Structural and optical properties of hydrated lanthanon uranyl arsenates

An investigation of the structural and optical properties of the lamellar lanthanon derivatives of hydrogen uranyl arsenate (HUO2AsO4 · 4H2O (HUAs)) is reported. The hydrated lanthanon uranyl arsenate compounds have the approximate composition Ln(UO2AsO4)3 · xH2O (LnUAs) (Ln ≡ La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, YbandLu, x ≈ 13–16) and are prepared by intercalation of the lanthanon ions into either HUAs or n-butylammonium uranyl arsenate. Upon intercalation of the tripositive guest ions, the lattice symmetry and crystallinity are reduced, as observed by X-ray powder diffraction. The LnUAs samples exhibit emission characteristic of the UO22+ moiety. Those samples having Ln ≡ Ce, Pr, Nd, Sm, HoandEr are very weakly emissive, whereas the remaining solids (with Ln ≡ La, Eu, Gd, Tb, Dy, Tm, YbandLu) emit strongly at 298 K when excited with near-UV light. The emissive LnUAs samples have photoluminescence (PL) decay curves that are exponential and yield lifetimes for the strongly emissive samples in the approximate range of 8–100 μs with quantum yields of about 0.02–0.09. From these data, unimolecular radiative and non-radiative rate constants have been calculated for uranyl PL, yielding values of about (5–30) × 102s−1 and about (1–10) × 104s−1 respectively. For EuUAs there is extensive host-to-guest energy transfer, resulting in long-lived emission from the europium guest species. Electronic absorption and PL properties of the LnUAs samples are similar to those of their Ln(UO2PO4)3 · xH2O counterparts