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

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

JOURNAL=Frontiers in Chemistry

VOLUME=7

YEAR=2019

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2019.00094

DOI=10.3389/fchem.2019.00094

ISSN=2296-2646

ABSTRACT=<p>We have investigated the incorporation of the luminescent Eu<sup>3+</sup> cation in different <italic>Ln</italic>PO<sub>4</sub> (<italic>Ln</italic> = Tb, Gd<sub>1−x</sub>Lu<sub><italic>x</italic></sub>, <italic>x</italic> = 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 TbPO<sub>4</sub> phase in an uncommon anhydrite-like crystal structure evoked by a pressure-induced preparation step (grinding). In the Gd<sub>1−x</sub>Lu<sub><italic>x</italic></sub>PO<sub>4</sub> solid solution series, several different crystal structures are observed depending on the composition. The TRLFS emission spectra of LuPO<sub>4</sub>, Gd<sub>0.3</sub>Lu<sub>0.7</sub>PO<sub>4</sub>, and Gd<sub>0.5</sub>Lu<sub>0.5</sub>PO<sub>4</sub> indicate Eu<sup>3+</sup>–incorporation within a xenotime-type crystal structure. TRLFS and PXRD investigations of the Gd<sub>0.7</sub>Lu<sub>0.3</sub>PO<sub>4</sub> 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 Gd<sup>3+</sup>–cation in this solid phase. Eu<sup>3+</sup>–incorporation occurs predominantly in the anhydrite-like structure with smaller contributions of Eu<sup>3+</sup> incorporated in monazite and xenotime. The electronic levels of the Eu<sup>3+</sup>–dopant in Gd<sub>0.3</sub>Lu<sub>0.7</sub>PO<sub>4</sub> and Gd<sub>0.5</sub>Lu<sub>0.5</sub>PO<sub>4</sub> 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 Eu<sup>3+</sup> in the xenotime structure is strongly suppressed. This phenomenon, however, is only visible for materials where Eu<sup>3+</sup> was predominantly incorporated within the xenotime structure. Single crystals of Eu<sup>3+</sup>–doped LuPO<sub>4</sub> 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 Eu<sup>3+</sup> migration within the crystal structure and Eu<sup>3+</sup> accumulation at grain boundaries or xenotime surface sites.</p>