Synthesis and Structural Characterization of the La0.7A0.3MnO3 (A=Sr,Ca,Ba and Na) Nanocrystalline Powders Prepared by a Simple Thermal Decomposition Route

Nanocrystalline powders of a manganite compound in the series of La(0.7)A(0.3)MnO(3) (LA3M), where A is Ca2+, Na1+, Sr2+ or Ba2+ are synthesized by a simple thermal decomposition method using related acetate salts as starting materials in aqueous solution. The precursors are analyzed by Thermogravimetric/Differential Thermogravimetric Analyzer (TG-DTG) techniques and then all of synthesized nanopowders are characterized at room temperature using X-ray diffraction (XRD) and X-ray absorption fine structure (EXAFS) techniques. The initial structural characterization shows that all the prepared samples exhibit non-stoichiometric perovskite structures. Only the La0.7Na0.3MnO3 (LN3B) sample contains oxygen deficiencies, whereas most samples contain excess oxygen, which results in cation vacancies in either the A- or B-site. The three regions of the crystal-structure transition are observed with variation of the average cationic radius in the A-site (aOE (c) r (A) >). The orthorhombic structure is observed when a smaller cation (Ca2+) is substituted for La3+. Likewise, the cubic structure is only obtained after substitution by the much larger Ba2+ ion, whereas the rhombohedral structure is still exhibited after substitution by Na1+ and Sr2+. The different structures are affected by the ionic radii of the A ion and the Mn4+ ion. Jahn-Teller distortions in the MnO6 octahedra composed of unequal in two or three bond lengths is observed for all samples except the LB3M sample, which exhibits equal values. The increasing of aOE (c) r (A) > can decrease this MnO6 distortion by shrinking the in the long bonds and stretching the in the short bonds; the distortion begins to vanish as aOE (c) r (A) > a parts per thousand yen1.292 .