K2Ln(PO4)(WO4):Tb3+,Eu3+ (Ln = Y, Gd and Lu) phosphors: highly efficient pure red and tuneable emission for white light-emitting diodes
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 K2Tb0.5Eu0.5(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.