The relationships between crystal structure of alkaline earth metal hexagonal aluminate and 4f–5d transitions of Ce3+ and Tb3+ ions

The Mg–Sr–Ba hexagonal aluminates (MgSrBa)Al12O19 doped with either Ce3+ or Tb3+ ion have been synthesized by solid-state reaction. Crystal structure variations with the increasing Ba2+ concentration have been thoroughly studied. When the concentration of Ba2+ reaches 0.39, the crystal structure becomes the ideal magnetoplumbite-type (No. PDF #80-1195), however, it becomes the same structure as BaMgAl10O17 with space group P63/mmc when Sr2+ is completely replaced by Ba2+ ions. In Ce3+ doped compounds, the emission peak of the Ce3+ ion is gradually red-shifted when the concentration of Mg2+ ion increases, while the 4f–5d transition of Ce3+ at 260 nm splits into five components. In the Tb3+ doped compounds, the spin-allowed and spin-forbidden 4f–5d transitions of Tb3+ have been observed. The intensity of the spin-forbidden 4f–5d transition is gradually increased with decreasing Sr2+ concentration and becomes dominant when Sr2+ is completely replaced by Ba2+. These experimental observations are also discussed in the context and match well with the theoretical calculations of 4f–5d transitions of Tb3+.