Unveiling the Phase Formations in the Sr–Zn–Eu3+ Orthophosphate System: Crystallographic Analysis and Photoluminescent Properties

This study investigates phase formation in the Sr–Zn–Eu3+ orthophosphate system, focusing on double- and triple-phosphates. The isomorphisms and phase formation in Sr3–1.5xEu1+х(PO4)3, Sr9–1.5xZn1.5Euх(PO4)7, Sr9.5–1.5xZnEux(PO4)7, Sr3‒xZnxEu(PO4)3, and Sr3‒xZnx(PO4)2 series were studied using powder X-ray diffraction and Rietveld refinement. A ternary phase diagram was constructed, identifying concentration limits for pure phases and multi-phase regions as well as areas of stabilization of strontiowhitlockite-, palmierite-, eulytite-, and strontiohurlbutite-type phases. The combinatorial complexity of Sr-based phosphates is discussed. The β-Sr3(PO4)2 isostructural to whitlockite was found to exhibit the highest isomorphic capacity for Eu3+ cations, which is advantageous for its application as a red-emitting phosphor. Photoluminescence properties were studied, and analyzed based on structural data. Photoluminescence studies confirmed intense red-emission dominated by the 5D0 → 7F2 transition of Eu3+, with the β-Sr3(PO4)2-based phosphor showing the highest emission intensity.