Role of valence state of dopant (Eu2+, Eu3+) and growth environment in luminescence and morphology of SrAl12O19 nano- and microcrystals

Incorporation of europium luminescent centre in its chosen valence state Eu2+ or Eu3+ has been effectively done in the complex strontium hexa aluminate SrAl12O19 by high temperature solid state reaction at regulated environment. Nanocrystalline SrAl12O19 doped with europium in its reduced form Eu2+, could be successfully synthesized by controlled autocombustion process from the nitrate salts of precursor components. The charge state of the dopant ion was ascertained from photoluminescence studies which show series of sharp emission lines in 590–710 nm range for Eu3+ activator whereas broad green emission for Eu2+ activator. Nanocrystalline SrAl12O19:Eu showed blue shifted broad green emission characteristic of Eu2+ 4f5d transition demonstrating the effectiveness of autocombustion process in producing Eu2+ charge state which usually requires a reducing environment. Intra-band gap excitation of SrAl12O19 between energy levels of rare earth dopant could be tailored effectively to obtain broad excitation transition in near UV/violet region and emission in the visible region. Striking dependence of morphology on valence state of Eu and growth environment was observed which could be explained due to creation of solid state defects in formation of lamellar structure in oxygen-rich environment.