Flux synthesis, crystal structure, and photoluminescence of a heterometallic uranyl-europium germanate with U=O-Eu linkage: K ₄[(UO₂)Eu₂(Ge₂O₇) ₂]

Crystals of a uranyl-europium germanate, K₄[(UO ₂)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.