Synthesis and Properties of Well-Crystallized Layered Rare-Earth Hydroxide Nitrates from Homogeneous Precipitation

We report the synthesis and characteristics of a rare-earth based layered family, Ln8(OH)20(NO3)4·nH2O with Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Y, synthesized through homogeneous precipitation of Ln(NO3)3·xH2O with hexamethylenetetramine. The products were uniform and of high crystallinity. Their morphology gradually changed from elongated hexagon (Sm, Eu, Gd) and hexagon (Tb, Dy) to rhombus (Ho, Er, Tm). Selected area electron diffraction revealed that the in-plane structure resembled that of the chloride counterpart, Ln8(OH)20Cl4·mH2O. Unit cell dimensions of the host layer, a and b, decreased with contracting size of lanthanide ions, whereas no such trend was observed for the interlamellar distance, c/2, which is dominated by hydration degree. Stability of the samples with temperature and relative humidity (RH) was examined. At high temperature or low RHs, hydrated water molecules could be removed, which afforded a phase with a basal decrease of ∼0.6 Å. The transition was reversible as revealed by an in situ powder X-ray diffraction study, but a RH hysteresis exists. The reversibility increased with an increase in atomic number or layer charge density. Nitrate anions of both phases could be quantitatively exchanged by other anions under ambient conditions.