Anion-Exchangeable Layered Materials Based on Rare-Earth Phosphors: Unique Combination of Rare-Earth Host and Exchangeable Anions

Layered materials, three-dimensional crystals built from stacking two-dimensional components, are attracting intense interest because of their structural anisotropy and the fascinating properties that result. However, the range of such layered materials that can exchange anions is quite small. Continuing efforts have been underway to identify a new class of anion-exchangeable materials. One major goal is the incorporation of rare-earth elements within the host because researchers expect that the marriage of rare-earth skeleton host and the exchangeable species within the interlayer will open up new avenues both for the assembly of layered materials and for the understanding of rare-earth element chemistry. Such lanthanide layered solids have industrial potential. These materials are also of academic importance, serving as an ideal model for studying the cationic size effect on structure stability associated with lanthanide contraction. In this Account, we present the work done by ourselves and others on this novel class of materials. We examine the following four subtopics regarding these layered anionic materials: (1) synthesis strategy and composition diversity, (2) structural features, (3) structure stability with relative humidity, and (4) applications. These materials can be synthesized either by hydrothermal reactions or by homogeneous precipitation, and a variety of anions can be intercalated into the gallery. Although only cations with a suitable size can form the layered structure, the possible range is wide, from early to late lanthanides. We illustrate the effect of lanthanide contraction on properties including morphology, lattice dimensions, and coordination numbers. Because each lanthanide metal ion coordinates water molecules, and the water molecules point directly into the gallery space, this feature plays a critical role in stabilizing the layered structure. In the 9-fold monocapped square antiprism structure, the humidity-triggered transition between high- and low-hydrated phases corresponds to the uptake of H(2)O molecules at the capping site, which provides further evidence of the importance of water coordination. Applications using this unique combination of rare-earth element chemistry and layered materials include ion-exchange, photoluminescence, catalysis, and biomedical devices. Further exploration of the compounds and new methods for functional modification would dramatically enrich the junction of these two fields, leading to a new generation of layered materials with desirable properties.