Hydrothermal Synthesis, Structure And Properties Of The Manganese Complex Oxides

Rare earth manganates are usually prepared by traditional high temperature solid-state methods which require calcination of oxide/carbonate/nitrate precursors at temperatures in excess of 1000 oC with frequent grindings. Although these methods are adequate for the synthesis of X-ray pure specimens, it is often difficult to control the total oxygen content in the mixed valence systems because of the high temperatures used in the synthesis. High temperature calcinations in some cases may lead to the decomposition of products of the formation of undesirable phases. Particle size reduction by milling can introduce chemical impurities into the ceramic product, which may severely influence the final properties of perovskite manganites.Soft chemical methods, such as the use of sol-gel precursors or molten salts as reaction media, have been adopted for the synthesis of oxide, but these methods involve often complex operating procedures. Hydrothermal technique has been widely applied to the synthesis of metastable phases and microporous solids. This method provides an attractive alternative to the synthesis of complex oxides and fluorides, since it is carried out under relatively mild conditions (autogeneous pressure and～240℃) with controllable particle size distribution of the product. 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 prepared using the traditional, high-temperature methods of solid-state chemistry: the use of a solvent would permit rapid mixing of several chemical elements, leading to homogeneous products, and also offers the potential for control of crystal growth leading to particles of desired morphologies, something rather difficult to achieve when using the high temperatures that must be employed in solid-state reactions. In particular, the hydrothermal method has great scope for the preparation of multinary oxide phases, i.e. containing two or more metals, where the rapid mixing of the constituent elements would provide a great synthetic advantage,as well as potentially leading to the discovery of new materials.In this study, we focus on the hydrothermal crystal growth of manganates. In the system that has been prepared by traditional methods, a more soft synthesis route is explored and the the relationship between the synthesis method and structure and properties of final crystalline products. New trivalent compounds have also been prepared by hydrothermal method in a finely controlled condition. La_0.7Ca_0.3-yK_yMnO₃ most interestingly shows the rectifying effect as atomic-scale p-n junctions. The purpose of the study is to investigate the influence of different synthesis methods on the structure and properties of final products.We have successfully grown several RMnO₃ (R=Sm-Ho) crystals by mild hydrothermal treatment of mixtures prepared from rare earth oxides, k-birnessite gel at 240℃for 72 hours. Two of these compounds are orthorhombic (R=Ho and Dy), whereas the hexagonal phases, which were competitive strongly with the orthorhombic phases in solid state reactions, are avoided in the hydrothermal systems. The pure metastable manganites may serve as a model for understanding the magnetismes of Jahn-Teller distortion and charge ordering. The variable temperature magnetization studies of these crystals show magnetic ordering at low temperatures and are in agreement with earlier reports. This new synthetic approach leaves many rooms for new doped or undoped RMnO₃ compounds.RMn₂O₅ (R=La, Pr, Nd, Tb, Bi) crystallites were prepared by a mild hydrothermal method and characterized by powder X-ray diffraction, scanning electron microscopy, X-Ray photoelectron spectroscopy (XPS) and magnetic measurement. The influences of various hydrothermal conditions such as reaction time and temperature on the formation, morphology and crystalline size of products were investigated. The max d.c. susceptibilities of LaMn₂O₅+δ(δ=0.01, 0.06, 0.08, 0.16, 0.17) were found to be variable and the excess oxygen (δ) in the compounds was influenced by the alkalinity used in the hydrothermal synthesis.The calcium-doped manganates, Pr_1-xCa_xMnO₃ (x=0.39, 0.46, 0.56, 0.70, 0.76, 0.87) and Nd_1-xCa_xMnO₃ (x=0.5, 0.8), were synthesized as cube-shaped crystalline phases under mild hydrothermal conditions for the first time. The crystals could be grown in one step from solutions of metal salts in aqueous potassium hydroxide solution at temperatures～240℃, and found to adopt perovskite-like structure (space group Pbnm). Samples were characterized by powder X-ray diffraction, scanning electron microscopy, inductively coupled plasma analysis and variable temperature dc/ac magnetic susceptibility. The magnetic properties show competing ferromagnetic and antiferromagnetic exchange interactions at low temperature for low Ca²⁺ doping compounds.La_0.7Ca_0.3-yK_yMnO₃ and La_1-yK_yMnO₃, which were substituted the A site with both alkali metal (K) and/or alkaline earth metal (Ca), has been directly prepared by a mild hydrothermal method in the system KOH-KMnO₄-MnCl₂-La(NO₃)₃-H₂O or KOH-MnO₂-La(NO₃)₃-H₂O. It was the first time to find three kinds of valent ions which competing the A site at the same time. La_0.7Ca_0.3-yK_yMnO₃ most interestingly shows the rectifying effect as atomic-scale p-n junctions (namely FY-Junctions) of single crystals and films. The new concept of the atomic-scale p-n junctions, based on the ideal rectification characteristic of the p-n junctions in the single crystal, basically originates from the structural linkages of [Mn³⁺-O-Mn⁴⁺-O-Mn⁵⁺], where Mn³⁺ (t_2g³e_g¹) and Mn⁵⁺ (t_2g²e_g⁰) in octahedral symmetry serve as a donor and an acceptor, respectively, corresponding to the localized Mn⁴⁺ (t_2g³e_g⁰). The product showed PM-FM transition at first and then to a spin-glass state at low temperature.