K2Ln(PO4)(WO4):Tb3+,Eu3+ (Ln = Y, Gd and Lu) phosphors: highly efficient pure red and tuneable emission for white light-emitting diodes

Abstract

A novel phosphate/tungstate family, K₂Ln(PO₄)(WO₄) (Ln = Y, Gd and Lu) doped with Tb³⁺ and Eu³⁺ is synthesized via a conventional high-temperature solid-state reaction to explore new pure red phosphors with high critical concentration for white light-emitting diodes (WLEDs). The results from the Rietveld method show that the crystal structures of the hosts are composed of phosphate layers and tungstate zigzags, and the Ln³⁺–Ln³⁺-units are isolated by the [PO₄]³⁻ groups in phosphate layers. The critical concentration of Tb³⁺ and Eu³⁺ is up to 40–50% in the singly doped phosphors, which is ascribed to the interaction of the isolated Ln³⁺ ions being mitigated by [PO₄]³⁻ and [WO₄]²⁻ groups, such that the special structure of K₂Ln(PO₄)(WO₄) helps the interaction of luminescence centres. The energy transfer from Tb³⁺ to Eu³⁺ in K₂Ln(PO₄)(WO₄) is demonstrated by fluorescence decay times. By adjusting the ratio of Eu³⁺ and Tb³⁺, we can tune the emission colour of K₂Ln(PO₄)(WO₄):Tb³⁺,Eu³⁺ from green to yellow, orange and pure red. For K₂Tb_0.5Eu_0.5(PO₄)(WO₄), the internal quantum efficiency is as high as 76.45% under an excitation of 394 nm, and the emission intensity at 150 °C is 92.2% of that at 25 °C.