High Mn3 Research Articles

Chemical and structural instability of the chemically delithiated (1 – z) Li[Li1/3Mn2/3]O2·(z) Li[Co1–yNiy]O2(0 ≤ y ≤ 1 and 0 ≤ z ≤ 1) solid solution cathodes

The delithiated samples obtained by chemically extracting lithium from a series of O3 type layered (1 – z) Li[Li1/3Mn2/3]O2·(z) Li[Co1–yNiy]O2 solid solution samples by reacting with the oxidizer NO2BF4 in acetonitrile medium have been chemically and structurally characterized. The oxygen loss in the delithiated samples has been found to be determined by the amount of lithium in the transition metal layer of the O3 type layered structure. However, high Mn4+ content in the samples causes a decrease in oxygen loss and the lithium content in the transition metal layer is sensitively influenced by the tendency of Ni3+ to get reduced to Ni2+ and the consequent volatilization of lithium during synthesis. The chemically delithiated samples were found to adopt either the parent O3 type structure or the new P3 or O1 type structures depending upon the composition and synthesis temperature of the parent samples and the proton content inserted into the delithiated sample. For example, a high proton insertion resulting from a good cation ordering or a larger surface area seems to favor the transformation of the initial O3 structure to the P3 or O1 structure. However, such phase transformations were not evident in the electrochemically delithiated solid solution samples.

Factors Influencing the Irreversible Oxygen Loss and Reversible Capacity in Layered Li[Li1/3Mn2/3]O2−Li[M]O2 (M = Mn0.5-yNi0.5-yCo2y and Ni1-yCoy) Solid Solutions

The electrochemical charge−discharge properties of the layered (1 − z)Li[Li1/3Mn2/3]O2·(z)Li[Mn0.5-yNi0.5-yCo2y]O2 (y = 1/12, 1/6, and 1/3 and 0.25 ≤ z ≤ 0.75) and (1 − z)Li[Li1/3Mn2/3]O2·(z)Li[Ni1-yCoy]O2 (0 ≤ y ≤ 1 and 0.2 ≤ z ≤ 0.7) solid solution series have been investigated with an aim to identify the factors that control the amount of oxygen loss from the lattice during the first charge and the reversible capacity values. The first charge profiles exhibit an initial sloping region A followed by a plateau region B around 4.5 V. The sloping region A is found to be determined by the initial average oxidation state of the transition metal ions and their oxidation to Ni4+ and Co3.6+ with Mn remaining as Mn4+. The plateau region B, which corresponds to an irreversible loss of oxygen from the lattice, is found to be determined by the amount of lithium η in the transition metal layer of the solid solution Li[LiηM1-η]O2. However, high Mn4+ content causes a decrease in oxygen mobility and loss. Moreover, the...

Study of La-deficient La1-x□xMnO3-δ perovskites with giant magnetoresistance in the composition x = 0.2

Magnetic, structural and electrical properties in self-doped La1-x□xMnO3-δ system are studied. X-ray diffraction shows that the materials with a composition range x = 0.0 -0.33 are single-phase perovskite-like compounds. Depending on sintering temperature and cooling speed, La-deficient phases undergo a complete transformation from orthorhom-bic to rhombohedral symmetry at high Mn4+ content. These phases exhibit a ferromagnetic to paramagnetic transition with increasing temperature (except for LaMnO3 which is antiferromagnetic (M ≈ 2 emu/g) at 115 K). On annealing at low temperature (800°C), the structural transition is accompanied by an important increase in both magnetization M and Curie temperature Tc . These experimental facts show a close relation of oxygen stoichiometry and Mn4+ stability with synthesis conditions. A sharp drop in resistivity and a magnetoresistance value (p0—P5T)/PO of 85% are observed close to Tc = 268 K, for an annealed sample with x = 0.2.

