Intercalation route-dependent orientation of a Tb³⁺-encapsulated Preyssler-type polyoxometalate in layered double hydroxides

Herein, a Tb³⁺-encapsulated Preyssler-type polyoxometalate (TbPOM) was intercalated into layered double hydroxides (LDHs) through four different routes: ion exchange (IE), exfoliation–restacking (ER), layer-by-layer (LbL) assembly, and reconstruction (Re) via the memory effect. Structural analyses based on powder X-ray diffraction and previously reported orientation models revealed that the orientation of TbPOM depends on its intercalation pathway into LDHs. The IE and Re methods predominantly produced the Form-1 structure, in which the fivefold symmetry (C₅) axis of TbPOM is oriented perpendicular to the LDH layers, whereas the ER method yielded a coexistence of Form-1 and Form-2 structures, corresponding to perpendicular and parallel orientations of the C₅ axis, respectively. By contrast, the LbL method selectively generated the Form-2 structure. Thermal annealing of the ER product caused a gradual decrease in the Form-2 phase, accompanied by the development of highly ordered Form-1 reflections and contraction of the interlayer spacing. These observations suggest that Form-2 is preferentially stabilized under hydrated conditions, whereas Form-1 becomes dominant upon dehydration. The results suggest that the orientation of TbPOM is influenced by the composition of the host LDHs and the intercalation process, which affects the hydration state of the interlayer galleries and thereby influences the preferred orientation of TbPOM. This study provides a strategy for controlling polyoxometalate (POM) orientation in layered materials and highlights the importance of the intercalation pathway as a key parameter influencing orientation formation in POM–LDH hybrid systems.