Luminescence Property Upgrading via the Structure and Cation Changing in AgxEu(2-x)/3WO4 and AgxGd(2-x)/3-0.3Eu0.3WO4

The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescence properties. AgxEu(2-x)/33+square(1-2x)/3WO4 and AgxGd(2-x)/3-0.3Eu0.33+square(1-2x)/3WO4 (x = 0.5-0) scheelite-type phases were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder synchrotron X-ray diffraction. Transmission electron microscopy also revealed the (3 + 1)D incommensurately modulated character of AgxEu(2-x)/33+square(1-2x)WO4 (x = 0.286, 0.2) phases. The crystal structures of the scheelite-based AgxEu(2-x)/33+square(1-2x)/3WO4 (x = 0.5, 0.286, 0.2) red phosphors have been refined from high resolution synchrotron powder X-ray diffraction data. The luminescence properties of all phases under near-ultraviolet (n-UV) light have been investigated. The excitation spectra of AgxEu(2-x)/33+square(1-2x)/3WO4 (x = 0.5, 0.286, 0.2) phosphors show the strongest absorption at 395 nm, which matches well with the commercially available n-UV-emitting GaN-based LED chip. The excitation spectra of the Eu-2/3 square 1/3WO4 and Gd0.367Eu0.301 square 1/3WO4 phases exhibit the highest contribution of the charge transfer band at 250 nm and thus the most efficient energy transfer mechanism between the host and the luminescent ion as compared to direct excitation. The emission spectra of all samples indicate an intense red emission due to the D-5(0) -> F-7(2) transition of Eu3+. Concentration dependence of the D-5(0) -> F-7(2) emission for AgxEu(2-x)/3 square(1-2x)/3WO4 samples differs from the same dependence for the earlier studied NaxEu(2-x/33+square(1-2x)/3MoO4 (0 <= x <= 0.5) phases. The intensity of the D-5(0) -> F-7(2) emission is reduced almost 7 times with decreasing x from 0.5 to 0, but it practically does not change in the range from x = 0.286 to x = 0.200. The emission spectra of Gd-containing samples show a completely different trend as compared to only Eu-containing, samples. The Eu3+ emission under excitation of Eu3+(L-5(6)) level (lambda(ex) = 395 nm) increases more than 2.5 times with the increasing Gd3+ concentration from 0.2 (x = 0.5) to 0.3 (x = 0.2) in the AgxGd(2-x)/3-0.3Eu0.33+square(1-2x)/3WO4, after which it remains almost constant for higher Gd3+ concentrations.