Reversible Valence Change of the Europium Ion Doped in Alkaline-earth Tetraborates

Valence state of the europium (Eu) ion doped in MB4O7 (M=Sr and Ba) was reversibly changed between the trivalent and divalent state by repeating the crystallization and vitrification cycle of the host lattices by heating even in air at 973-1273 K and such valence change was discussed on the basis of their structural connection manner for the BO3 and/or BO4 units in the networks of crystalline and vitreous forms.