| Perovskite oxides have been extensively investigated due to their particular electrical and magnetic propertites.These important properties make them become the possible candidates as electrolytes used in solid-oxide fuel cells,high-temperature oxygen sensorsa and catalysts. The discovery of superconducing and colossal magnetoresistance was the starting point for a number of research studies related to those perovskites.Perovskite-type manganese oxides with a formula RMnO3 are a speciese of important compound,which have been widely used in magnetic, electronic ,optics ,catalysis. The most interesting issue is the creation of the complicated mixed valence states.,which may rise the change of their propertites. Rare earth manganites are traditionally synthesized by solid state reaction.Though high temperature solid state reaction has the advantages of simple reaction and easy operation, it requires relatively high temperature erature (above 1000℃) and in some cases may lead to the formation of undesirable phases. In contrast, in the hydrothermal system the boiling point of water increases and great pressure is generated, both of which improve the solubility of agents. Thus the reaction can be carried out in a homogeneous system at much lower temperature. And the low temperature of hydrothermal method also favors the formation of some metastable phases. The differences between hydrothermal method and solid state reaction lie in the fact that the key step of the latter reaction is the atom diffusion and the key step of the former is the process of dissolution and re-crystallization which favors the preparation of inorganic compounds with high purity and perfect crystallization.In our work ,we have hydrothermally synthesized two oxidation states of manganese and three oxidation states of manganese,studied the mechanism of the three oxidation states of manganese in thermodynamics,discussed the effect of mol ratio of agents,alkalinity and temperature to two oxidation states of manganese.In the system KOH-KMnO4-MnCl2-La(NO3)3-Ca(NO3)2-H2O,our group successfully synthesized three oxidation states of manganese La0.6Ca0.32K0.08MnO3,, which was substituted the A site with both alkali metal (K) and alkaline earth metal (Ca),the B site with three oxidation states of manganese(Mn3+,Mn4+,Mn5+), It was the first time to find three kinds of valent ions competing the A site and the triplet valence states at the B site. at the same timeIn the system KOH-MnO2-La(NO3)3-Ca(NO3)2-H2O, La0.66Ca0.29K0.05MnO3 was obtained by hydrothermal synthesis,which was also three oxidation states of manganese.Both of the two methods can synthesize three oxidation states of manganese,so we consider disproportionation reaction may happen in the system KOH-MnO2-La(NO3)3-Ca(NO3)2-H2O ,and redox or disproportionation reaction may happen in the system KOH-KMnO4-MnCl2-La(NO3)3-Ca(NO3)2-H2O.The two reaction mechanism are both allowed in thermodynamics after calculation and we need make further study to their reaction mechanism in the hydrothermal synthesis.By hydrothermal synthesis, two oxidation states of manganese La0.45 Sr0.55MnO3, La0.36 Sr0.64MnO3,La0.19 Sr0.81MnO3 have been prepared. The influence of nominal materials, KOH concentration and the crystalline temperature on the formation of pure phase products were investigated. It was found that the average valence of Mn is related to the concentration of KMnO4 and MnCl2.All of these mentioned above shows the excellence of the hydrothermal syntheses.Two oxidation states of manganese LaMn2O5 has been directly prepared by a mild hydrothermal method in the system KOH- KMnO4-MnCl2-La(NO3)3-H2O.The influences of various hydrothermal conditions such as reaction time and temperature on the formation, morphology and crystalline size of products were investigated. The magnetic susceptibilities showed that the transition from PM to FM at first and then to a spin-glass state at low temperature.
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