Effect of carbon shell on stabilization of single-phase lanthanum and praseodymium hexaaluminates prepared by a modified Pechini method

A modified Pechini method involving the formation of carbon shell was proposed and used for the synthesis of the unsubstituted lanthanum and praseodymium hexaaluminates with the magnetoplumbite structure. The peculiarities of the LaAlO₃ and LaAl₁₁O₁₈ phases' formation were studied in a wide temperature range starting from their appearance at 700 °C and finishing at 1300 °C when the complete phase formation occurs. It was found that the size of the formed hexaaluminate's crystallites is a crucial factor affecting the phase purity of the final product. The critical size of the thermodynamic stability of the LaAlO₃ phase is supposed to be 10 nm, which corresponds to the specific surface area of 90 m²/g. The use of a carbon template approach leading to the formation of a core-shell structure during the synthesis procedure allows stabilizing the oxide particles in size and preventing their agglomeration at high temperatures. Depending on the carbon template nature, the carbon content in the samples was found to be 13 wt% (low-carbon-shelled samples) and 40 wt% (high-carbon-shelled samples). In the case of high carbon content, the resulting LaAl₁₁O₁₈ and PrAl₁₁O₁₈ hexaaluminates possess the surface areas as high as 160 m²/g.