Effect of Na-, K-, Mg-, and Ga dopants in A/B-sites on the optical band gap and photoluminescence behavior of [Ba0.5Sr0.5]TiO3 powders

In this work, the role of the representative metal dopants (Na, K, Mg and Ga) in A/B-sites of [Ba0.5Sr0.5]TiO3 powders (in short BST) synthesized by sol–gel method have been investigated. As revealed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–visible spectroscopy, Na and K can be occupied into A-site, while Mg and Ga can be substituted on B-site of BST powders. It was found that the optical band gap energies of modified B-site are higher than modified A-site of BST powders. The possible mechanisms of intermediate energy levels between optical band gaps were suggested by photoluminescence (PL) behavior. The four major optical emissions in visible range were found to be 2.95, 2.80, 2.55, and 2.33 eV. The 2.95 eV in violet PL emissions is related to the electron transfer in octahedral [TiO6] clusters. Moreover, this energy level is attributed to the charge compensation process due to acceptor substitution defects in order to preserve the overall charge neutrality in the BST crystal. The 2.85 eV in blue PL emissions is attributed to the fully-ionized oxygen vacancy (VO••) in BST powders. The 2.55 and 2.33 eV in green PL emission are assigned to the charge-transfer of singly-ionized oxygen vacancy (VO•) and the charge transfer vibronic excitons (CTVE) in BST perovskite, respectively. In additional, as revealed by electron paramagnetic resonance (EPR), modified B-site of BST powders facilitated titanium vacancy more than modified A-site. The X-ray photoelectron spectroscopy (XPS) results indicated that modified B-site of BST powders were easily created oxygen deficiency than modified A-site. However, the oxygen deficiency significantly affected on A-site of the Sr atoms site more than A-site of the Ba atoms, it might be a result of the strength of chemical bond of Sr–O bonds which is lower than Ba–O bonds.