further increase of temperature, however, less voids and higher leakage currents are observed for the LMO samples sintered at higher temperature. As the physical properties of manganites depend of structure, composition and oxidation states of Mn ions, XPS analysis are done to study the valence states. XPS analysis shows that LMO ceramics sintered at higher temperature have lower Mn valence and values approaches towards +2 as well as the loss of oxygen stoichiometry. All the samples show colossal magnetoresistance (CMR) behavior, however, the LMO prepared by sintering at 1300 °C for 2 h shows the highest MR% ∼60%.

Electronic properties of LiMn2−x Ti x O4

Ti-substituted LiMn2O4 (LiMn2−x Ti x O4, x=0, 0.15, 0.30, 0.45, 0.60, and 0.75) has been synthesized using solid-state reactions. Their crystal and electronic structures were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). XRD data suggested that the lattice parameters of LiMn2−x Ti x O4 increase due to the replacement of Mn by Ti ions. XPS results indicated that the substituted Ti ions were in +4 oxidation state; consequently, the normal oxidation state of Mn ions has been detected by measuring the binding energy splitting of Mn 3s states, which decreases with the content of substituted Ti. The valence band spectra suggested that the intensity of e g level of Mn 3d orbitals increased due to the increase of the Mn3+/Mn4+ ratio.

Investigations on 0.5 Li ( Ni0.8Co0.15Zr0.05 ) O2 – 0.5 Li ( Li1 ∕ 3Mn2 ∕ 3 ) O2 Cathode for Li Rechargeable Batteries

In the present work we have reported the electrochemical characteristics of 0.5–0.5 cathodes for lithium rechargeable batteries. In the pristine cathodes the formal oxidation state of Mn ions are , whereas Ni and Co are in the valence state. Rietveld refinement of the powders indicates a hexagonal layered structure (space group ) and minimal cation disorder between lithium and transition-metal layers. Raman analysis confirms the composite nature of pristine cathode laminates which contain (LMO)- and (LNCZO)-rich regions. The galvanostatic charge–discharge characteristic of these composite cathodes is reported in various cutoff voltage limits. When these composite cathodes are charged to , electrochemical extraction of Li takes place from active (LNCZO) as well as inactive (LMO) components. The charge and discharge capacitates of these cathodes are found to be due to , , and redox couples. Reasonably good cyclability of these composite cathodes is obtained when cycled in the cutoff voltage limits of . The cyclability of these composite cathodes are deteriorated when these composite cathodes are charged above and deep discharged up to , followed by repeated cycling in these voltage limits. The probable reasons for such findings are discussed.

Study on structural, magnetic and transport properties of La0.7Ca0.3Mn1-xCoxO3 (x = 0.01-0.05) thin films

During the last few years the studies on the divalent ion substituted rare earth manganites R1-xAxMnO3 (R = rare earth, A = alkaline earth ions), gained impetus after the compound were found to show colossal magnetoresistance. The physical properties of the substituted manganites depend on their structure, composition and oxidation states of Mn ions. Doping of the divalent metal ions in LaMnO3 induces ferromagnetism due to the change in the oxidation state of Mn ions. This indicates that the valence state of Mn ions play a key role in the tuning of the physical properties of manganites. In the present work, we study the effect of Co ions substitution at Mn side in La0.7Ca0.3Mn1-xCoxO3 (x = 0.01-0.05) thin films. Thin films of La0.7Ca0.3Mn1-xCoxO3 were grown epitaxially at 700°C under the oxygen pressure of 300 mTorr using pulsed laser deposition technique on LaAlO3 (001) substrate. X-ray diffraction study infers that all films grow epitaxially along the c-axis. The field cooled magnetization measured as a function of temperature indicates that all film exhibits ferromagnetic (FM) to paramagnetic (PM) transition with increase in the temperature. Moreover, the doping of Co affects the colossal magnetoresistance (CMR) effect in the present materials.

First-principles investigation on extraction of lithium ion from monoclinic LiMnO 2

Changes with Li extraction in the crystal structure and electronic structure of monoclinic LiMnO 2 have been investigated by first-principle calculations using spin-polarized generalized gradient approximation method. It is found that the Li extraction has changed the oxidation state of Mn ion from 3+ to 4+ and thereby induced a phase transition from the monoclinic structure to rhombohedral symmetry, which opens channels for the migration of Mn ions from their own octahedral sites to the interlayer Li vacancies. It is the migration of Mn ions that responds for the phase transition to a spinel-like structure and the severe capacity loss during the electrochemical cycles of monoclinic LiMnO 2.

