We have investigated the incorporation of the luminescent Eu³⁺ cation in different LnPO₄ (Ln = Tb, Gd_1-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 Gd_1-xLu _x PO₄ solid solution series, several different crystal structures are observed depending on the composition. The TRLFS emission spectra of LuPO₄, Gd_0.3Lu_0.7PO₄, and Gd_0.5Lu_0.5PO₄ indicate Eu³⁺-incorporation within a xenotime-type crystal structure. TRLFS and PXRD investigations of the Gd_0.7Lu_0.3PO₄ 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 Gd_0.3Lu_0.7PO₄ and Gd_0.5Lu_0.5PO₄ 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.

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