| Summary: | Gd<sup>3+</sup> and Sm<sup>3+</sup> co-activation, the effect of cation substitutions and the creation of cation vacancies in the scheelite-type framework are investigated as factors influencing luminescence properties. Ag<i><sub>x</sub></i>Gd<sub>((2−<i>x</i>)/3)−0.3−<i>y</i></sub>Sm<i><sub>y</sub></i>Eu<sup>3+</sup><sub>0.3</sub>☐<sub>(1−2<i>x</i>)/3</sub>WO<sub>4</sub> (<i>x</i> = 0.50, 0.286, 0.20; <i>y</i> = 0.01, 0.02, 0.03, 0.3) scheelite-type phases (A<i><sub>x</sub></i>GS<i><sub>y</sub></i>E) have been synthesized by a solid-state method. A powder X-ray diffraction study of A<i><sub>x</sub></i>GS<i><sub>y</sub></i>E (<i>x</i> = 0.286, 0.2; <i>y</i> = 0.01, 0.02, 0.03) shows that the crystal structures have an incommensurately modulated character similar to other cation-deficient scheelite-related phases. Luminescence properties have been evaluated under near-ultraviolet (n–UV) light. The photoluminescence excitation spectra of A<i><sub>x</sub></i>GS<i><sub>y</sub></i>E demonstrate the strongest absorption at 395 nm, which matches well with commercially available UV-emitting GaN-based LED chips. Gd<sup>3+</sup> and Sm<sup>3+</sup> co-activation leads to a notable decreasing intensity of the charge transfer band in comparison with Gd<sup>3+</sup> single-doped phases. The main absorption is the <sup>7</sup>F<sub>0</sub> → <sup>5</sup>L<sub>6</sub> transition of Eu<sup>3+</sup> at 395 nm and the <sup>6</sup>H<sub>5/2</sub> → <sup>4</sup>F<sub>7/2</sub> transition of Sm<sup>3+</sup> at 405 nm. The photoluminescence emission spectra of all the samples indicate intense red emission due to the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> transition of Eu<sup>3+</sup>. The intensity of the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> emission increases from ~2 times (<i>x</i> = 0.2, <i>y</i> = 0.01 and <i>x</i> = 0.286, <i>y</i> = 0.02) to ~4 times (<i>x</i> = 0.5, <i>y</i> = 0.01) in the Gd<sup>3+</sup> and Sm<sup>3+</sup> co-doped samples. The integral emission intensity of Ag<sub>0.20</sub>Gd<sub>0.29</sub>Sm<sub>0.01</sub>Eu<sub>0.30</sub>WO<sub>4</sub> in the red visible spectral range (the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> transition) is higher by ~20% than that of the commercially used red phosphor of Gd<sub>2</sub>O<sub>2</sub>S:Eu<sup>3+</sup>. A thermal quenching study of the luminescence of the Eu<sup>3+</sup> emission reveals the influence of the structure of compounds and the Sm<sup>3+</sup> concentration on the temperature dependence and behavior of the synthesized crystals. Ag<sub>0.286</sub>Gd<sub>0.252</sub>Sm<sub>0.02</sub>Eu<sub>0.30</sub>WO<sub>4</sub> and Ag<sub>0.20</sub>Gd<sub>0.29</sub>Sm<sub>0.01</sub>Eu<sub>0.30</sub>WO<sub>4</sub>, with the incommensurately modulated (3 + 1)D monoclinic structure, are very attractive as near-UV converting phosphors applied as red-emitting phosphors for LEDs.
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