Second harmonic generation and Yb3+ cooperative emission used as structural probes in size-driven cubic-tetragonal phase transition in BaTiO3 sol-gel nanocrystals

Nanocrystalline powders of 1% Yb3+-doped BaTiO3 sintered at different temperatures were obtained by the sol-gel method. The structure of the powders covering crystallite sizes from 25 to 60 nm depending on the annealing temperature (700-1200 °C), was determined from the Scherrer's equation. Second harmonic generation (SHG) intensity magnitude is compared in each samples of the tetragonal BaTiO3 whereas the isotropic cubic phase is detected by the absence of the SHG signal. Cooperative emission, indicating the Yb3+ ions pairs, was easily detected in the cubic phase and not seen in the ferroelectric tetragonal structure.