Optical absorption and photoluminescence properties of anadium ions in ‘lithium-tungsten-borate’ oxide glasses

Abstract

40Li2O–5WO3–(55–x) B2O3: xV2O5 glasses for x = 0.0, 0.2, 0.4, 0.6 and 0.8 (all in mol %) were prepared by melt-quenching method. The glasses were characterized by X-ray diffraction spectra, optical absorption spectra and photoluminescence spectra. The XRD patterns confirmed the amorphous nature of the glass samples. The optical absorption spectra of the glasses have exhibited two broad absorption bands of VO2+ ions with meta-centers around 630 nm and 990 nm corresponding to 2B2g→2B1g and 2B2g→2Eg transitions respectively. From the observed absorption edges, we have evaluated the optical band gap (Eo) of the glasses by drawing Tauc's plots. The photoluminescence spectra (excited at 639 nm) of the glasses have exhibited a broad emission band with meta-center at around 800 nm. The interpretation of results shows that as the content of V2O5 increases up to 0.6 mol % in the glass system, the optical band gap (Eo) decreases and the intensity of the emission peak increases with red shift due to gradual conversion of vanadium ions from V5+ state to V4+ state. Thus, these investigations point out that the vanadium ions coexist in V4+ state with V5+ state. More precisely, when x ≤ 0.6 mol %, the V4+ ions are expected to be dominant in the glass system which act as network modifiers; whereas when x = 0.8 mol %, the V5+ ions seem to prevail, acting as network formers with VO5 structural units in the glass matrix.