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⁺.