Self-propagating high-temperature synthesis of aluminum substituted lanthanum ferrites LaFe1−xAlxO3 (0 ≤ x ≤ 1.0)

Abstract

Pure and aluminum substituted lanthanum ferrites LaFe_1−xAl_xO₃ (x = 0–1.0) were synthesized in air by self-propagating high-temperature synthesis (SHS) using iron or aluminum as sources of fuel. Two series of samples were produced: I – SHS in the absence of an external magnetic field (zero field SHS); II – SHS in a magnetic field of 0.27 T (applied field SHS); both series of samples were sintered at 1400 °C for 65 h with intermediate grinding. Scanning electron microscopy, energy dispersive analysis of X-rays, X-ray powder diffraction and IR spectroscopy were carried out for both series of samples. XRD showed that for all the combustion products single phase orthorhombic ferrites were produced with a decrease in lattice parameters and unit cell volume with aluminum content (e.g., for series I: x = 0, V = 242.4 ų; x = 1.0, V = 217.3 ų) as well as with magnetic field (e.g., for series I: x = 0.3, V = 233.8 ų; for series II x = 0.3, V = 232.8 ų). ⁵⁷Fe Mössbauer indicated that at low Al concentrations (x ≤ 0.3) more than 92–93% of Fe atoms experience a perturbative disruption to their interatomic exchange interactions, consistent with the random distribution of Al³⁺ ions on the B sublattice. From x = 0.3 the Mössbauer spectra of both the series showed the presence of a paramagnetic doublet together with a sextet, the percentage of doublet component increased with x from 6.7% for x = 0.3 to 28.3% for x = 0.8. Room temperature magnetic measurements demonstrate weak ferromagnetism for all the iron-containing samples (x = 0–0.9) with maximal magnetization in the range of 1.02–0.13 emu g⁻¹ whereas LaAlO₃ samples exhibit paramagnetic behavior. The FT-IR spectra of both series I and II samples show predominantly two broad bands at 450–460 and 550–670 cm⁻¹. Our experiments on a weakly magnetic system LaFe_1−xAl_xO₃ (0 ≤ x ≤ 1.0) have provided evidence that the reorganisation of the green mixture along the field lines is one way in which the field acts. Reactions on powders and pellets have provided that evidence as well indicating that the magnetic field is important in determining the structural and magnetic characteristics of the product from SHS reactions.