Site Occupation Engineering toward Giant Red-Shifted Photoluminescence in (Ba,Sr)2LaGaO5:Eu2+ Phosphors

Exploring oxide-based red-emitting phosphors is essential for improving the color rendering index (R-a) and reducing the correlated color temperature (CCT) of white-light-emitting diode (LED) lighting sources. Especially, it is challenging to design Eu2+ red emission in inorganic solids. Here, the Eu2+-activated oxide phosphor Sr2LaGaO5:Eu2+ was synthesized with red emission peaking at 618 nm under 450 nm excitation. The crystal structure and spectral analysis indicate that Eu2+ tends to occupy [Sr1/LaO8] polyhedrons with a smaller coordination number, resulting in a large crystal field splitting at the 5d level and realizing the broadband 4f-5d red emission. When Sr is substituted by Ba atoms, density functional theory calculations verify that Ba tends to enter [Sr2O(10)] with a large coordination number, further giving rise to the lattice distortion and a giant spectral redshift (618-800 nm). The white LED device fabricated by mixing red Sr1.8Ba0.2GaO5:Eu2+ and green Lu3Al5O12:Ce3+ phosphors exhibits a high color rendering index (R-a = 92.1) and a low color-dependent temperature (CCT = 4570 K). This study will give guidance for exploring new Eu2+ activated oxide-based red phosphors as well as achieving tunable emission through cations' substitution.