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In this study the complexation of U(VI) with orthosilicic acid (H 4 SiO 4 ) was investigated between pH 3.5 and 5 by combining electrospray ionization mass spectrometry (ESI-MS) and laser-induced luminescence spectroscopy. The ESI-MS experiments performed at a total silicon concentration of 5 · 10 -3 M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO 2 OSi(OH) 3 species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10 + species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10 -6 M, the silicon concentration was varied between 1.3 · 10 -4 -1.3 · 10 -3 M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO 4 . The results confirmed the formation of the aqueous UO 2 OSi(OH) 3 + complex. The conditional complexation constant at 25 °C, log *β = -(0.31 ± 0.24), was extrapolated to infinite dilution using the Davies equation, which led to log *β 0  = -(0.06 ± 0.24). Further experiments at different temperatures (1-25 °C) allowed the calculation of the molal enthalpy of reaction Δ r H m 0  = 45.8 ± 22.5 kJ·mol -1 and molal entropy of reaction Δ r S m 0  = 152.5 ± 78.8 J·K -1 ·mol -1 using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Abstract: In this study, the effect of host cations on the local structure around the dopant site of materials from the xenotime family is systematically studied on the molecular level. A series of six Eu3+‐doped xenotime‐type single crystals (Tb, Y, Ho, Er, Yb, and LuPO4) have been grown and spectroscopically analyzed by using polarization‐dependent laser‐induced luminescence spectroscopy (p‐TRLFS). Our results demonstrate that the structural disorder changes in a non‐linear manner with a structural break between Yb3+ and Lu3+. Despite adopting identical crystal structures, the solid solutions of these materials vary significantly, and differ from monazite solid solutions. Similar Eu3+ incorporation behavior with a strongly distorted dopant site is found for the early members of the xenotime family, whereas LuPO4 with the largest host versus dopant radii mismatch is anomalous in that it contains the most symmetrical lattice site. This goes along with a significantly stronger crystal field, indicating a shorter Eu−O bond length, as well as a strong vibronic coupling to external translational lattice vibrations. The p‐TRLFS analysis confirms the breakdown of the crystallographic site symmetry from D2d to C1 in YPO4, whereas a small distortion of the crystallographic site in LuPO4 results in an S4 point symmetry for the Eu3+ cation. The lattice with the smallest cation host site is no longer sufficiently flexible to make room for Eu3+ and instead “forces” the guest ion to occupy a less distorted Lu3+ site. [ABSTRACT FROM AUTHOR]

In the present study, we have investigated the luminescent properties of Eu 3+ as a dopant in a series of synthetic lanthanide phosphates from the monazite group. Systematic trends in the spectroscopic properties of Eu 3+ depending on the size of the host cation and the dopant to ligand distance have been observed. Our results show that the increasing match between host and dopant radii when going from Eu 3+ -doped LaPO 4 toward the smaller GdPO 4 monazite decreases both the full width at half maximum of the Eu 3+ excitation peak, as well as the 7 F 2 / 7 F 1 emission band intensity ratio. The decreasing Ln⋯O bond distance within the LnPO 4 series causes a systematic bathochromic shift of the Eu 3+ excitation peak, showing a linear dependence of both the host cation size and the Ln⋯O distance. The linear relationship can be used to predict the energy band gap for Eu 3+ -doped monazites for which no Eu 3+ luminescent data is available. Finally, mechanisms for metal-metal energy transfer between host and dopant lanthanides have been explored based on recorded luminescence lifetime data. Luminescence lifetime data for Eu 3+ incorporated in the various monazite hosts clearly indicated that the energy band gap between the guest ion emission transition and the host ion absorption transition can be correlated to the degree of quenching observed in these materials with otherwise identical geometries and chemistries. [ABSTRACT FROM AUTHOR]