Effect of secondary heat treatment on the spectroscopic properties of Na2O-Nb2O5-P2O5 glasses

We have studied the optical absorption and luminescence spectra of 45Na(2)O center dot xNb(2)O(5) center dot (55 - x)P2O5 glasses containing 5, 10, 20, 25, 30, and 35 mol % Nb2O5. The results indicate that the absorption band around 26000 cm(-1), responsible for the yellow color of the glasses, is due to the [Nb(5+)--O-] center and disappears upon secondary heat treatment. Heat treatment of europium-doped glasses increases the concentration of Eu3+ centers in an asymmetric environment, which is accompanied by an increase in luminescence efficiency. The reason for this is that the Eu3+ ions are located outside the niobate subsystem of the glass matrix. The europium in the glasses studied acts as a protector ion.