Synthesis and structural characterization of trimetallic perovskite-type rare-earth orthoferrites, LaxSm1-xFeO3

Ultrafine powders of trimetallic orthoferrites containing lanthanum and sam arium in various ratios were synthesized by thermal decomposition at low te mperatures of the corresponding hexacyano complexes to modulate the functio nal properties of these perovskite-type oxides. The precursors and their de composition products were analyzed by simultaneous thermogravimetric and di fferential thermal analyses, X-ray diffractometry, and Raman spectroscopy. Single-phase trimetallic precursors and oxides were obtained. The crystal s tructure of the perovskite-type oxides was orthorhombic, and the lattice pa rameters were affected by the ionic size of the rare-earth elements present in the oxides. Raman spectra showed a broadening of the vibrational bands with increased lanthanum content, This was ascribed to some disorder in the oxygen sublattice, related to distortions of the cation-oxygen coordinatio n, and to a reduction of the orthorhombic distortion in the unit-cell basal plane. Most of the Raman modes above 200 cm(-1), associated with the vibra tion of oxygen ions, showed a frequency increase with effective cation mass , defined as m(eff) = xm(La) + (1 - x)m(Sm), i.e., with samarium content, T his was explained by assuming that the force constants increased with decre ased Ln-O and Fe-O interatomic distances observed for high samarium content .