Rare Earth Metal Oxalatophosphonates: Syntheses, Structure Diversity, and Photoluminescence Properties

Hydrothermal reactions of N-piperidinomethane-1-phosphonic acid (H₂pmp) and oxalate anion with trivalent rare earth metal ions yielded 13 rare earth oxalatophosphonates with three closely related structural types, ranging from two-dimensional (2D) networks to three-dimensional (3D) frameworks. They are formulated as [Ln₃(Hpmp)₃(C₂O₄)₂(H₂O)₅](ClO₄)₂·9H₂O [Ln = Gd (1), Eu (2), Tb(3), type I structure], [Ln₂(Hpmp)(C₂O₄)_2.5(H₂O)₃]·H₂O [Ln = Gd (5), Sm (6), Eu (7), Tb (8), Dy (9), Ho (10), Er (11), Yb (12), Y (13), type II structure], and [Gd₃(Hpmp)(C₂O₄)₄(H₂O)₆]·5H₂O [(4), type III structure]. Their structures were characterized by single crystal X-ray diffraction, powder X-ray diffraction (PXRD), infrared (IR) spectroscopy, elemental analysis (EA), and thermogravimetric analysis (TGA), except that the structures of compounds 2 and 3 were confirmed by PXRD, IR, and EA. The effects of reaction conditions including pH value, reaction temperature, counterion, and stoichiometry of the starting materials on the structures have been discussed. Furthermore, the solid-state photoluminescence properties of the Eu (2 and 7) and Tb (3 and 8) compounds have been measured at room temperature. The two types of Eu and Tb compounds display intense red and green luminescence emissions, respectively, with lifetimes at the millisecond order.