Structural and photoluminescent properties of fully-hydrated lanthanon uranyl phosphates

A re-investigation of the lamellar lanthanon derivatives of hydrogen uranyl phosphate (HUO2PO4.4H2O (HUP)) is reported. When prepared from both HUP and n-butylammonium uranyl phosphate (n-C4H9NH3UO2-PO4.3H2O (BAUP)) the fully-hydrated LnUP compounds have the approximate composition Ln1/3UO2PO4.xH2O (Ln = La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Yb; χ = 3.7-4.6). Interlamellar spacings parallel the trend in lanthanon size and range from 11.1 A for the lanthanum derivative to 10.7 A for the ytterbium compound; these distances decrease by 2.5-2.8 A if the LnUP compounds are air dried. With the exception of cerium, praseodymium, neodymium and europium derivatives, which are only weakly emissive, the other LnUP samples emit efficiently at 295 K when excited with blue or near-UV light. Photoluminescence decay curves for the fully-hydrated samples are exponential and yield lifetimes for the strongly emissive samples of about 20-100 μs; their radiative quantum yields are typically about 0.1-0.2. These data have been used to calculate unimolecular radiative and non-radiative rate constants for excited-state decay. Values of about (1-3)x103 s-1 and (1-4)x104 s-1 have been obtained for the radiative and non-radiative rate constants respectively. (orig.)