Exploring the Crystallization of Lanthanum(III) and Neodymium(III) Hydroxides from Solution

Understanding the mechanics of crystallization from solution is crucial for advancing material discovery and design. Studying these complex processes requires a combination of experimental techniques. Here, the crystallization of lanthanum(III)- and neodymium(III) hydroxides was studied with in situ and ex situ X-ray techniques in combination with pair distribution function analysis, scanning electron microscopy, light scattering, pH titrations, simulations, and optical spectroscopy. Starting from the Ln(III) aqua ions in nitric acid, the pH is increased to start the precipitation of hydroxides. In situ optical spectroscopy and potentiometry revealed that at pH = 6, an initial gel phase with a composition of [Ln(NO₃)(OH)(H₂O)_<i>z</i>]OH was formed. At pH > 10, the nitrate ligands were replaced by hydroxides, resulting in gels with a composition of [Ln(OH)₂(H₂O)₇]OH. Upon washing and dehydration, X-ray scattering and Rietveld analysis showed that the gels crystallize into Ln(OH)₂(NO₃)(H₂O)_<i>z</i> at pH < 10 and Ln(OH)₃ at pH > 10. Ln(OH)₃ was obtained at all pH values if hydrothermal treatment was performed prior to the dehydration. In situ total X-ray scattering and pair distribution function analysis was used to show that no crystallization occurs in solution and that the hydrothermal treatment removes water from the gel. The size and morphology of the isolated lanthanide(III) hydroxides were found to be dependent on the pH value, but our results showed that the gel structure is found in all cases, suggesting that crystallization occurs within gel particles and not in solution.