Preparation Of Perovskite Manganites With Mixed Valence Via A Molten Hydroxide Method

After the discovery of colossal magnetoresistance （CMR）, the physical propertiesof the hole-doped manganites Re_1-xA_xMnO₃（Re=La,Nd and A=Ca,Ba） have beeninvestigated intensively. Mixed-valence manganites because of its excellent physicalproperties （The compound structure, magnetic behavior and transport properties aredepending on the average size of the A-site cations<rA> and the doping concentrationof Mn） are expected to offer various technical applications such as spintronics-baseddevices.Mixed-valence especially the triple valence state were reported little. In thisarticle simple, practical and environmentally friendly molten hydroxide（KOH system）method was used to prepare a new type of mixed valence perovskite manganites. InChapter1Introduction we introduced the basic physical concepts of stronglycorrelated electron system such as the theoretical knowledge of the perovskitemanganites. Also,chemical synthesis means plays an important role in the physicalproperties of the product. In Chapter II we use MnO₂,Ca （OH）₂, La （OH）₃as thestarting materials for the preparation of novel perovskite materials in an inertatmosphere during400℃to800℃in a molten KOH system and we get Ca, Kdoped La_1-x-yCa_xK_yMnO₃nano-materials. The product structure,element valence state,morphology and magnetic properties have been characterized by XRD, XPS, SEMand SQUIDS. In Chapter III, we use Mn2O₃instead of MnO₂as a manganese sourceand the other reaction conditions remain the same. We get the La_1-x-yCa_xK_yMnO₃microcrystals with the orthorhombic structure. The I/V curves obtained by the Conductive AtomForce Microscopy（C-AFM） showed obvious rectification of the microcrystal.In this paper, experimental results show that the molten alkali method is a simpleand green chemistry synthesis method. Also this technology is convenient to generate the metastable phase compounds in molten alkali system and provide the existence ofa metastable valence state such as Mn⁵⁺.Moreover it’s conducive to the growth of theperfect crystal with good rectifying behavior.