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

Henry Lösch,Antje Hirsch,Jacqueline Holthausen,Moritz Schmidt,Nina Huittinen,Lars Peters,Bin Xiao,Stefan Neumeier

doi:10.3389/fchem.2019.00094

Abstract

We have investigated the incorporation of the luminescent Eu3+ cation in different LnPO4 (Ln = Tb, Gd1−xLux, 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−xLuxPO4 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.

Highlights

A large challenge yet to be resolved by many countries is the safe disposal of high-level radioactive waste materials, such as spent reactor fuel and various waste streams from, e.g., fuel reprocessing facilities and dismantled nuclear weapons
We have combined powder X-ray diffraction (PXRD), Raman spectroscopy, and site-selective TRLFS investigations to understand the incorporation of Eu3+ in LnPO4 ceramics predominantly crystallizing in the xenotime structure
The investigations of Eu3+–doped LnPO4 in the xenotime structure by PXRD, Raman spectroscopy, and site-selective TRLFS provide molecular insights of the incorporation of Eu3+ as a chemical analog of the trivalent actinides in a possible ceramic host material for radioactive waste disposal

Summary

Introduction

A large challenge yet to be resolved by many countries is the safe disposal of high-level radioactive waste materials, such as spent reactor fuel and various waste streams from, e.g., fuel reprocessing facilities and dismantled nuclear weapons. The naturally occurring LnPO4–minerals xenotime and monazite have been shown to contain varying quantities of primordial actinides (U, Th), in some cases up to more than 20 wt– % (Gramaccioli and Segalstad, 1978; van Emden et al, 1997; Seydoux-Guillaume et al, 2004; Förster et al, 2008). These crystalline minerals have existed for millions of years, showing good chemical durability and radiation tolerance. The lanthanide-orthophosphates GdPO4, TbPO4, and DyPO4 show polymorph properties depending on pressure and temperature (Celebi and Kolis, 2002; Boakye et al, 2008)

Results

Discussion

Conclusion