Propriedades estruturais e magnéticas de soluções sólidas do Tipo Bi2(MxAl1-x)4O9 (M=Fe, Ga)

Abstract

Substituted mullites of the Bi2(FexAl1-x)4O9 and Bi2(GaxAl1-x)4O9 types - with 0 ≤ x ≤ 1.0 - were synthesized by high-energy ball milling, followed by heat treatment in free atmosphere. The as-treated iron containing compounds were milled once again aiming to change the original cationic distributions of the iron and aluminum cations throughout the octahedral and tetrahedral sites of the mullite structure. The prepared samples were characterized by X-ray diffraction, Mössbauer spectroscopy (57Fe), magnetization versus temperature, infrared spectroscopy, nuclear magnetic resonance (27Al) and energy dispersive X-ray spectroscopy. The Rietveld refinement of the diffractograms revealed that, for both series of solid solutions, the crystalline structure is orthorhombic (space group Pbam) and that the lattice parameters increase linearly with x. Specifically for the Bi2(FexAl1-x)4O9 solid solution, the magnetic measurements showed a antiferromagnetic behavior, with the Néel temperature decreasing and the saturation magnetization increasing with the aluminum content. The Mössbauer spectra for these samples presented two paramagnetic components at room temperature (i.e., two doublets), one belonging to the octahedral site and the other to the tetrahedral site. For x ≥ 0.5, the hyperfine parameters, together with those Rietveld refined and the infrared results, showed a random distribution of aluminum and iron cations over both sites, whereas for x < 0.5 a slight preference of the iron for an octahedral coordination was observed. For the Bi2(GaxAl1-x)4O9 series of the samples, the Rietveld refinement and the NMR spectra revealed the preference of gallium cations for the tetrahedral sites. Finally, for the re-milled samples, the diffraction analyses showed that, for periods of up to 1 hour of extra milling, the samples preserved the same crystallographic structure. However, the Mössbauer spectroscopy confirmed that an effective increase in the tetrahedral site occupation by iron, to 70%, took place.