Luminescent dynamics of magnesium tantalite

Text in English

Solar, bio-imaging, toy decoration, safety, and road marking are just a few of the many uses for Persistent luminescent (PersL) materials. By providing PersL materials that do not require external sources, these materials are intended to address the growing demand for energy-saving. In this work, Magnesium Tantalite (MgTa2O6), MgTa2O6: 1 mol% Cr3+, x mol % Al3+ (x =2, 4, 6, 8, 10, and 12) and MgTa2O6: 1 mol% Cr3+, x mol Si4+ (x = 2, 4, 6, 8, 10 and 12) compounds were prepared with the intention of developing suitable materials for persistent luminescence, which, by increasing the number of electron trapping centers, may be tuned. After milling the combined precursors for an hour, MgTa2O6 was synthesized using a traditional solid-state chemical reaction method at 800 ⁰C for 8 hours. This moderate temperature processing thus allows synthesis with low energy consumption and a more environmentally friendly method compared to the typical high temperature oxide synthesis methods. X-ray diffraction (XRD) confirmed the presence of the MgTa2O6 compound. The material exhibits a strong absorption band around 283 nm (4.34 eV), which corresponds to band-to-band electron transitions as observed from the diffuse reflectance spectrum. The compound showed different types of electron trapping centres, and the depth of the electron trapping centres was approximated as 0.75 ± 0.04 eV, 1.28 ± 0.07, and 1.55 ± 0.02 eV, respectively, for peaks (i), (ii), and (iii). By XRD, MgTa2O6 co-doped with Cr3+ (1 mol% %) and Al3+ (6 mol% %) ions was confirmed to be a single phase of the material without impurities. Incorporating Cr3+ and Al3+ into the host reduced the optical energy band gap of the material from 4.34 to 4.31 eV. The results showed that adding Al3+ can improve the photon absorption of the Al3+ co-doped MgTa2O6: 1mo% Cr3+, enhancing its emission intensity. Moreover, a traditional solid-state chemical reaction was used to synthesize Magnesium Tantalite (MgTa2O6), at different preparation temperatures of 600 ⁰C ≤ x ≤ 1600 ⁰C for 8 hours, after ball milling the mixed precursors for 1 hour. The MgTa2O6 compound’s phase was confirmed using XRD. The optical band gap of different materials, which was estimated from diffuse reflection measurements, showed variations due to the different sample preparation temperatures, from 4.42 ± 0.09 eV to 4.28 ± 0.04 eV as the preparation temperature was increased. The photoluminescence (PL) spectra showed a red shift in the emission bands from 571 nm to 595 nm on increasing the annealing temperature. The thermoluminescence (TL) glow curves were also significantly affected by the preparation temperature, which led to the complete fading of the high-temperature signal of the sample prepared at 1600 ◦C. A conventional solid-state chemical reaction method was used to prepare a composite MgTa2O6/Mg4Ta2O9:1 mol% Cr3+ phosphor. The PL Spectra displayed a broad emission between 700 and 850 nm when the samples were excited with a 320 nm beam, with the maximum at 800 nm. Finally, the first and second-order exponential decay equations were used to fit the phosphorescence decay curves, which showed improved lifetimes in milliseconds. In this thesis, we attempt to unravel some of the remaining mysteries of MgTa2O6, in particular the features of the trap system and electron trapping process. The results obtained in this thesis demonstrate that MgTa2OMgTa2O6 is a promising material for the future development of PersL applications, according to experiments conducted in this study.

D. Sc. (Physics)

College of Engineering, Science and Technology