Abstract
Perovskite (ABO3) structures offer several advantages, including compositional flexibility, non-stoichiometry of the cations and anions, and the distortion of cation-anion polyhedra. This implies that this family of materials has wide-ranging applications. ABO3 are prospective hosts for the development of novel phosphors. Hence, there is a substantial interest in developing perovskites with intentional rare-earth (RE) ions doping to form multifunctional materials (i.e., having both luminescent and piezo/ferroicbehavior). The optical characteristics of a doped material depend on the site wherein the dopant resides. In mixed A-site perovskites, the dopant has several possibilities. This, in turn, correlates with the emission spectrum of the doped material. Here, sodium bismuth titanate (Na0.5Bi0.5TiO3-NBT) is selected as a host due to its ability to accommodate Eu over large concentration windows while maintaining optical activity. RE ions (Eu, Gd, Dy) are chosen to be the dopants.
In RE doped NBT structures, the dopant chose Ti-site besides Bi-site after some concentration. This behavioris known as “amphotericity”, where simultaneous dopant substitution occurs in two competing sites. RE occupancy in Bi and Ti-sites are quantitatively estimated using Neutron diffraction studies and Raman spectroscopy. Here, we derive a tolerance factor equation (tR) for the rhombohedral ABO3 perovskites similar to the Goldschmidt tolerance factor(tG) for the cubic ABO3 perovskites. We have also predicted the amphotericity domains (i.e., range of dopants that show this amphoteric behavior) in NBT.Further, the ground-state properties of NBT are studied using first-principles calculations based on density functional theory (DFT). Substitution of RE dopants induces crystal structure changes in NBT. So, strain-induced electronic and optical properties of NBT is studied. The results shown here are promising for luminescence-based applications, light-harvesting, charge separation, and charge storage applications.