Titanium-doped sample
Titanium ions can exist in glass as trivalent and tetravalent ions [31,32].
Ti3+ ions belong to the 3d1 configuration and thus exhibit a single absorption band at 440–540 nm and sometimes show a
shoulder at 650–750 nm because of Jahn–Teller distortion [32,33].
The tetravalent titanium (Ti4+) ions belong to d0 configuration and as such will give no visible bands but exhibit ultraviolet
absorption.
The observed optical result reveals the same spectrum as the undoped sample which indicates the absence of trivalent titanium ions and the presence of an extra UV absorption due to Ti4+ ions is not clearly identified because of the presence of strong UV-near visible absorption due to combined presence of strong absorption of both Fe3+impurities and absorption due to Bi3+ ions.
The shielding behavior upon irradiation and the stability of UVnear visible spectrum is related to the presence of high content of heavy metal Bi3+ ions and the resolution of two new visible bands at 560 and 620 nm due to Ti3+ in distorted octahedral coordination which are generated by photochemical reduction of Ti4+ by accepting librated electrons during the irradiation process.