Synthesis and luminescent properties of SrNb2O6:Eu3+ and MGe4O9 (M = Li2, LiNa and K2): Mn4+ red emitting phosphors

Abstract

A series of Eu3+-doped SrNb2O6 phosphors have been successfully prepared by a high-temperature solid-state reaction technique. The as-prepared samples exhibit strong red emission peak around 612 nm, which is attributed to the 5D0-7F2 transition of the Eu3+ ion. Both the emission intensity and color rendering effect can be obviously improved in SrNb2O6:Eu3+ phosphors by self-compensation or co-doping with Li+ ions. Meanwhile, the decay time of phosphors can also be extended by charge compensation. The Judd-Ofelt theory is used to calculate the optical transition strength parameters and quantum efficiencies of the obtained samples. In addition, Eu3+ and Li+ concentration-dependent excitation and emission spectra are investigated in detail. The critical distance is determined to be about 11.48 Å and the strongest red emission intensity is achieved in the Sr0.7Nb2O6:0.15Eu3+,0.15Li+ phosphor. The CIE-1931coordinate (0.633, 0.366) of this sample is very close to that of the standard red light (0.67, 0.33). All of the results indicate that charge compensation approach can greatly improve the photoluminescence properties of Eu3+-doped SrNb2O6 phosphors, which will further promote their applications in solid state lighting. viii And others of Mn4+ions doped red-emitting phosphors such as Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 red-emitting phosphors were prepared by a facile solid-state reaction technique. The crystal structure, microstructure, luminescent performance and thermal stability of the synthesized phosphors are investigated in detail with the aids of X-ray diffraction, scanning electron microscope, X-Ray photoelectron spectroscopy, diffuse reflection spectra, photoluminescence spectra, decay curves and temperature-dependent spectra. For the Mn4+ ions doped Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 phosphors, the emission bands are centered at 669 (14948 cm-1), 661 (15129 cm-1) and 664 (15060 cm-1) nm, respectively. The studied compounds possess good CIE chromaticity coordinate, high color purity and high quantum yield. The crystal field strength (Dq) and Racah parameters (B and C) are estimated to evaluate the nephelauxetic effect of Mn4+ ions in Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 host lattices. The calculated results well conform to show linear dependence of E(2Eg) on β1 parameter which helps to predict emission position of Mn4+ ions. In addition, the doping concentration-dependent emission spectra of K2Ge4O9:xMn4+ red-emitting phosphors are investigated to find out the optimal doping concentration. All of the results indicate that Mn4+ ions doped Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 red-emitting phosphors are suitable for solid state lighting.