Unusual properties and features of oxygen nonstoichiometry of La-Sr manganites with manganese replacement by a combination of nickel and germanium

When studying manganites of the system La1-cSrcMn1-x(Ni2+ 0.5Ge4+ 0.5)xO3+γ (x = 0.05; 0.10; 0.15; 0.15 c 0.35), it was experimentally established that after reductive annealing, which leads the oxygen content to the stoichiometric one, unit cell volume (v) of the samples decreases and at c 0.19 it is less than after annealing in oxygen. Based on the analysis of various charge compensation mechanisms using derived dependences of v on the oxygen content and taking into account the changes in magnetic and electrical parameters, it is established that experimental data can be explained only with the allowance for formation of singly charged oxygen ions. The relationships between magnetic parameters of the manganites and their composition, structural characteristics and deviations from oxygen stoichiometry are explained. The possibility of local ferromagnetic ordering of uncompensated spins of Ni2+ ions due to their interaction with singly charged O- anions is considered. © Published under licence by IOP Publishing Ltd

Effects of long storage in La-Sr manganites with pared divalent and quadrivalent substituents for manganese

The results of investigation on the ageing of ceramic manganites La1-cSrcMn0.90Ni0.05Ge0.05O3+γ and La1-cSrcMn0.90Zn0.05Ti0.05O3+γ (c = 0.15, 0.17, 0.19) during long-term storage at normal conditions are presented. Unit cell volume of all manganites having rhombohedral structure decreased over time. The width of X-ray diffraction reflexes from (Ni,Ge)-compositions varied irregularly within the measurement error, and the width of reflexes from (Zn,Ti)-contained manganites decreased significantly. Magnetization of all samples increased in the end of storage period. Curie point of compositions with c > 0.17 exhibited some trend of increase. Ageing leaded to the lowering of "metal-semiconductor" transition temperature in (Ni,Ge)-substituted manganites. (Zn,Ti)-contained compositions showed only semiconductor behavior in investigated temperature range from 100 to 300 K, and the changes of their magnetic parameters were most considerable. The increase in magnetization and Curie point reached 7.5 % and 11 %, respectively. Possible mechanisms of ageing in considered materials are discussed in terms of oxidation processes, formation of microinhomogeneities and relaxation of internal stresses. © Published under licence by IOP Publishing Ltd

Mechanisms of Substituting Quadrivalent Ions Influence on the Properties of La-Sr Manganites

Regularities of the influence of quadrivalent ions and oxygen concentrations on the structural characteristics, saturation magnetization, Curie point, temperature dependence of electrical conductance of ceramic manganites $La_{1-c-x}Sr_{c+x}Mn_{1-x}Me^{4+x}O_{3+γ}$ (Me = Ti, Ge; 0.17 ≤ c ≤ 0.35; 0.025 ≤ x ≤ 0.125) were established. The relation between experimental data on lattice parameters, electromagnetic properties and crystal chemistry characteristics (nonstoichiometry index, concentrations of ions residing at different charge states) was determined using the modified model of effective cation-anion distances. The data obtained provide some evidence that in Ti-containing manganites when Sr concentration is more than 0.375, single-charged oxygen ions can arise after reductive annealing

Ageing phenomena in La-Sr manganites with divalent substituents for manganese

Variations of electromagnetic and structural characteristics of La_{1-c+x}Sr_{c-x}Mn_{1-x}Me_{x}O_{3+γ} manganites (Me = Zn, Ni) during storage time up to 36000 hours at room temperature and after frequentative thermal cycling in the 300-573 K range were investigated. Changes of magnetization of all samples were within the error of measurement. In most cases Curie point (T_{C}) showed irregular variations within the range of 20%, while zinc-substituted manganites with relatively high values of "c" exhibited some trend of T_{C} rise. The resistance of all samples increased during storage period. The cell volume of single-phase manganites decreased over time, that may indicates an increase in oxygen content. However, the sample with the highest Ni content (x=0.125) at c=0.19 suffered phase transition "rhombohedral-orthorhombic structures", which cannot be explained by the oxygen absorption. Probably, this effect was due to the rearrangement of the ions and vacancies between crystal sublattices of manganite

Effect of ageing on magnetic and electrical properties of Ti–substituted La-Sr manganites

The changes of structural, magnetic and electrical parameters of ceramic La0.8-xSr0.2+xMn1-xTixO3+γ (0.025 ≤ x ≤ 0.150) manganites during long-term storage at room temperature and after thermal cycling were investigated. The unit cell volume (V) of all manganites decreased over time. Most considerable decrease of V and broadening of X-ray diffraction lines width were observed in the samples with high quantity of titanium. During the storage period magnetization (σ) and Curie point (Tc) of manganites with relatively low Ti content decreased, while the compositions with high values of “x” exhibited some trend of σ and Tc increase. The resistance of the samples with x < 0.100 increased over time. Fluctuations of magnetization of all thermal-aged samples were within the error of measurement, and Curie point of manganites with x = 0.150 showed the rise. From the data obtained it might be inferred that ageing phenomena in Ti-substituted manganites can be associated with oxidation processes, the transfer of Ti4+ to vacant sites in rare-earth sublattice and formation of microinhomogeneities