Valence State Of Manganese Ions Research Articles

Electrical, dielectric and spectroscopic studies on MnO doped LiI–AgI–B2O3 glasses

LiI–AgI–B2O3 glasses doped with different concentrations of MnO (ranging from 0 to 0.8 mol%) were prepared. Electrical and dielectric properties have been studied over a wide frequency range of 10−2 – 106 Hz and in the temperature range from 173 to 523 K. The valence states of manganese ions and their coordination in the glass network have been investigated using optical absorption, luminescence, and ESR spectroscopy. The analysis of the spectroscopic results has indicated that the manganese ions exist in both Mn2+ and Mn3+ states and occupy octahedral and tetrahedral positions. With increasing MnO concentration there is a gradual increase in the tetrahedral occupancy of Mn2+ ions at the expense of octahedral occupancy in the glass network. The results of dc conductivity have indicated that when T > θD/2, the small polaron hopping model is appropriate and the conduction is adiabatic in the nature. Further, the analysis of experimental data indicates that there is a mixed, ionic and electronic, conduction. It has been observed that the electrical conductivity decreases as the concentration of MnO increases suggesting the electronic conduction controlled by polaron hopping between manganese ions. In the low temperature region, up to 250 K, the ac conductivity is nearly temperature independent and varies linearly with frequency, which can be explained by the quantum mechanical tunneling (QMT) model. The dielectric properties have been analyzed in the framework of complex dielectric permittivity and complex electrical modulus formalisms. The evolution of the complex permittivity as a function of frequency and temperature has been investigated.

Investigation on Oxygen Potential Distribution in a ZrO2-Based Solid Electrolyte by Using In-Situ Micro XAS Technique

Oxygen potential distribution in a ZrO2-based solid electrolyte in the vicinity of a dense film platinum electrode under a current flow was investigated by using in-situ micro x-ray absorption spectroscopy. The oxide ion conductor (Y2O3)0.08(ZrO2)0.92 doped with 2.5 mol% of Mn2O3, in which the valence state of manganese ions could be used as an indicator for evaluating the oxygen potential, was chosen as a model electrolyte. As results, a drastic drop of oxygen potential was apparently observed near the triple phase boundary. The proposed method was found to be principally applicable for the direct observation of the oxygen potential distribution in an oxide ion conductor under a current flow.

Catalytic activity of Mn-substituted barium hexaaluminates for methane combustion

The catalysts of hexaaluminate (BaMn x Al12−x O19−δ , x = 1.0, 2.0, 3.0, 4.0) to be used in methane combustion have been successfully synthesized by co-precipitation method and supercritical drying. The crystalline structure and surface area of catalyst were characterized by X-ray diffraction (XRD) and nitrogen adsorption analysis of BET method. BET analysis revealed that the preparing and drying method proposed here provides stable materials with higher surface area of 51.4 m2/g in comparison to materials prepared using conventional ambient drying method for BaMn x Al12−x O19−δ calcined at 1200° under oxygen. XRD analysis indicated that formation of a pure single phase BaMn x Al12−x O19−δ occurred up to x = 3 in the case of Mn-substituted barium hexaaluminates. Incorporation of Mn in excess leads to BaAl2O4 phase formation. As far as the valence state of Manganese ions was concerned, the introduced Mn ions were either divalent or trivalent. The first Mn ions were introduced in the matrix essentially as Mn2+ and only for BaMn3Al9O19−δ does manganese exist exclusively as Mn3+; the higher the Mn concentration, the higher the proportion of Mn3+. Catalytic activity for methane combustion has been measured for Mn-substituted barium hexaaluminates, light-off temperature was observed in the 512−624°C range. The highest activity was obtained for catalysts containing 3 Mn ions per unit cell, which reveals that the BaMn x Al12−x O19−δ catalyst was a promising methane combustion catalyst with high activity and good thermal stability. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.

