Study by electrical conductivity and infrared spectrometry of the thermal stability of defective iron spinels with aluminium and chromium

Abstract

Precipitation in defective chromium- or aluminium-substituted magnetites,$\gamma - \left( {Fe_{\left( {8/3} \right) - \left( {8/9} \right)x}^{3 + } M_{\left( {8/9} \right)x}^{3 + } \square _{1/3} } \right)O_4^{2 - } \left( {M^{3 + } = Al^{3 + } ,Cr^{3 + } ;0< x< 2} \right)$, and defective iron aluminium chromium spinels,$\gamma - \left( {Fe_{8/9}^{3 + } Al_{\left( {8/9} \right)\left( {2 - x} \right)}^{3 + } Cr_{\left( {8/9} \right)x}^{3 + } \square _{1/3} } \right)O_4^{2 - }$ has been investigated by electrical conductivity and infrared spectrometry in the temperature range 600 to 1200° C. For highly γ-AI-substituted magnetites and γ-iron aluminium chromium spinels the transformation of the spinel lattice into an α-rhombohedral lattice has been found to be preceded by the formation of an intermediate phase at about 900° C with a high alumina content, approximately identical to disordered γ-Al₂O₃. It is only at higher temperatures (> 1100° C) that the formation of an α-rhombohedral phase is observed. In the case of γ-Cr-substituted magnetites, temperatures of only about 700° C are required for the transformation γ → α.