How does crystalline solids form from solution? Crystallization of lanthanide(III) hydroxides

Abstract Technology and innovation requires new high performance materials, however our knowledge of how crystalline materials are formed is limited.1-5 We can control the growth of single crystals from melts and make semiconductors, optical fibers, and jet turbine blades.6 And we can with atomic precision make molecular architectures in the lab,7 or by exploiting biotechnological advances.8 However, where we for the creation of molecular species have a detailed mechanistic understanding, we do not have a succinct model that describes the formation mechanism of crystalline materials from solutions containing the molecular building blocks.9-11 Here we show that the formation of lanthanide(III) hydroxide crystallites does not take place in solutions,12-17 but happens upon dehydration on the filter. The data reveals that no solids are present in solution and that no crystallization occur. In solution a strong, washable gel phase is formed. Using total scattering,18-21 it is documented that no long range order is present in solutions, and that the gel is constructed from clusters of lanthanide(III) ions. Despite the emergence of studies of the structure of crystallites in solutions and in melts,22-31 the early stages of formation of solids from ions and molecules in solution remains poorly understood.21, 32-34 The process from solvated ion to cluster to agglomerate can be described like any other chemical transformation, and must follow a reaction profile with energy barriers, intermediates, and transition states—and will be governed by reaction kinetics and thermodynamics. The first step is to map all species involved between solute and nanocrystal, and here studying materials made from lanthanide(III) ions are useful as luminescence, electron miscopy and scattering is strong with these ions.