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Nanocrystalline spinel lithium manganese oxide (LiMn 2 O 4 ) powders with narrow-size-distribution, pure-phase particles, and high crystallinity with an average crystallite size of about 70 nm were synthesized at 600 o C for 6 h in air by freeze drying method. Spinel LiMn 2 O 4 is also prepared by sol-gel using citric acid as a chelating agent. The influence of different parameters such as pH conditions, solvent, molar ratio of citric acid to total metal ions, calcination temperature, starting material on the structure, morphology and purity of this oxide was investigated. The results of sol-gel method show that pure LiMn 2 O 4 with average crystallite size of about 130 nm can be produced from nitrate salts as starting materials at 800 °C for 6 h in air. The optimum pH and molar ratio of chelating agent to total metal ions are 4 ≤ pH ≤ 6 and 1.0, respectively. A possible mechanism on the formation of the nanocrystallines synthesized by sol-gel was also discussed. At the end a comparison of the differences between two methods was made on the basis of x-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) tests.

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We studied Pr substitution for Y in a 123 high Tc superconductor related system, Y1−xPrxBa2Fe3O8 using of x-ray diffraction, magnetization, and 57Fe transmission Mössbauer spectroscopy. The Fe sublattices are magnetically ordered through the whole system. The Pr substitution, however, suppressed the Fe ordering temperature. The low temperature susceptibility across the system increases with increased Pr concentration x, and an additional antiferromagnetic ordering at 24 K is observed for x=1, indicating a possible Pr ordering. The averaged magnetic hyperfine field at Fe sites measured at 15 K is also reduced as x increases. The dramatic effect of Pr substitution for Y in this system, which is not observed in studies of Y substitution by other rare-earth elements, is interpreted as the effect of the hybridization between the Pr 4-f electrons and the electrons in the Fe(2)O2 layers, similar to the Pr hybridization in Y1−xPrxBa2Cu3O7−y system.

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