Achieving white emission by site-selective occupation of Eu2+ in K2BaSr(PO4)2−x(BO3)x:Eu2+

Abstract

Efficient regulation of activator site occupation is crucial for developing white light phosphors with excellent luminous properties. Herein, we report the synthesis of new borophosphate K₂BaSr(PO₄)_2−x(BO₃)_x:0.04Eu²⁺ phosphors via an anion group substitution strategy. In the pristine sample, there are three different cationic sites (Ba²⁺, Sr²⁺, and K⁺) with different coordination environments where Eu²⁺ can occupy the Ba²⁺ and/or Sr²⁺ sites. In the process of (BO₃)³⁻ partially substituting (PO₄)³⁻ units, owing to the effect of increased cation disorder generated from the difference in geometric configurations between the two anion groups, a new emission band centering at around 535 nm appears in the emission spectra. The intensity of this band increases as more (PO₄)³⁻ units are substituted by (BO₃)³⁻, resulting in the phosphor emission tuned from blue to white by controlling the (BO₃)³⁻ concentration. This emission band can be attributed to the occupation of Eu²⁺ at the K⁺ sites induced by cation disorder through the analysis of structural and photoluminescence properties and first-principles calculations, except for the co-occupation of Eu²⁺ with Ba and Sr sites. A phosphor-converted LED (pc-LED) has been successfully fabricated using the phosphor at optimized compositions, K₂BaSr(PO₄)_1.8(BO₃)_0.2:0.04Eu²⁺, and a 365 nm near-ultraviolet chip, exhibiting a color rendering index of 81.3 and a correlated color temperature of 7024 K. Our findings demonstrate the effectiveness of the anion group modulation for developing new high-performance LED phosphors.