A Spectroscopic Investigation of Eu(3+) Incorporation in LnPO(4) (Ln = Tb, Gd(1-x)Lu(x), X = 0.3, 0.5, 0.7, 1) Ceramics

We have investigated the incorporation of the luminescent Eu(3+) cation in different LnPO(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.