Effect of Magnesium Substitution into LaMnAl11O19 Hexaaluminate on the Activity of Methane Catalytic Combustion

The effect of the substitution of manganese by magnesium on the activity and physicochemical properties of a LaMn1-xMgxAl11O19 hexaaluminate catalyst was investigated. The samples were prepared using a coprecipitation method and characterized by X-ray diffractometry (XRD), hydrogen temperature-programmed reduction (TPR), oxygen temperature-programmed desorption (TPD), and nitrogen adsorption. The results show that the Mg-substitution enhances the catalytic activity. A sample with a composition of LaMn0.7Mg0.3Al11O19 has a very good activity and T10% = 704 K. XRD analysis suggests that the partial substitution of manganese by magnesium effectively suppresses the crystal growth along the [001] direction, which leads to an improved surface area. Although the increase in activity is partially attributable to the high surface area, TPD and TPR results confirm that the substitution also affects the oxygen adsorption property and the oxidation state of Mn ions in the hexaaluminate lattice. The reducibility, the ...

X-ray photoelectron spectroscopy of LiM 0.05Mn 1.95O 4 (M = Ni, Fe and Ti)

LiMn 2O 4 and LiM 0.05Mn 1.95O 4 (M = Ni, Fe and Ti) were synthesized by using solid-state reactions and their surface stoichiometries were confirmed by XPS data. The crystal and electronic structures were investigated by using X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). XRD data suggested that LiM 0.05Mn 1.95O 4 possesses nearly no any variations in lattice parameters compared with LiMn 2O 4 for slight substitution of Ni, Fe and Ti; the substituted Ni, Fe and Ti ions were located on the 16d octahedral sites in the spinel crystal lattice. The XPS results suggested that Fe and Ti ions were at + 3 and + 4 oxidation states, respectively; while Ni ions are mixed with + 2 and + 3 oxidation states. The normal oxidation state of Mn ions in the above four materials is almost the same and calculated as + 3.55 according to the splitting energies of Mn3s states.

Mn-substituted Ca–La–hexaaluminate nanoparticles for catalytic combustion of methane

Mn-substituted Ca–La–hexaaluminate rod-like nanoparticles (Ca1−xLaxMnAl11O19−α) with high surface area ranging from 47 to 80m2/g for catalytic combustion of methane have been prepared using alumina sol as the (NH4)2CO3 coprecipitation precursor and a supercritical drying (SCD) method. Ca substitution gave rise to the maximum combustion activity (T10%=459°C) at x=0 owing to the highest surface area. Meanwhile, Ca substitution affects the oxygen sorption property and the oxidation state of Mn ions in the hexaaluminate lattice. Ca0.6La0.4MnAl11O19−α catalyst with high catalytic activities was obtained owing to the excellent performance of activating oxygen.

X-ray photoelectron spectroscopy of La 0.5Sr 0.5MnO 3

Cathode electrode for solid oxide fuel cells (SOFC) consisting of strontium-doped lanthanum manganite (La 0.5Sr 0.5MnO 3: LSM) was prepared with a modified sol–gel process. Its surface chemical composition was studied by X-ray photoelectron spectroscopy (XPS). As prepared LSM, the oxidation state of Mn ions is +3.5, which is consistent with oxidation state calculated from the chemical formula. The ratio of La, Sr and Mn ions is about 1/1.1/2. After the sample was heated in ultrahigh vacuum (UHV) chamber with oxygen partial pressure of 10 −6 mbar at 600 °C for about 3 h, the oxidation state of Mn ions decreases from +3.5 to +3.3 probably due to the generation of oxygen vacancy on the surface. A little Sr-rich phase was formed on the surface duo to the formation of SrO.

