Let us pass from crystals to nanoparticles. "Barium titanate nanoparticles were synthesized via a low temperature sol-gel technique using acetylacetone as a chelating agent. The X-ray diffraction pattern confirmed the formation of tetragonal perovskite structure of the material. The average crystallite size was found to be 49 nm … Raman spectroscopic features typical for the tetragonal phase were identified for barium titanate powder. … Barium titanate displays polychromatic luminescent peaks in UV-visible regions, and it was attributed to the near-band-edge transition and defect states in the optical band gap of the perovskite structure" (Sherlin .