Thermal Evolution Crystal Structure and Fe Crystallographic Sites in LaFexAl12-xO19 Hexaaluminates

Compared with BaFe12O19 hexaferrite, La-magnetoplumbite (MP) hexaaluminates with a high-substituted concentration have rarely been investigated. LaFexAl12-xO19 catalysts at various Fe loadings (x = 0-12) were synthesized by coprecipitation. Thermal evolution of the samples from 700 up to 1400 degrees C has been followed by X-ray diffractometry. Fe distribution in different crystallographic sites in the final MP structure was clarified by means of Rietveld refinement and room-temperature and low-temperature Fe-57 Mossbauer spectroscopy. The results showed that Fe doping (x = 1-9) lowered the threshold temperature of La-hexaaluminates. When x >= 6, the segregation of LaFeO3 intermediates essentially hindered the formation of monophasic MP phase and even led to the absence of the MP structure when x = 12. Compared with the unsubstituted sample, Fe-substituted La-hexaaluminates (x = 4, Al(1) and Al(4) sites in the spinel block started to be systematically occupied up to x = 11. The reasonably constant turnover frequencies ((0.16 to 0.47) x 10(2) h(-1) m(-2)) indicated a correlation between the N2O catalytic decomposition performance and Fe3+. ions in the mirror plane (Al(3), Al(5))