Resolving intermediates during the growth of aluminum deuteroxide (Hydroxide) polymorphs in high chemical potential solutions

Aluminum hydroxide polymorphs are of widespread importance yet their kinetics of nucleation and growth remain beyond the reach of current models. Here we attempt to unveil the reaction processes underlying the polymorphs formation at high chemical potential. We examine their formation in-situ from supersaturated alkaline sodium aluminate solutions using deuteration and time-resolved neutron pair distribution function analyses, which indicate the formation of individual Al(OD)₃ layers as an intermediate particle phase. These layers ultimately stack to form gibbsite- or bayerite-like layered heterostructures. Ex-situ characterization of the recovered precipitates using ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy, Raman, X-ray diffraction, and scanning electron microscopy, suggests the presence of additional intermediate states, an amorphous compound bearing both tetrahededrally- and penta-coordinated Al³⁺. These observations reveal the complex pathways to form Al(OD)₃ monolayers via either transient pentacoordinate species or amorphous-to-ordered transitions. The subsequent crystallization of admixed gibbsite/bayerite is followed by an Al(OD)₃ monolayer attachment process.