Nonstoichiometry defects and electromagnetic properties of La0.55Sr0.35Ce0.10Mn0.90Co0.10O3+γ manganites with different oxygen content

Abstract

Single-phase ceramic samples of La_0.55Sr_0.35Ce_0.10Mn_0.90Co_0.10O_3+γ manganites with rhombohedral structure were synthesized. Different oxygen concentrations ($\gamma =0$, $\gamma =-0.022$, $\gamma =0.002$) were achieved by heat treatment of sintered (initial) samples at the temperature of 1223 K and different partial pressure of oxygen in the gas phase. By comparing calculated and experimental values of the unit cell volume, concentrations of Co²⁺ and Co³⁺ ions in the obtained samples were found. In the stoichiometric manganite ($\gamma =0$) and in the sample annealed in oxygen ($\gamma =0.002$), cobalt is present in the Co³⁺ state. The sample annealed in high vacuum ($\gamma =-0.022$) contains Co²⁺ (10%) and Co³⁺ (90%) ions. Manganite annealed in oxygen has the highest values of magnetization, Curie point, metal-semiconductor transition temperature, and conductivity, which is due to the higher concentration of Mn³⁺ and Mn⁴⁺ ion pairs in this sample. Manganite annealed in high vacuum, containing significant concentration of oxygen vacancies ($\delta =0.022$) and Co²⁺ ions, has a minimum Curie temperature and a fairly low metal-semiconductor transition temperature. The maximum width of the temperature range of ferromagnetic--paramagnetic transition indicates a high inhomogeneity of the distribution of different valence ions in this sample due to the Coulomb interaction. The initial oxygen-deficient sample ($\gamma =-0.006$) also has similar characteristics.