Synthesis and structural characterization of trimetallic perovskite-type rare-earth orthoferrites, La_xSm_1-xFeO₃

Ultrafine powders of trimetallic orthoferrites containing lanthanum and samarium in various ratios were synthesized by thermal decomposition at low temperatures of the corresponding hexacyano complexes to modulate the functional properties of these perovskite-type oxides. The precursors and their decomposition products were analyzed by simultaneous thermogravimetric and differential thermal analyses, X-ray diffractometry, and Raman spectroscopy. Single-phase trimetallic precursors and oxides were obtained. The crystal structure of the perovskite-type oxides was orthorhombic, and the lattice parameters 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 coordination, 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 vibration 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. This was explained by assuming that the force constants increased with decreased Ln-O and Fe-O interatomic distances observed for high samarium content.