Eu2+ luminescence in the borates X(2)Z(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 berates to establish a correlation between the energy of the 4f(7)-->4f(6)5d(1) 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.