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

Abstract

Nanocrystalline powders of a manganite compound in the series of La0.7A0.3MnO3 (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 (〈rA〉). 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 \(d_{\mathrm{Mn}\mbox{--}\mathrm{O}}\) in two or three bond lengths is observed for all samples except the LB3M sample, which exhibits equal \(d_{\mathrm{Mn}\mbox{--}\mathrm{O}}\) values. The increasing of 〈rA〉 can decrease this MnO6 distortion by shrinking the \(d_{\mathrm{Mn}\mbox{--}\mathrm{O}}\) in the long bonds and stretching the \(d_{\mathrm{Mn}\mbox{--}\mathrm{O}}\) in the short bonds; the distortion begins to vanish as 〈rA〉≥1.292 A.