RESUMEN
Ceramic powders of Sr1-1.5xYbxTiO3 (x = 0.0, 0.0125, 0.025, 0.05 and 0.075) solid solutions were synthesized by the polymeric complex method. The crystal structure, microstructure and optical properties of the powders annealed at 800 °C for 1 h were investigated by X-ray diffraction, scanning electron microscopy, and diffuse reflectance and photoluminescence spectroscopy, respectively. All the solid solutions exhibit a cubic perovskite-like structure. The reflectance spectra show a broadband below 400 nm ascribed to the ligand-to-metal charge transfer (LMCT) O2- â Ti4+ fundamental state. The Yb3+ (λem = 980 nm) excitation spectra show a broadband being also compatible with the LMCT O2- â Ti4+ state, indicating the energy transfer from the host to the Yb3+. The sample with x = 0.025 presents the highest emission intensity upon near UV excitation, which is further enhanced when the powder is treated under an oxygen-rich atmosphere. The luminescence quenching of Yb3+ is explained as due to defects associated with O2- and Sr2+ vacancies. Finally, it is shown that the solid solutions may downshift photons from UV to wavelengths where a crystalline-silicon photovoltaic solar cell has a higher spectral responsivity.
RESUMEN
Lead-free ferroelectric ceramics based on the Na(0.5)Bi(0.5)TiO3 system, in solid solution with Pr(3+) and La(3+), are synthesized by the Pechini method at low temperature. The powders calcined at 700 °C are found to be below 200 nm in mean crystal size. The dielectric properties reveal a diffuse phase transition. In Na(0.5)Bi(0.5)TiO3 ceramics the rhombohedral to tetragonal transition temperature (T(R-T)) appears near 190 °C, and the Curie temperature (T(C)) at 280 °C. These transitions are shifted when Pr(3+) and La(3+) cations are introduced into the lattice. The ferroelectric properties are also affected, causing the remnant polarization and coercive field to diminish with the increase in La(3+) content. In addition, the piezoelectric properties decrease in the same way with La(3+) concentration. Finally, the red luminescence of Pr(3+) is improved when lanthanum concentration is increased.