Eu2+ luminescence in strontium aluminates†
The luminescence properties of Eu2+ doped strontium aluminates are reported and reviewed for a variety of aluminates, viz. SrAl12O19, SrAl4O7, Sr4Al14O25, SrAl2O4 and Sr3Al2O6. The aim of the research is to investigate the role of local coordination and covalency of the aluminate host lattice, related to the Sr/Al ratio, on the optical properties of the Eu2+ ion. The UV and VUV excited luminescence spectra as well as luminescence decay curves were recorded to characterize the luminescence properties of the investigated aluminates. The emission of Eu2+ ions varies over a wide spectral range, from ultraviolet (UV) to red, for the series of aluminates. The variation in emission color can be related to the crystal-field splitting of the 5d levels and the covalent interaction with the surrounding oxygen anions. In the least covalent material, viz. SrAl12O19:Eu2+, narrow line emission due to the 6P7/2–8S7/2 transition occurs at 4 K, indicating that the 4f65d excited state is situated above the 6P7/2(4f7) excited state around 360 nm. The most alkaline material, viz. Sr3Al2O6:Eu2+ is the most covalent host and exhibits several d–f emission bands in the yellow to red spectral range due to the Eu2+ ions located on different crystallographic Sr2+ sites. The Eu2+ emission spectra in the other aluminates confirm the trend that with increasing Sr/Al ratio the Eu2+ emission shifts to longer wavelengths. Interesting differences are observed for the Eu2+ from different crystallographic sites which cannot always be related with apparent differences in the first oxygen coordination sphere. The discussion gives insight into how in a similar class of materials, strontium aluminates, the emission color of Eu2+ can be tuned over a wide spectral region.