Layered rare earth hydroxides (LREHs): synthesis and structure characterization towards multifunctionality.

Layered rare earth hydroxides (LREHs) represent a new family of layered host compounds that integrate attractive physicochemical properties of rare earth elements with the wide tunability of guest anions. The compounds have attracted significant research attention, and potential applications have been found in various fields such as optics, catalysis, bio-medicine and so on. In this perspective, we describe our recent progress in the synthesis, structure characterization, and development of functionalities of the LREH compounds. A unique homogeneous alkalization method, in which RE ions are precipitated from a solution containing RE salt, concentrated target anions and hexamethylenetetramine, has been employed to effectively produce highly crystalline LREH samples. A range of anionic forms including chloride-, nitrate-, sulfate- and organodisulfonate-series, have been synthesized and structurally characterized. Two types of cationic rare earth hydroxide layers, {[RE2(OH)5(H2O)2](+)}∞ for the chloride- and nitrate-series and {[RE(OH)2(H2O)](+)}∞ for the sulfate- and organodisulfonate-series, are classified. Unique dehydration/rehydration behaviors or thermal phase evolution of the LREH compounds have been revealed and discussed in relation to the crystal structures. An outlook for potential applications of LREH compounds as anion exchangers, precursors to unique functional oxides, and optical phosphors is described.