Optical investigation of electronic states of Mn4+ ions in p-type GaN

The electronic states of manganese in p-type GaN are investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. A series of sharp PL lines at 1.0 eV is observed in codoped GaN and attributed to the intra d-shell transition 4T2(F)-4T1(F) of Mn4+ ions. PLE spectrum of the Mn4+ [4T2(F)-4T1(F)] luminescence reveals intra center excitation processes via the excited states of Mn4+ ions. PLE peaks observed at 1.79 and 2.33 eV are attributed to the intra-d-shell 4T1(P)-4T1(F) and 4A2(F)-4T1(F) transitions of Mn4+, respectively. In addition to the intra shell excitation processes, a broad PLE band involving charge-transfer transition of the Mn4+/3+ deep level is observed, which is well described by the Lucovsky model. As determined from the onset of this PLE band, the position of the Mn4+/3+ deep level is 1.11 eV above the valence band maximum, which is consistent with prior theory using ab initio calculations. Our work indicates 4+ is the predominant oxidation state of Mn ions in p-type GaN:Mn when the Fermi energy is lower than 1.11 eV above the valence band maximum.

X-ray photoelectron spectroscopy of La 0.5Sr 0.5MnO 3

Cathode electrode for solid oxide fuel cells (SOFC) consisting of strontium-doped lanthanum manganite (La 0.5Sr 0.5MnO 3: LSM) was prepared with a modified sol–gel process. Its surface chemical composition was studied by X-ray photoelectron spectroscopy (XPS). As prepared LSM, the oxidation state of Mn ions is +3.5, which is consistent with oxidation state calculated from the chemical formula. The ratio of La, Sr and Mn ions is about 1/1.1/2. After the sample was heated in ultrahigh vacuum (UHV) chamber with oxygen partial pressure of 10 −6 mbar at 600 °C for about 3 h, the oxidation state of Mn ions decreases from +3.5 to +3.3 probably due to the generation of oxygen vacancy on the surface. A little Sr rich phase was formed on the surface duo to the formation of SrO.

On magnetic properties of LaMnO3±δ phase within its domain

Magnetic behaviour of LaxMnyO(3x+yV)/2 samples with different La/Mn ratios (0.91:1, 1:1, and 1:0.9) has been studied by dc-, and ac-susceptibility measurements. The samples were prepared under various sintering conditions with the aim to stabilize appropriate structure forms of this phase, i.e. the rhombohedral (Rh), the low-temperature orthorhombic (O*) and the high-temperature orthorhombic (O), each characterized by specific oxidation state of Mn ions (3.0–3.6) and specific distribution of the structure components. The results obtained have allowed to clarify more precisely complex magnetic properties of lanthanum manganite phase and their evolution versus oxidation state of Mn ions within the structure forms available.

Electrochemical behavior of Li intercalation processes into a Li[Ni xLi (1/3−2 x/3) Mn (2/3− x/3) ]O 2 cathode

Li[Ni x Li (1/3−2 x/3) Mn (2/3− x/3) ]O 2 ( X=0.17, 0.25, 0.33, 0.5) compounds are prepared by a simple combustion method. The Rietvelt analysis shows that these compounds could be classified as having the α-NaFeO 2 structure. The initial charge–discharge and irreversible capacity increases with the decrease of x in Li[Ni x Li (1/3−2 x/3) Mn (2/3− x/3) ]O 2. Indeed, Li[Ni 0.50Mn 0.50]O 2 compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni 0.17Li 0.22Mn 0.61]O 2 and Li[Ni 0.25Li 0.17Mn 0. 58]O 2 compounds exhibit high initial discharge capacity over 245 mAh/g and stable cycle performance in the voltage range of 4.8 –2.0 V. On the other hand, XANES analysis shows that the oxidation state of Ni ion reversibly changes between Ni 2+ and about Ni 3+, while the oxidation state of Mn ion sustains Mn 4+ during charge–discharge process. This result does not agree with the previously reported ‘electrochemistry model’ of Li[Ni x Li (1/3−2x/3)Mn (2/3−x/3)]O 2, in which Ni ion changes between Ni 2+ and NI 4+. Based on these results, we modified oxidation-state change of Mn and Ni ion during charge–discharge process.

