Rapid Synthesis of a Green Emitting Phosphor by Sulfidation\nof Intermetallic EuAl<sub>2</sub> and its Use in a Hybrid Material

Ternary sulfides\nare usually synthesized from the binary sulfidic\nprecursors at elevated temperatures using prolonged reaction times\nand sometimes H<sub>2</sub>S atmosphere to remove residual oxygen.\nThe direct synthesis from the elements is usually hampered by the\nhighly exothermic reaction vaporizing the sulfur. One way to suppress\nthis exothermic reaction is to use a prereacted metallic precursor.\nIn this work, the well-defined and crystalline intermetallic compounds\nCaAl<sub>2</sub>, SrAl<sub>2</sub>, and EuAl<sub>2</sub> have been\nreacted with elemental sulfur in less than 24 h at 1173 K achieving\nphase pure samples of CaAl<sub>2</sub>S<sub>4</sub>, SrAl<sub>2</sub>S<sub>4</sub>, and EuAl<sub>2</sub>S<sub>4</sub>, according to powder\nX-ray diffraction. While the first two are extremely sensitive to\nmoisture, the latter is air and moisture stable and could therefore\nbe characterized with respect to its magnetic and luminescent properties\nas well as by <sup>151</sup>Eu Mössbauer spectroscopy. All\nmethods clearly confirm the divalent oxidation state of the Eu atoms.\nSince CaAl<sub>2</sub>S<sub>4</sub> and SrAl<sub>2</sub>S<sub>4</sub> are diamagnetic materials, we have investigated these by <sup>27</sup>Al solid state MAS NMR to verify the crystal structure and gain further\ninformation about the local Al environment. Subsequently, three polysiloxane-polysilsesquioxane-based\nmaterials with phenyl, naphthyl, and phenanthrenyl groups were used\nas a water impermeable material to embed powdered EuAl<sub>2</sub>S<sub>4</sub> and investigate the luminescent properties of the resulting\nhybrid material.