The influence of ceria surface additions on manganese oxidation at high temperatures

The kinetics, scale composition, and growth mechanism of ceria-coated and blank specimens of manganese oxidation in air were examined. The scale growth obeys the parabolic rate law at 700°C for all specimens. Lower parabolic rate constants for coated specimens are attributed to the presence of a CeO2 external scale. It constitutes a limiting factor of the oxygen activity at the gas-oxide interface. This lower-oxygen activity leads to a less-metal-deficient state of the scale. Due to this, the inner-MnO scale becomes more adherent to the substrate. Preheating at 700°C, in hydrogen (PO 2=10−24 atm), was performed in order to be placed in the MnO stability domain and try to introduce cerium in the manganese-oxide scale. This pretreatment promotes macroscopic bonding in the layer formed during subsequent oxidation in air. It ensures a better scale adherence. A new diffusional-transport mechanism in manganosite is proposed in accordance with all experimental observations of the literature and with the cerium-manganese-oxygen system studied in the present work. This model considers the high Mn3+ stability in octahedral sites of the MnO oxygen ion body. Low-oxygen partial pressure conditions permit the formation of an adherent inner-MnO scale on coated specimens. A CeO2 scale formed above the MnO scale; MnO is present as a minor component in this scale and it is located mainly at the internal interface. The difficulties in forming the cerium-orthomanganite are attributed to the very high stability of MnO related to this wide range of nonstoichiometry and to the low manganese diffusivity through the cerium-containing scale.

Effect of Additions on Conduction Properties of ZnO Varistors

In this paper, several substitutive and defect reactions have been analyzed to describe the conduction properties in ZnO varistors. The composition of ZnO varistors with added Co and Mn ions of high valence, i.e., Co3+ and Mn4+, was deduced to result in higher donor concentration, trap density, barrier height and leakage current, but a lower nonlinearity coefficient in comparison with that of ZnO varistors with added Co and Mn ions of low valence, i.e., Co2+ and Mn2+. The capacitance-voltage (C-V) and current-voltage (I-V) measurements were employed in the experiment, respectively. Theoretical inference was found to be consistent with the experimental results of electrical characteristics.

Dielectric Properties of the Solid Solution Formed in the System LiTao3 - “Li2MnO4 ”

AbstractImpedance spectroscopy methods were used to characterize the electrical properties of the solid solution formed in the system LiTaO3 -“Li2Mn4O9”.Involved parameters were calculated using equialent circuits. Activation energy values were decreasing from around 1.3 eV for lowest concentration of Mn4+ until around 0.8 eV for higher Mn4+ contents. Results will be discused in terms of Z' vs Z" and Arrhenius plots. The ferroelectric character was stablished using the real part of the permittivity against temperature. Curie temperature values Tc were found decreasing toward higher concentration of Mn4+ in the range 630-580°C.

Crystal Structure Phase Transitions in the Perovskites A(Cu1−xMnx)Mn4O12(A = Na+, Ca2+, Y3+, La3+)

AbstractX‐Ray diffraction and neutronographical studies on the crystal structure of the perovskitelike compounds A(Cu1−xMnx)Mn4O12 have been undertaken. It has been shown that the samples with 50% of Mn3+ in the octahedral sites have a cubic unit cell, while the samples with higher Mn3+ composition may have a monoclinic or rhombohedral unit cell. The temperature increase raises the symmetry to the cubic one. The concentration ranges with different symmetry type have been determined. The lowering of the symmetry of the unit cell is due to the cooperative Jahn‐Teller effect.

Magnetic and neutron diffraction study of the phase transition in TmFe1−xMnxO3

AbstractMagnetic and neutron diffraction investigations on polycrystalline samples of TmFe1−xMnxO3 compounds (x = 0.1 and x = 0.3) reveal some interesting features of the Mn3+‐ion influence on the thulium orthoferrite magnetic properties. Preliminary X‐ray diffraction data indicate an increase of the unit cell distortion with growing content of Mn3+ ions. A decrease of the Néel temperature with higher Mn3+ amount is observed. For the compound with x = 0.1 a reorientation transition from GxFz to GzFx (Γ4 − Γ2) spin configuration is observed, whereas in the case of the compound with x = 0.3 a phase transition to antiferromagnetic phase (Γ1) is established. The neutron diffraction results point out to the occurrence of a complicated transitions sequence in the last case.