Microstructures and photoluminescence of barium titanate nanocrystals synthesized by the hydrothermal process

Nanocrystalline barium titanate was synthesized by the hydrothermal technique at low temperature and atmospheric pressure. The lattice vibration, phase structures, microstructures and luminescence spectra were examined by X-ray diffraction and Raman spectroscopy. Grain sizes of 28-48 nm were obtained from the (2 0 0) diffraction peak via the Scherrer equation. A critical size of 48 nm was found, below which the ferroelectric transition vanishes. The c/a ratio equals 1.003, much smaller than 1.01 for a single crystal, showing a weakening of ferroelectricity. A strong photoluminescence (PL) spectrum centered at 696 nm was observed in nanocrystal barium titanate. Both the annealing-temperature and annealing-time dependence of the luminescence spectrum were investigated, the origin for the PL being attributed to charge transfer via intrinsic defects inside the oxygen octahedron. An optimum synthesizing temperature in the hydrothermal technique is found at 80 °C, at which the as-prepared nanocrystal barium titanate shows an excellent lattice structure and the strongest PL at room temperature.