Accessing ( Ba1-xSrx) Al2Si2O8: Eu Phosphors for Solid State White Lighting via Microwave- assisted Preparation: Tuning Emission Color by Coordination Environment

The preparation of Eu2+-substituted barium aluminum silicates is achieved using a rapid microwave-assisted preparation. The phase evolution of two BaAl2Si2O8:Eu2+ polymorphs, the higher temperature hexagonal phase (hexacelsian), and the lower temperature monoclinic phase (celsian), is explored by varying the ramp time and soak time. This preparation method significantly reduces the reaction time needed to form these phases compared to conventional solid state routes. The luminescent properties of the two phases are identified under UV excitation with the hexagonal phase emitting in the UV region ((em) = 372 nm) and the monoclinic phase emitting in the blue region ((em) = 438 nm). The differences in optical properties of the two polymorphs are correlated to the coordination number and arrangement of the alkali earth site. The optical properties of the monoclinic phase can be further tuned through the substitution of Sr2+, forming the solid solution (Ba1-xSrx)Al2Si2O8:Eu2+. Changes in the crystal structure due to Sr2+ substitution produce a surprising blue-shift in the emission spectrum, which is explained by a greater dispersion of bond lengths in the (Ba/Sr)-O polyhedra. The entire monoclinic solid solution exhibits excellent quantum yields of nearly 90%, owing to the structural rigidity provided by the highly connected tetrahedral network.