New localized/delocalized emitting state of Eu²⁺ in orange-emitting hexagonal EuAl₂O₄

Eu²⁺-activated phosphors are being widely used in illuminations and displays. Some of these phosphors feature an extremely broad and red-shifted Eu²⁺ emission band; however, convincing explanation of this phenomenon is lacking. Here we report a new localized/delocalized emitting state of Eu²⁺ ions in a new hexagonal EuAl₂O₄ phosphor whose Eu²⁺ luminescence exhibits a very large bandwidth and an extremely large Stokes shift. At 77 K, two luminescent sites responsible for 550 nm and 645 nm broadband emissions are recognized, while at room temperature only the 645 nm emission band emits. The 645 nm emission exhibits a typical radiative lifetime of 1.27 μs and an unusually large Stokes shift of 0.92 eV. We identify the 645 nm emission as originating from a new type of emitting state whose composition is predominantly that of localized 4f⁶5d character but which also contains a complementary component with delocalized conduction-band-like character. This investigation provides new insights into a unique type of Eu²⁺ luminescence in solids whose emission exhibits both a very large bandwidth and an extremely large Stokes shift.