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

在纯氧气氛中，在1300-1350ﾟC的温度范围内合成了钙钛矿型(Ln_&lt；1-x&Gt；Ca_x)MnO_3(Ln：La，Nd，Gd)。X射线粉末衍射谱表明这些锰酸盐为钙钛矿型结构。在低温下，锰酸盐为n型半导体，其电阻率符合莫特定律，表明由于安德森局域化，可能会发生电子的变程跃迁。在室温以上，锰酸盐具有正的温度系数，并且表现出金属-绝缘体的转变，没有任何晶体学变化。磁化率测量表明，在金属-绝缘体转变温度下，Mn~(2+)~(3+)&Gt~(3+)离子的自旋态由低向高转变。根据这些结果，认为金属-绝缘体转变是由于(Ln_&lt；1-x&gt；Ca_x)MnO_3中Mn^&lt；3+&gt；离子自旋态的变化引起的。(La^&lt；1-x&gt；Ca_x)MnO_3的合成如下：一是前驱体a…的分解二是利用乙酸酯进行的固相反应。采用共沉淀法制备了前驱体，其化学式为(Ca&lt；0.5&gt；Mn&lt；0.5&gt；)CO3。合成了(La&lt；1-X&gt；CaX)MnO_3微粉：一种是前驱体的分解，另一种是醋酸酯的固相反应。用共沉淀法制备了CaMnO_3前驱体，其配方为(Ca_&lt；0.5&gt；Mn_&lt；0.5&gt；)CO_3。在950ﾟC下焙烧得到CaMnO_3。另一方面，将醋酸盐加权混合，在900ﾟC下焙烧，得到的锰酸盐颗粒尺寸为0.8-0.9m，比表面积为3-5m^2/g，用这些方法合成的(La_&lt；1-x&Gt；Ca_x)MnO_3的合成温度比传统的固相反应降低了300~400ﾟC。只有CaMnO_3的电阻率随H_2O蒸汽压的增加而降低，这可能归因于CaMnO_3表面吸附的水层的离子电导率。

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