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

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