Site preference of Eu²⁺ dopants in the (Ba,Sr)_13−xAl_22−2xSi_10+2xO₆₆ phosphor and its effect on the luminescence properties: a density functional investigation

First principles density functional calculations were carried out for model compounds Ba₁₂EuAl₂₂Si₁₀O₆₆ and Sr₁₂EuAl₂₂Si₁₀O₆₆ to account for the luminescence properties of Eu²⁺ doped Ba_13−xAl_22−2xSi_10+2xO₆₆ and Eu²⁺ doped (Ba,Sr)_13−xAl_22−2xSi_10+2xO₆₆. Both host lattices have three different crystallographic alkaline earth sites, namely, AE(1), AE(2) and AE(3), at which the dopants Eu²⁺ are located. The fluorescence properties of Eu²⁺ doped Ba_13−xAl_22−2xSi_10+2xO₆₆ are not explained if the dopant ions Eu²⁺ are considered to randomly occupy the three different Ba sites, but are explained if the dopants are considered to preferentially occupy the Ba(3) sites. In contrast, the fluorescence properties of Eu²⁺ doped (Ba,Sr)_13−xAl_22−2xSi_10+2xO₆₆ are explained only when both Sr²⁺ and Eu²⁺ cations are considered to occupy the AE(3) sites, which suggests that the activators are more randomly distributed over the three AE sites in Eu²⁺ doped (Ba,Sr)_13−xAl_22−2xSi_10+2xO₆₆ than in Eu²⁺ doped Ba_13−xAl_22−2xSi_10+2xO₆₆. Calculations evidence the strong tendency for the Ba(3) sites to accommodate defects in Ba_13−xAl_22−2xSi_10+2xO₆₆, namely, Eu dopants, Sr substituents, and Ba vacancies. This arises from the higher malleability of the Ba(3) site compared to the Ba(1) and Ba(2) sites.