Synthesis and Water Adsorption Properties of Perovskite-type Oxides Containing Manganese

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Perovskite-type (Ln_<1-x>Ca_x)MnO_3 (Ln: La, Nd, Gd) were synthesized at 1300-1350ﾟC in the flow of pure oxygen gas. X-ray powder diffraction patterns of these manganates were indexed as the perovskite-type structure. At low temperature, manganates are n-type semiconductors and the electrical resistivity follows Mott's T^<-1/4> law, indicating the possible occurrence of variable range hopping of electron due to Anderson localization. Above room temperature, manganates have positive temperature coefficient, and exhibit a metal-insulator transition without any crystallographic change. The magnetic susceptibility measurement suggests that spin state of Mn^<3+> ions change from low to high at the metal-insulator transition temperature. From these results, it is considered that the metal-insulator transition is caused by a change in the spin state of Mn^<3+> ion in (Ln_<1-X> Ca_X)MnO_3.Fine particle (La^<1-x> Ca_x)MnO_3 was synthesized as follows: one is the decomposition of the precursor a … More nd the other is the solid state reaction by using acetates. The precursor was prepared from the co-precipitation method and its formula was found to be (Ca_<0.5> Mn_<0.5>)CO_3.Fine particle (La_<1-X> Ca_X)MnO_3 was synthesized as follows: one is the decomposition of the precursor and the other is the solid state reaction by using acetated. The precursor was prepared from the co-precipitation method and its formula was found to be (Ca_<0.5> Mn_<0.5>)CO_3. Firing this precursor at 950ﾟC, CaMnO_3 was obtained. On the other hand, acetates were weighted and mixed, then fired at 900ﾟC. The particle size of manganates thus obtained was 0.8-0.9 m, and the specific surface area was 3-5 m^2/g. By using these methods, the synthesis temperature of (La_<1-x> Ca_x)MnO_3 was lowered by 300-400ﾟC compared with the conventional solid state reaction.The electrical resistivity of (La_<1-x> Ca_x)MnO_3 under various relative pressure of H_2O vapor was measured. Only the electrical resistivity of CaMnO_3 decreased with increasing vapor pressure of H_2O, and this decrease was assumed to be ascribed to the ionic conductivity of water layer adsorbed on CaMnO_3. Less