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Electron spin resonance (ESR) spectroscopy was used to determine the magnetic state transitions of nanocrystalline La 0.8 Sr 0.2 MnO 3 at room temperature, as a function of crystallite size. Ferromagnetic nanoparticles having an average crystallite size ranging from 9 to 57 nm are prepared by adopting the autocombustion method with two-step synthesis process. Significant changes of the ESR spectra parameters, such as the line shape, resonance field (Hr), g-factor, linewidth (∆Hpp), and the low-field microwave absorption (LFMA) signal, are indicative of the change in magnetic domain structures from superparamagnetism to single-domain and multi-domain ferromagnetism by increase in the crystallite size. Samples with crystallite sizes less than 24.5 nm are in a superparamagnetic state. Between 24.5 and 32 nm, they are formed by a single-domain ferromagnetic. The multi-domain state arises for higher sizes. In superparamagnetic region, the value of g-factor is practically constant suggesting that the magnetic core size is invariant with decreasing crystallite size. This contradictory observation with the core-shell model was explained by the phenomenon of phase separation that leads to the formation of a new magnetic state that we called multicore superparamagnetic state.

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The current work principally treats the significant aspects of solid electrolytes based on cerium oxide in the absence and presence of potassium bicarbonate. The classic oxide electrolyteCe0.7La0.15Ca0.15normalO2-δ (LCDC) and the bicarbonate nanocomposite electrolyteCe0.7La0.15Ca0.15normalO2-δ @KHCO3 (LCDC@KHC) are synthesized separately via self‐combustion and co‐precipitation techniques. Structural, thermal, electro‐morphological and electrochemical properties of pure LCDC and nanocomposite material LCDC@KHC are carefully examined. In particular, the influence of the heavily coupling amongst LCDC oxide and KHCO3 bicarbonate on the microstructures and ionic conductivities of KHCO3‐coated nanocrystalline LCDC is studied by TG/DTA, Raman, FEGSEM and AC impedance spectroscopy. Thermal analyses show that the LCDC@KHC nacomposite is stable at a temperature below 122 °C. Beyond this temperature, the LCDC@KHCO3 nanocomposite is transformed into a LCDC@normalKnormalHnormalCnormalO3/normalK2normalCnormalO3 nanocomposite. XRD data confirms that the LCDC phase and the various nanocomposite materials LCDC@KHC, sintering at different temperatures, adopt the fluorite structure. Lattice parameters and bond lengths are determined by Rietveld refinement. The ionic conductivity of bicarbonate nanocomposite electrolyte LCDC@KHC is 100 to 1000 times higher than that of the novel classic electrolyte LCDC. The remarkable enhancement of conductivity as a function of temperature rise is correlated to the presence of potassium in two forms: bicarbonate and carbonate in the LCDC@normalKnormalHnormalCnormalO3/normalK2normalCnormalO3 nanocomposite electrolyte.