The complex synthesis and solid state chemistry of ceria–lanthana solid solutions prepared via a hexamethylenetetramine precipitation

Mixed oxide solid solutions are becoming ever more commercially important across a range of applications. However, their synthesis can be problematical. Here, we show that ceria–lanthana solid solutions can be readily prepared via simple precipitation using hexamethylenetetramine. However, the solution chemistry can be complex, which results in the precipitated particles having a complex structure and morphology. Great care must be taken in both the synthesis and characterisation to quantify the complexity of the product. Even very high heat treatments were not able to produce highly homogeneous materials and X-ray diffractions reveals the non-equilibrium form of particles prepared in this way. Unexpected crystal structures are revealed including a new metastable cubic La 2O 3 phase. The suggested mechanism for the formation of dual fluorite phase particles, where Step 1 corresponds to room temperature aging, Step 2; heating the solution to 90 °C, Step 3; cooling of the solution to room temperature, Step 4; calcination to 500 °C, Step 5; calcination to 700 °C and Step 6; calcination to 1300 °C. The terminology of e.g. La 1− x Ce x (OH) 3 is used to indicate the formation of a mixed oxy-hydroxy participate rather than a definitive assignment of stoichiometry. Similarly, La 1− y Ce y O 2 only implies a mixed solid solution [Display omitted] . ► Mol% of prepared Ce–La oxides did not follow that of reactant mol%. ► Complex reaction pathway found to be dependent on metal solution concentrations. ► At certain concentrations core shell particles were found to form. ► A reaction model was produced based on cationic solubility. ► Report lanthana solubility higher than previously reported in CeO 2.