Hydrothermal Synthesis And Characterization Of The Perovskite Manganites With Triplet Valence States

The manganese perovskite oxide with complicated mixed valence states is a strong electron correlation system, the interaction among spin, charge, orbit and lattice of this system is very strong, such as spin-spin interaction, spin-lattice interaction, charge-lattice interaction and so on. These interactions coexist and compete, which resulted in complicated physical effect on electrics, magnetism, thermodynamics, thus this subject is widely investigated. New substances and materials were synthesized owing to the breakthrough of synthesis, preparation, assemble and study of relationship between structure and characterization, which will arise a new revolution of the material industry.The solid compound with mixed valence states is usually synthesized by high-temperature solid state reactions such as the manganese perovskite oxide. One synthesis route of the manganese perovskite oxide is that mixing up the initial materials, then heating to 1000 oC to get the product, which is commonly used. Another synthesis route is charge disproportionation, which synthesized two interval oxidation states. For example, CaFeO3 and YNiO3 was found having charge disproportionation at certain temperature. The structure of CaFeO3 change from orthorhombic to monoclinic below 17°C as well as charge disproportionation reaction 2Fe⁴⁺→Fe³⁺ + Fe⁵⁺. The structure of YNiO3 change from orthorhombic to monoclinic below 309°C as well as charge disproportionation reaction 2Ni³⁺→Ni³⁺δ+ Ni3-δ,δ=0.35.Hydrothermal method, involving heating metal salts, oxides or hydroxides as a solution or suspension in a liquid elevated temperature and pressure, offers an alternative synthesis route for the multicomponent ceramic oxides. Because the reactions are carried out in solution, the physicochemical properties of many reactions will change. Compared to high temperature solid state reactions, hydrothermal reactions need relatively low temperatures, which favors the formation of low agglomeration, homogeneity and good sinterability ceramic powder. In addition, hydrothermal method is an open route to metastable phases. With the basis of large experiments, we investigated the characterization of this materials such nanocrystalline, colossal magnetoresistance and superconducting.In this study, two manganese perovskite oxide with triplet valence states of manganese(Mn³⁺, Mn⁴⁺,Mn⁵⁺) was synthesized by hydrothermal reaction following investigating the synthesis condition such as the ratio of initial materials, basis quantum, reaction time and reaction temperature. We also study the structure, morphology, magnetism, I-V curves and photovoltaic characterization of these materials.1. The marvelous triplet mixed valence states of Mn³⁺, Mn⁴⁺ and Mn⁵⁺ in B sites results from the partial substitution of Ca2+ and K+ for La³⁺ in A sites. The molecular formula is La0.29Ca0.70K0.01MnO3 and La0.66Ca0.29K0.05MnO3. The study indicates that the stability of hydrothermal reaction and the ratios of different oxidants and reducers are the most important reasons for changing the mixed valence states of the products. Valence states of Mn in this two products are Mn³⁺, Mn⁴⁺ and Mn⁵⁺, which was not reported as far as our known. In the SEM photos we observed the products have high purity and the particles were composed by cubic sheets about 10⁵0μm. The results indicate that hydrothermal reaction compared with other synthesis methods has the advantages of synthesizing metastable manganites, stable valence states of Mn⁵⁺ and perfect crystals.2. Analysis of the single crystal structure shown that the crystal structure of the two products is cubic with high symmetric.3. Magnetics study indicates that La0.29Ca0.70K0.01MnO3 shows PM-AFM transition at 265K while La0.66Ca0.29K0.05MnO3 shows PM-FM transition at 308K. 4. I-V curves of products measured by C-AFM indicates that La0.66Ca0.29K0.05MnO3 has a ideal rectifying characteristics of p-n junction .This study reveals the extraordinary phenomena and features of the atomic-scale p-n junctions of the atomic-scale p-n junction.5. Photovoltaic measurement indicates that the calcined products have short photovoltaic response time and high photovoltaic intensity. The most important reason is that: when the product is calcined, Mn ionic valence states are coming stable, which is advantage for forming intact energy band resulted in the easy transferring in the energy band; and after calcining, the defects of the crystal surface decreases, the contact of the crystals becomes tight, which making the transfer of photo-charge easily.The marvelous triplet mixed valence states systems as one of the new subjects still contains lots of unknown physical phenomenon. The synthesis and the intrinsic synthesis mechanism of the marvelous triplet mixed valence materials need to be further investigated in future. These two products with complicated mixed valence states will bring new potential application of the atomic-scale electric devices. Further studies may open to other transition metal oxides and theoretical considerations as well.