Optical, FTIR and ESR Spectral Investigations of Tungsten Ions in Barium Phosphate Host Glass and Effects of Gamma Irradiation

Abstract

Tungsten oxide (0.25 – 3% WO₃) doped barium phosphate glasses were prepared by the conventional melting annealing procedure. Spectroscopic FTIR infrared and ESR measurements were applied at 0 and 10 Mrad doses of gamma irradiations. The UV-Visible spectrum of the base glass consists of distinct ultraviolet peaks that can be correlated with trace ferric (Fe³⁺ ions) impurities. UV-Visible absorption spectra of WO₃ – doped glasses show similar ultraviolet absorption as the base glass and a very broad visible band settled at about 720 nm and with the high content sample (3% WO₃) two peaks are identified at 602 and 790 nm on the very broad visible absorption band. These additional broad band accompanied by the bluish color to the samples support the assumption of the formation or presence of pentavalent tungsten ions because of the reducing nature of the phosphate host glass. Gamma irradiation produces some extension of the UV absorption of the undoped glass spectrum accompanied by the generation of an induced visible broad band centered at 520 nm. These extra induced changes are related to some photochemical reactions leading to the partial oxidation of some ferrous ions to ferric ions through the radiation effect. The induced broad visible band is assumed to be related to the formation of phosphorus oxygen hole centers (POHC) or nonbridging oxygen hole centers (NBOHC) from interaction of gamma rays on the phosphate network. Gamma irradiation of WO₃ doped glasses reveals the increase of the intensity of the very broad band and in the high content sample (3% WO₃), a new induced band is generated at about 411 nm. These changes are assumed to be related to some photochemical reactions involving the conversion of some W⁶⁺ ions to W⁵⁺ ions by capturing electrons or by photo-reduction of W⁶⁺ ions to W⁴⁺ ions and thus imparting the additional peak at 411 nm. FTIR results of the studied glasses reveal the appearance of vibrational modes due to phosphate network groups mainly of metaphosphate units or P–O–P linkages as the response of the chemical composition 50% BaO – 50% P₂O₅. The infrared results of the radiative samples have no notable changes due to compactness of the phosphate with noticeable amount of heavy Ba²⁺ ions which seem to retard the passage of liberated electrons or positive holes during the irradiation process and thus the network structure remains unaffected. ESR spectra of WO₃ – doped glasses reveal distinct signals due to the unpaired pentavalent ions which are stable towards irradiation due to the existence of a high content of the heavy Ba²⁺ ions which block the passage of negative and positive holes during irradiation.