Optical properties of Eu3+-doped TiO2 nanocrystalline under high pressure

The sample of Eu3+-doped TiO2 (Eu3+/TiO2) nanocrystals is synthesized by the sol–gel method with hydrothermal treatment. Raman and photoluminescence (PL) spectra of the sample have been investigated under high pressure up to 18 GPa using a diamond anvil cell. At ambient pressure, a strong PL signal associated with the 5D0→7FJ (J=0,1,2,3,4) transition of Eu3+ under the excitation wavelength at 514.5 nm is observed. The emission peaks red shift slightly and the intensity decreases with the increase of pressure, while the intensity ratio between 5D0→7F2 and 5D0→7F1 decreases dramatically. A phase transition is observed at 8.2–10.6 GPa by the Raman spectra analysis. Reducing of the intensity ratio between 5D0→7F2 and 5D0→7F1 value indicates the rising of the site symmetry and the weakening of the covalency of Eu–O bond.