Local Structural Effects of Eu3+ Incorporation into Xenotime-type Solid Solutions with Different Host Cations.

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.

Temperature-dependent luminescence spectroscopic and mass spectrometric investigations of U(VI) complexation with aqueous silicates in the acidic pH-range.

In this study the complexation of U(VI) with orthosilicic acid (H4SiO4) 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 · 10-3M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO2OSi(OH)3+ species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10-6M, the silicon concentration was varied between 1.3 · 10-4-1.3 · 10-3M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO4. The results confirmed the formation of the aqueous UO2OSi(OH)3+ complex. The conditional complexation constant at 25 °C, log *ß = -(0.31 ± 0.24), was extrapolated to infinite dilution using the Davies equation, which led to log *ß0 = -(0.06 ± 0.24). Further experiments at different temperatures (1-25 °C) allowed the calculation of the molal enthalpy of reaction ΔrHm0 = 45.8 ± 22.5 kJ·mol-1 and molal entropy of reaction ΔrSm0 = 152.5 ± 78.8 J·K-1·mol-1 using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process.

Association of Eu(III) and Cm(III) onto an extremely halophilic archaeon.

In addition to geological, geochemical, and geophysical aspects, also, microbial aspects have to be taken into account when considering the final storage of high-level radioactive waste in a deep geological repository. Rock salt is a potential host rock formation for such a repository. One indigenous microorganism, that is, common in rock salt, is the halophilic archaeon Halobacterium noricense DSM15987T, which was used in our study to investigate its interactions with the trivalent actinide curium and its inactive analogue europium as a function of time and concentration. Time-resolved laser-induced fluorescence spectroscopy was applied to characterize formed species in the micromolar europium concentration range. An extended evaluation of the data with parallel factor analysis revealed the association of Eu(III) to a phosphate compound released by the cells (F2/F1 ratio, 2.50) and a solid phosphate species (F2/F1 ratio, 1.80). The association with an aqueous phosphate species and a solid phosphate species was proven with site-selective TRLFS. Experiments with Cm(III) in the nanomolar concentration range showed a time- and pCH+-dependent species distribution. These species were characterized by red-shifted emission maxima, 600-602 nm, in comparison to the free Cm(III) aqueous ion, 593.8 nm. After 24 h, 40% of the luminescence intensity was measured on the cells corresponding to 0.18 µg Cm(III)/gDBM. Our results demonstrate that Halobacterium noricense DSM15987T interacts with Eu(III) by the formation of phosphate species, whereas for Cm(III), a complexation with carboxylic functional groups was also observed.

A Spectroscopic Investigation of Eu3+ Incorporation in LnPO4 (Ln = Tb, Gd1-xLux, X = 0.3, 0.5, 0.7, 1) Ceramics.

We have investigated the incorporation of the luminescent Eu3+ cation in different LnPO4 (Ln = Tb, Gd1-xLu x , x = 0.3, 0.5, 0.7, 1) host phases. All samples were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, and site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS) directly after synthesis and after an aging time of one year at ambient conditions. The PXRD investigations demonstrate the formation of a TbPO4 phase in an uncommon anhydrite-like crystal structure evoked by a pressure-induced preparation step (grinding). In the Gd1-xLu x PO4 solid solution series, several different crystal structures are observed depending on the composition. The TRLFS emission spectra of LuPO4, Gd0.3Lu0.7PO4, and Gd0.5Lu0.5PO4 indicate Eu3+-incorporation within a xenotime-type crystal structure. TRLFS and PXRD investigations of the Gd0.7Lu0.3PO4 composition show the presence of anhydrite, xenotime, and monazite phases, implying that xenotime no longer is the favored crystal structure due to the predominance of the substantially larger Gd3+-cation in this solid phase. Eu3+-incorporation occurs predominantly in the anhydrite-like structure with smaller contributions of Eu3+ incorporated in monazite and xenotime. The electronic levels of the Eu3+-dopant in Gd0.3Lu0.7PO4 and Gd0.5Lu0.5PO4 xenotime hosts are strongly coupled to external lattice vibrations, giving rise to high-energy peaks in the obtained excitation spectra. The coupling becomes stronger after aging to such an extent that direct excitation of Eu3+ in the xenotime structure is strongly suppressed. This phenomenon, however, is only visible for materials where Eu3+ was predominantly incorporated within the xenotime structure. Single crystals of Eu3+-doped LuPO4 show no changes upon aging despite the presence of vibronically coupled excitation peaks in the excitation spectra measured directly after synthesis. Based on this observation, we propose a lattice relaxation process occurring in the powder samples during aging, resulting in Eu3+ migration within the crystal structure and Eu3+ accumulation at grain boundaries or xenotime surface sites.