Structure and microstructure of Ti-doped gahnite

Zinc aluminate, ZnAl2O4 (gahnite), has been widely known as a mineral and also as a ceramic and catalytic material. It is also suitable host material for transition metal cations and rare earth cations providing luminescence properties. To the best of our knowledge, there is no literature data on studies of Ti-doped gahnite by now. Ti-doped gahnite samples with doping levels of 0, 1.8, 3.8, 5.4 and 11.6 at% Ti in relation to Al were prepared by a sol-gel technique followed by thermal treatement. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), EPR spectroscopy, UV-vis reflectance spectroscopy and photoluminescence (PL) studies. The XRD patterns showed that all prepared powder samples were nanocrystalline, with a cubic spinel-type structure4, space group Fd m. The unit-cell parameter was determined using silicon powder as an internal standard, and refined by the WPPF program. It increased with the increase in Ti-doping level. EPR measurements indicated that titanium was present as Ti4+ ion in the samples, while results of UV-vis reflectance spectroscopy evidenced that Ti4+ was situated on octahedral cation site in the gahnite structure. The crystal structure of Ti-doped gahnite samples was refined by the Rietveld method, simultaneously with the analysis of diffraction line broadening. TEM investigations showed that Ti-doped gahnite samples contained evenly shaped particles of about 20 nm in size, and confirmed that the samples had the spinel-type structure. Ti-doped gahnite samples showed a strong UV absorption and the blue emission under UV excitation. This indicates that polycrystalline gahnite doped with titanium may be a promising luminescent material for application in various optoelectronic devices.