Nanocrystalline BaTiO 3, doped with Eu 3+ and Cr 3+: Fluorescence spectroscopy and spontaneous ordering effects

The results of optical studies of Cr ³⁺ and Eu ³⁺ ions doped nanocrystalline ferroelectric BaTiO ₃ produced by the sol-gel process (particle size 20-40 nm) are reported. The Cr ³⁺ impurity ions ²E- ⁴A ₂ (R-lines) fluorescence spectra in BaTiO ₃ revealed significant differences from that reported in literature for the bulk material. At least three types of Cr ³⁺ centers were found in the spectra. The temperature dependence of optical second harmonic generation in nanocrystalline BaTiO ₃: Eu ³⁺ shows a strong hysteresis in C _4v-O _h 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 Eu ^{3+ 5}D ₀ excited level reveal some hysteresis too. The possibility of influence of the ordering in the system of BaTiO ₃:Eu ³⁺ nanocrystals on the effective index of refraction of the medium and thus on the Eu ^{3+ 5}D ₀ radiative lifetime, due to the local field effects is discussed.