Polymorphic forms and defect structure formation within homogeneity domain of LaMnO 3 phase

A detailed crystallochemical characterization of air annealed LaMnO 3+ δ samples of different La/Mn ratios is presented. This characterization comprises the temperature dependencies of the lattice parameters, the valence of manganese ( V Mn), and sample densities. The latter are compared with densities calculated for various models of possible distribution of the structure components. The data obtained have allowed: (a) to determine temperature stabilities of previously known individual structure forms appearing within the homogeneity domain of perovskite-related LaMnO 3 phase, (b) to confirm the presence of cationic deficiency, created on La and Mn sites in quantities which depend on both, the nominal La/Mn ratio and the temperature-dependent oxidation state of Mn ions greater than 3+, (c) to provide strong evidence for the presence of some excess oxygen ions. Their presumable locations may be either at vacant positions of the metal-sublattices mentioned above or at some interstitial positions available. Similarly to the cation deficiency, quantities of the excess oxygen have occurred highly dependent on the nominal La/Mn ratio and on the oxidation state of Mn ions, the latter variable versus temperature. For La/Mn=0.91:1 stoichiometry samples, a distribution characterized by a certain fraction of Mn ions on the La site has also been determined within some temperature range. Analytical formulae describing the appropriate distribution models found within the homogeneity domain of LaMnO 3+ δ versus temperature are given.

Thermoluminescence studies on Li 2O–CaF 2–B 2O 3 glasses doped with manganese ions

Thermoluminescence studies on X-ray irradiated Li 2O–CaF 2–B 2O 3 glasses doped with different concentrations (0–0.4 mol%) of MnO have been carried out. The thermoluminescence light output has been observed to decrease with increase in MnO concentration. Results of these studies (coupled with the data on optical absorption and infrared spectra) are analyzed in the light of different oxidation states of Mn ions.

Stability of LiMn 2O 4 and new high temperature phases in air, O 2 and N 2

We investigate the reversible and irreversible transformations, the Li, Mn spinel undergoes under different atmospheres (air, O 2, and N 2) when heated up to 1050 °C. In air and O 2, a substantial reversible cation exchange occurs. For T≥800 °C, Mn 2+ ions substitute Li + at the tetrahedral (8a) position, and Li + shifts to interstitial octahedral (16c) site. Charge balance is achieved by a decrease in the Mn 3+ fraction, which is partially reduced to Mn 2+ in the regular octahedral (16d) site, according to the charge distribution (Li 1−2 y +Mn 2 y 2+) 8a[Li 3 y +] 16c[Mn 5 y 2+Mn 1−5 y 3+Mn 1 4+] 16dO 4 ( y increases with T). Under N 2 flow, a first decomposition occurs between 600 and 800 °C and yields Mn 3O 4, orthorhombic LiMnO 2 and O 2. A minor O 2 release occurs when heating above 900 °C, which is accompanied by the transformation of o-LiMnO 2 into a cubic Li x Mn 1− x O solid solution ( x≤0.5), consistent with a decrease in the average oxidation state of Mn ions. This cubic phase is stable at high temperature and decomposes upon cooling (T≤800 ° C) leaving just the Mn 3O 4 and o-LiMnO 2 phases.

Synthesis and characterization of lithium aluminum-doped spinel (LiAl xMn 2− xO 4) for lithium secondary battery

LiAl x Mn 2− x O 4 has been synthesized using various aluminum starting materials, such as Al(NO 3) 3, Al(OH) 3, AlF 3 and Al 2O 3 at 600–800°C for 20 h in air or oxygen atmosphere. A melt-impregnation method was used to synthesize Al-doped spinel with good battery performance in this research. The Al-doped content and the intensity ratio of (3 1 1)/(4 0 0) peaks can be important parameters in synthesizing Al-doped spinel which satisfies the requirements of high discharge capacity and good cycleability at the same time. The decrease in Mn 3+ ion by Al substitution induces a high average oxidation state of Mn ion in the LiAl x Mn 2− x O 4 material. The electrochemical behavior of all samples was studied in Li/LiPF 6-EC/DMC (1:2 by volume)/LiAl x Mn 2− x O 4 cells. Especially, the initial and last discharge capacity of LiAl 0.09Mn 1.97O 4 using LiOH, Mn 3O 4 and Al(OH) 3 complex were 128.7 and 115.5 mAh/g after 100 cycles. The Al substitution in LiMn 2O 4 was an excellent method of enhancing the cycleability of stoichiometric spinel during electrochemical cycling.

