Optical band gap studies on (55-x)Na2O-xPbO-45P2O5 (SLP) glass system

Sodium lead phosphate (SLP) glasses in the series (55 - x)Na2O-xPbO-45P(2)O(5) with x varying from 55 to 0 were prepared by the single step melt quenching process with Na2CO3 (sodium carbonate),PbO (lead monoxide) and NH4H2PO4 (ammonium dihydrogen phosphate - ADP) as starting materials. The 12 samples prepared were grouped into two batches with batch A having 55 less than or equal to x less than or equal to 30 for increasing Na2O and batch B having 0 less than or equal to x less than or equal to 25 for increasing PbO to understand the roles of Na2O and PbO in phosphate network. The optical absorption spectra of these glasses were recorded in the range 240-300 nm. The fundamental optical absorption edges were observed for all glasses except for Na2O-P2O5 glass with x = 0. From these absorption edge studies the optical band gap (E-opt) and the Urbach energy (DeltaE) values have been evaluated by following the available semi-empirical theories. The E-opt values were found to vary from 4.33 to 4.49 eV for batch A with increasing Na2O content and from 4.55 to 4.40 eV for batch B with increasing PbO content. The E-opt behavior with increasing Na2O was found to be complementary to increasing PbO in the glass network. The role of Na2O is found to increase the E-opt value by causing decrease in the number of bridging oxygens affected. (C) 2000 Elsevier Science B.V. All rights reserved.