Synthesis and Characterization of LaCrAl11O19 (Magnetoplumbite) Through Solid State Reaction

LaCrA_l1O₁₉ powders, having the magnetoplumbite crystal structure (Fig. 1a), were prepared by solid state synthesis in a reducing environment.  Powders were synthesized using a multi-step process in which starting nitrate precursors (La(NO₃)₃-6H₂O, Cr(NO₃)₃-9H₂O and Al(NO₃)₃ -9H₂O were first heat treated to cause evaporation of water and pyrolysis of the nitrate groups. Thermogravimetric analysis of precursors revealed complete water evaporation did not occur until 300°C.   A subsequent high temperature heat treatment under a 4%H₂-Ar flow promoted the reduction of Cr³⁺ to Cr²+ that stabilizes the magnetoplumbite structure, allowing for the formation of LaCrA_l1O₁₉.  X-ray diffraction analysis verifies LaCrA_l1O₁₉ formation at temperatures as low as 1200°C, although there is the presence of Al₂O₃ and LaCrO₃ phases indicating that the reaction may not have gone to completion. Scanning electron microscopy (Fig. 1b) shows LaCrA_l1O₁₉ to have a porous plate-like morphology with a high surface area. This morphology may be ideal for catalytic combustion applications where a large surface area is necessary for high efficiency.

Figure 1.  (a) Magnetoplumbite crystal structure with blue spheres representing oxygen, red aluminum, purple lanthanum and green chromium, and (b) scanning electron microscopy image of synthesized powders; light phase - LaCrO₃, gray phase - LaCrA_l1O₁₉ and dark phase - Al₂O₃.