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

Luminescence properties are reported for the Eu2+-doped materials Ba2Mg(BO3)2, Ba2Ca(BO3)2, and Sr2Mg(BO3)2; they exhibit red, green, and yellow emission, respectively. Results from these materials have been coupled with those from another extensive series of Eu2+-doped borates to establish a correlation between the energy of the 4f7 → 4f65d1 transition of Eu2+ 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.