Topological Understanding of the Narrow Emission Band of β-Ca3(PO4)2:Eu2+-Type Phosphors: The Importance of Pinwheel Structural Units.

In whitlockite-type compounds, the Eu2+ ion can emit in a wide region from blue to yellow, but its luminescent mechanism remains unclear. Here, we performed a topological crystallography analysis of the whitlockite structure and first clarified the origin of the blue narrow emission band. Specifically, close-packing theory was used to reveal the topological character of β-Ca3(PO4)2 and describe its evolution from M3(PO4)2 (M = Ba, Sr). Following these analyses, we determined the relationships of unit cells and cation sites in two crystal structures and confirmed their common structural unit, the pinwheels, composed of shared corners of the octahedron and tetrahedron. Combined with the luminescent similarity of β-Ca3(PO4)2:Eu2+ and M3(PO4)2:Eu2+, the origin of the blue emission band in both materials was assigned to the Eu2+ ion at the cation site with pinwheel character. This assignment was further confirmed by comparative emission width analyses with Cs2CaP2O7:Eu2+, indicating that the consideration of the pinwheel structural unit, with ududud topological notation, is important to understand the emission energy and half-width of the Eu2+ ion in inorganic solids. This work will not only provide the baseline for understanding the complex Eu2+ luminescence in whitlockite-type phosphors but also benefit the design of narrow-emission luminescent materials via crystal site engineering.