Rapid Synthesis of a Green Emitting Phosphor by Sulfidation of Intermetallic EuAl₂ and its Use in a Hybrid Material

Ternary sulfides are usually synthesized from the binary sulfidic precursors at elevated temperatures using prolonged reaction times and sometimes H2S atmosphere to remove residual oxygen. The direct synthesis from the elements is usually hampered by the highly exothermic reaction vaporizing the sulfur. One way to suppress this exothermic reaction is to use a prereacted metallic precursor. In this work, the well-defined and crystalline intermetallic compounds CaAl2, SrAl2, and EuAl2 have been reacted with elemental sulfur in less than 24 h at 1173 K achieving phase pure samples of CaAl2S4, SrAl2S4, and EuAl2S4, according to powder X-ray diffraction. While the first two are extremely sensitive to moisture, the latter is air and moisture stable and could therefore be characterized with respect to its magnetic and luminescent properties as well as by 151Eu Mössbauer spectroscopy. All methods clearly confirm the divalent oxidation state of the Eu atoms. Since CaAl2S4 and SrAl2S4 are diamagnetic materials, we have investigated these by 27Al solid state MAS NMR to verify the crystal structure and gain further information about the local Al environment. Subsequently, three polysiloxane-polysilsesquioxane-based materials with phenyl, naphthyl, and phenanthrenyl groups were used as a water impermeable material to embed powdered EuAl2S4 and investigate the luminescent properties of the resulting hybrid material.