Spin and Valence States of Manganese Ions in Mn-Doped BaTiO<sub>3</sub> Ceramics by Electron Paramagnetic Resonance

A number of paramagnetic defects and defect complexes in BaTiO3 have been detected by the Electron Paramagnetic Resonance (EPR) technique. The signal intensity, the g factors and the hyperfine splitting constants A of Mn2+ at the different temperatures were distributed with temperatures in a U-shape. In this paper, a new direct evidence for the presence of manganese with an electron spin of S=5/2 (Mn2+) in all four phases (R, O, T and C) and the high spin state of the electron configuration in MnTi² at the high temperature cubic phase is presented by EPR method in La-Mn codoped BaTiO3 PTCR ceramics.

EPR INVESTIGATION OF Bi2O3-As2O3-MnO GLASSES

Glasses of the system Bi 2 O 3· As 2 O 3 doped with MnO up to 20 mol% were studied by means of EPR. The distribution of Mn 2+ ions on different structural units of the matrix was studied as function of the MnO content of the glass. The revealed valence states of manganese ions and the interactions involving them are discussed.

Magnetic susceptibility studies on Bi 2O 3–PbO–As 2O 3–MnO glasses

The valence states of manganese ions and the interactions involving them were investigated for xMnO·(100− x)[Bi 2O 3·PbO·As 2O 3] glasses with 0< x≤30 mol% by means of the temperature dependence of the magnetic susceptibility. Up to x=5 mol% of both Mn 2+ and Mn 3+ ions are evidenced. For higher MnO content ( x>5 mol%), the manganese ions in bivalent valence state dominate. For the most part, manganese ions participate in negative superexchange interactions as MnO content exceeds 3 mol%.

Spin and valence states of manganese ions in manganese-doped yttrium orthoaluminate

Two paramagnetic charge states of manganese with total electron spin of S=5/2 (Mn 2+) and S=3/2 (Mn 4+) were identified by electron paramagnetic resonance (EPR) method in Mn doped YAlO 3. In addition to the allowed ΔM S =1 spin transitions, low field forbidden ΔM S =2 transitions were observed for both Mn 4+ and Mn 2+ and ΔM S =3 transitions for Mn 2+. Relative intensities of forbidden transitions increase with a decrease of temperature and decrease with an increase of Mn concentration. The effect is due to the increase of spin–lattice and/or spin–spin relaxation times at low temperature and the decrease of spin–spin relaxation time at higher dopant concentration. Optically induced disproportionation reaction 2Mn 4+→Mn 3++Mn 5+ in YAlO 3 was studied by EPR.

Reversible change in the valence state of manganese ions between tetra valent and divalent states on a sodium-mounted magnesium oxide surface

Paramagnetic manganese cation species have been investigated for Mn-added Na MgO by ESR. It has been found that a Mn 4+ species different from those reported previously is present on the surface of Na MgO . The Mn 4+ is converted to surface Mn 2+ by degassing at 773 K and restores when exposed to oxygen.

Reversible change in the valence state of manganese ions between tetravalent and divalent states on a sodium-mounted magnesium oxide surface

Paramagnetic manganese cation species have been investigated for Mn-added Na/MgO by ESR. It has been found that a Mn 4+ species different from those reported previously is present on the surface of Na/MgO. The Mn 4+ is converted to surface Mn 2+ by degassing at 773 K and restores when exposed to oxygen.

EXAFS study of polymorphs of manganese dioxides

An attempt has been made to use extended X-ray absorption fine structure (EXAFS) to study the polymorphs (beta, gamma and alpha types, and ramsdellite) of manganese dioxides. The ratio of (1 × 1) tunnels to (1 × 2) tunnels for the intergrowth structure of gamma-MnO2 was found to be as high as 46.2%. The relation between the valence state of manganese ions and the threshold absorption energy (E0) was clarified andE0 was found to be in the following order: beta>gamma>alpha (K)>alpha (Ba). EXAFS presented a possibility of nondestructive analysis of the valence state of manganese ions in manganese dioxides.