Magnetic and Electric Properties of Ca2Fe2-xAlxO5 Brownmillerites Obtained by Ball Milling High-Energy Process

Abstract

The   structural,    magnetic,   and    electric    properties   of    the Ca₂Fe_2-xAl_xO₅ (0 ≤ x ≤ 1.2) brownmillerites prepared by the method of mechanochemical synthesis were examined. It was confirmed that these compounds, at least for the Al doping level x ≤ 0.8, adopt the orthorhombic Pnma structure. Undoped Ca₂Fe₂O₅ brownmillerite exhibits ferromagnetic-like properties up to room temperature. In contrast, only a marginal ferromagnetic contribution to magnetic properties is observed in samples with Al addition. The Mössbauer results also suggest that Al atoms substitute for Fe in both tetrahedral and octahedral positions, resulting in a reduction of the hyperfine field. These results are consistent with density functional theory calculations, confirming the Al substitution at two positions in the crystal structure and the presence of an energy barrier between the favored antiferromagnetic and ferromagnetic ordering in both Ca₂Fe₂O₅ and Ca₂Fe_1.75Al_0.25O₅ compounds. A set of complex impedance measurements on frequency and temperature, along with the analysis of the obtained data within the Nyquist plot and the models of Arrhenius and Jonscher, yielded information on the conductivity mechanisms, as well as the contributions of grains and grain boundaries. The addition of Al significantly enhances the conductivity of Ca₂Fe₂O₅ ceramics, particularly in grain boundaries. However, at high doping levels, the localization of oxygen holes takes place, thereby increasing the activation energy. The conductivity mechanism also changes character from diffusive-like below 380 K to hopping-like at higher temperatures.