Microwave-Employed Sol-Gel Synthesis of Scheelite-Type Microcrystalline AgGd(MoO4)2:Yb3+/Ho3+ Upconversion Yellow Phosphors and Their Spectroscopic Properties

AgGd(MoO4)(2):Ho3+/Yb3+ double molybdates with five concentrations of Ho3+ and Yb3+ were synthesized by the microwave employed sol-gel based process (MES), and the crystal structure variation, concentration effects, and spectroscopic characteristics were investigated. The crystal structures of AgGd1-x-yHoxYby(MoO4)(2) (x = 0, 0.05; y = 0, 0.35, 0.4, 0.45, 0.5)at room temperature were determined in space group I4(1)/a by Rietveld analysis. Pure AgGd(MoO4)(2) has a scheelite-type structure with mixed occupations of (Ag,Gd) sites and cell parameters a = 5.24782 (11) and c = 11.5107 (3) angstrom, V = 317.002 (17) angstrom(3), Z = 4. In doped samples, the sites are occupied by a mixture of (Ag,Gd,Ho,Yb) ions, which provides a linear cell volume decrease with the doping level increase. Under the excitation at 980 nm, AGM:0.05Ho,yYb phosphors exhibited a yellowish green emission composed of red and green emission bands according to the strong transitions F-5(5) -> I-5(8) and S-5(2)/F-5(4) -> I-5(8) of Ho3+ ions. The evaluated photoluminescence and Raman spectroscopic results were discussed in detail. The upconversion intensity behavior dependent on the Yb/Ho ratio is explained in terms of the optimal number of Yb3+ ions at the characteristic energy transfer distance around the Ho3+ ion.