Hydrothermal Synthesis And Properties Of The Metastable Perovskite Manganites

The manganese perovskites exhibit magnetic, orbital, and charge orderings caused colossal magnetoresistance (CMR), multiferroics, and superconducting. The manganese perovskites RMnO3 in which R is a trivalent rare earth ion .When it comes to the heavy rare earth atoms such as Ho-Lu, Y and Sc, RMnO3 usually exists in the form of a metastable and non-centrosymmetric hexagonal structure (space group P63cm), and when it is the light rare earth atoms from La to Dy, one can get it in the orthorhombic structure (space group Pbnm) .Both the hexagonal and the orthorhombic structures are derived from distorted cubic perovskite structure. The hexagonal phase usually shows ferroelectromagnetic behavior which possesses both magnetic and ferroelectric orderings, whereas the orthorhombic phase exhibits incommensurate antiferromagnetic structure at low temperature.The synthesis for perovskite manganites could be carried out in many ways, and conventionally used ones were the solid-state reaction, sol-gel method, and co-precipitation. However, these methods need high temperatures. A method with the advantages of lower temperature, easy handling and one-step synthesis was employed to prepare manganites in hydrothermal system. As we know that, the formation of hexagonal RMnO3 strongly competes with the orthorhombic phase in those conventional synthesis methods. For example, the orthorhombic manganites RMnO3 (R=Ho-Lu) synthesized by solid-state reactions at 1573K and 6GPa in a belt-type press must be rapidly cooled down to room temperature to protect the phase transition. By the citrate method, HoMnO3 and YMnO3 were prepared as hexagonal phase We have reported previously the synthesis for the pure orthorhombic phases of RMnO3 (R=Sm-Ho) by hydrothermal method at relative lower temperature compared to other methods. Here we report that the stable orthorhombic phase and the metastable hexagonal phase of RMnO3 (R=Er, Tm) were prepared selectively under hydrothermal conditions at relative low temperature and so many influencing factors for hydrothermal conditions were discussed. Using these new synthesis methods, four new products were prepared and most of them were potassium-doped manganates.Hexagonal and orthorhombic perovskite RMnO3 (R=Er, Tm) were selectively prepared by hydrothermal method, using Mn2O3 andγ-MnO2 as precursor, respectively. Pure phase of hexagonal and orthorhombic RMnO3 (R=Er, Tm) were respectively synthesized by selecting different reaction temperatures and alkaline hydroxides. Powder XRD data of hexagonal and orthorhombic RMnO3 (R=Er, Tm) were refined and SEM photographs show that their homogenous crystal size ranged from 20 to 25 um. DC susceptibility measurements indicated their antiferromagnetic ordering at low temperatures.The potassium-doped manganates, La0.69K0.31MnO3,La0.69K0.31MnO3,La0.80K0.20MnO3 and Pr0.71Ca0.19K0.1MnO3 were synthesized under hydrothermal conditions for the first time.The crystal was synthesized by the so-called hydrothermal reaction of MnO2 under the condition of strong alkali media. The study indicate that formation of the material is dependent on the different precursors and reaction temperature. SEM photographs show that their homogenous crystal size ranged from 15 to 20 um .