Nanocrystalline BaTiO3, doped with Eu3+ and Cr3+: Fluorescence spectroscopy and spontaneous ordering effects

The results of optical studies of Cr3+ and Eu3+ ions doped nanocrystalline ferroelectric BaTiO3 produced by the sol–gel process (particle size 20–40 nm) are reported. The Cr3+ impurity ions 2E–4A2 (R-lines) fluorescence spectra in BaTiO3 revealed significant differences from that reported in literature for the bulk material. At least three types of Cr3+ centers were found in the spectra. The temperature dependence of optical second harmonic generation in nanocrystalline BaTiO3: Eu3+ shows a strong hysteresis in C4v–Oh ferroelectric phase transition region, which was explained by the ordering effects in the system of electric dipole moments of dipole nanocrystals. The temperature dependencies of radiative lifetime of Eu3+ 5D0 excited level reveal some hysteresis too. The possibility of influence of the ordering in the system of BaTiO3:Eu3+ nanocrystals on the effective index of refraction of the medium and thus on the Eu3+ 5D0 radiative lifetime, due to the local field effects is discussed.