Structural Refinement and Optical Properties of Strontiowhitlockite-Based Mixed Phosphate-Vanadates

A series of Sr-based whitlockites─Strontiowhitlockites─doped with Eu³⁺ ions was synthesized by using the high-temperature solid-state method in air. The possibility of obtaining anion mixed salts, namely, Sr₈ZnEu(PO₄)_7-x(VO₄)_x phosphate-vanadates, in strontiowhitlockite-type (Sr-WHT) structure was shown for the first time. These compounds exhibit a continuous solid solution series with a Sr-WHT structure for 0 ≤ x ≤ 6. The formation limit of Sr-WHT (x = 6) was explained by structural features. A novel structural approach was applied for the Rietveld refinement of Sr₈ZnEu(PO₄)₇ phosphate. The refined structure was confirmed by IR and Raman spectroscopy and photoluminescence (PL) data, collected at room temperature and 80 K. The mixed salts demonstrate stable PL properties in the red region of the visible spectrum due to 4f-4f Eu³⁺ transitions. Luminescence decay curves of Eu³⁺ photoluminescence were investigated, and color coordinates were calculated for all single-phase mixed salts. These results demonstrate that the described mixed phosphate-vanadate salts represent a novel class of phosphors and are promising candidates for applications as the red component in white light-emitting diodes (w-LED) due to their narrow red band.