(765e) 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.