Eu²⁺ Luminescence in the Borates X₂Z(BO₃)₂ (X = Ba, Sr; Z = Mg, Ca)

Luminescence properties are reported for the Eu 2+ -doped materials Ba 2 Mg(BO 3 ) 2, Ba 2 Ca(BO 3 ) 2, and Sr 2 Mg(BO 3 ) 2; they exhibit red, green, and yellow emission, respectively. Results from these materials have been coupled with those from another extensive series of Eu 2+ -doped borates to establish a correlation between the energy of the 4f 7 → 4f 6 5d 1 transition of Eu 2+ and the coordination environments of the O atoms in these hosts. The transition occurs at low energies in those borates containing O atoms that are highly coordinated by regular arrangements of Ba or Sr atoms. Covalency effects are primarily responsible for the low-energy position of the excitation.