Flux Synthesis, Crystal Structure, and Photoluminescence of a Heterometallic Uranyl-Europium Germanate with UO–Eu Linkage: K4[(UO2)Eu2(Ge2O7)2]

Crystals of a uranyl-europium germanate, K4[(UO2)­Eu2­(Ge2O7)2], have been grown at high temperature from a KF-MoO3 flux and structurally characterized by single-crystal X-ray diffraction. The structure contains PaCl5-type chains formed of edge-sharing EuO7 pentagonal bipyramids that are connected by digermanate groups such that layers of europium germanate are formed. Neighboring layers are further linked by UO6 tetragonal bipyramids through uranyl ion oxygen atoms to generate a 3D framework with 6-ring channels where the K+ cations are located. The structure contains an unusual heterometallic UO–Eu linkage. Photoluminescence studies show strong red emission at room temperature. The relative intensities of the 5D0 → 7F1 and 5D0 → 7F2 transitions confirm that the europium site lacks an inversion center. All of the emission lines show similar decay curves, which can be well-fitted by a single exponential decay function with radiative lifetimes of 0.53 ± 0.03 ms. No emission is observed in the region from 450 to 550 nm typical of the uranyl cation, indicating that, upon uranyl excitation, the energy is either transferred to the Eu3+ centers or lost to nonradiative processes.