Hexagonal BaTiO_3:Eu~(3+)Nanoparticles: A Kinetically Stable Phase Prepared at Low Temperatures

摘要

Nanoparticles of Eu~(3+)-doped BaTiO_3 having a hexagonal structure, which is a thermodynamically stable phase above 1432℃, have been prepared at a low temperature of 110℃ based on the coprecipitation of Ti~(4+), Ba~(2+), and Eu~(3+) in ethylene glycol medium by adding urea. Based on differential thermal analysis studies, it is inferred that the as-prepared hexagonal phase of BaTiO_3:Eu~(3+) is unstable and decomposes to the anatase form of TiO_2 and barium carbonate above 420℃. Luminescence properties of the hexagonal form of BaTiO_3:Eu~(3+) nanoparticles have been compared with cubic BaTiO_3:Eu~(3+) nanoparticles prepared by the combustion method. Based on the detailed luminescence studies of Eu~(3+) in both cubic and hexagonal form of BaTiO_3 samples, it is inferred that Eu~(3+) ions mainly occupy the Ti~(4+) sites in hexagonal BaTiO_3 and both Ba~(2+) and Ti~(4+) sites in cubic BaTiO_3. A comparison of the luminescence properties of Eu~(3+) in hexagonal and cubic form of BaTiO_3 suggested that there is increased distortion around Eu~(3+) in hexagonal BaTiO_3 phase compared with cubic BaTiO_3 phase, and this has been explained in terms of the distorted environment created around larger ionic radii Eu~(3+) ions occupying the smaller ionic radii Ti~(4+) site. There is a significant difference in the decay constant for ~5D_0 level of Eu~(3+) for these two forms of BaTiO_3:Eu~(3+) nanoparticles.