Oxygen Reduction Activities of Praseodymium Manganites in Alkaline Solution

A series of praseodymium manganites, Pr1-xAxMnO3 (A=Ca, Sr, Ba, 0≤x≤1.0), were examined for their electrode-catalytic activities as incorporated into a gas diffusion-type oxygen reduction electrode working in an alkaline solution. Among the oxides tested, Pr0.6Ca0.4MnO3 gave the highest electrode performance (320mA·cm-2 at -150mV vs. Hg/HgO) in 8mol·dm-3 KOH aqueous solution at 60°C under air flow. The electrode performances were found to be well correlated with the activities of the oxides for the 4-electron reduction of oxygen (O2+2H2O+4e-→4OH-) evaluated from a rotating ring-disk electrode analysis. The oxidation state of Mn ions of the oxides was investigated by means of iodometry, X-ray photoelectron spectroscopy, temperature-programmed oxygen desorption and electron spin resonance. The results obtained suggest that the active sites for the 4-electron reduction of oxygen involve Mn3+ ions and that their activities increase profoundly as the Mn3+ ions neighbor Mn4+ to form Mn3+-Mn4+ pairs. The electrode loaded with Pr0.6Ca0.4MnO3 was confirmed to continue to work rather stably for a test period of 200h under the galvanostatic condition of 300mA·cm-2, the oxide being kept intact as revealed by X-ray diffraction analysis after the test. The same electrode was shown to exhibit a promising performance when incorporated into a zinc-air cell.

Local structures around Mn, Co and Ni atoms in glasses of the systems CaMgSi2O6-CaMnSi2O6, CaMgSi2O6-CaCoSi2O6 and CaMgSi2O6-CaNiSi2O6 determined by the XAFS spectroscopy.

Local structures around transition-metal atoms in silicate glasses of the systems CaMgSi2O6 (Di)-CaMnSi2O6(CaMnPx), Di-CaCoSi2O6 (CaCoPx) and Di-CaNiSi2O6 (CaNiPx) were investigated with the XAFS (X-ray Absorption Fine Structure) spectroscopy, by making use of SR (Synchrotron Radiation). The obtained distances suggest that the preference to the octahedral coordinations is in the decreasing order Ni2+, Co2+ and Mn2+. This order can be explained in view of the difference between the crystal-field stabilization energies for octahedral and tetrahedral coordinations in the high-spin states. ELNES (Energy-Loss Near Edge Structure) spectra were also measured around the Mn L2,3 edges to determine the oxidation states of Mn ions in glasses of the system Di-CaMnPx. Chemical shifts of the L2,3 edge threshold-energy and the ratios of L3/L2 threshold peaks in all the examined glasses were similar to those in MnO but unlike those in MnO2. The observed tendency of chemical shifts in the XANES spectra of Mn K-edges was identical with that in ELNES. The shifts of Co K- and Ni K-edges in the XANES spectra of all the examined glasses were similar to those of Co-olivine and NiO, respectively. These facts suggest that Mn, Co and Ni ions in the glasses are in divalent states.

Effect of structural modification on the catalytic property of Mn-substituted hexaaluminates

The effects of structural modification on the catalytic properties of Mn-substituted hexaaluminates were investigated with a view toward developing a high-temperature combustion catalyst. The surface areas and catalytic activities of Mn-substituted hexaaluminates are significantly enhanced when the mirror plane cations are partially replaced by aliovalent cations of the same size. In the Sr 1- x La x MnAl 11O 19-α system calcined at 1300°C, Sr substitution gave rise to the maximum surface area, 23.8 M 2/g, and the maximum combustion activity ( T 10% = 500°C) at x = 0.2. Transmission electron microscopy revealed that the substitution thinned the planar crystals of hexaaluminate and thus increased the surface area. Although the increase in catalytic activity corresponds to that in surface area, Sr substitution also affects the oxygen sorption property and the oxidation state of Mn ions in the hexaaluminate lattice. In particular, oxygen species desorbed at high temperatures (650–800°C) appear to play an important role in catalytic reaction. These oxygen species are related to the oxidation/reduction of Mn ion, which is brought into the highest oxidation state by the partial substitution.