Nanocrystalline SrTiO₃:Eu³⁺ and BaTiO₃:Eu³⁺: fluorescence spectroscopy and optical studies of structural phase transitions

Abstract

The results of optical studies of Eu 3+ ions doped nanocrystalline materials which undergo structural phase transitions: SrTiO 3 :Eu 3+ and BaTiO 3 :Eu 3+ are presented. Nanocrystalline SrTiO 3 and BaTiO 3 samples with crystallite sizes about 25 -50 nm were produced by the sol-gel process. The EuF n fluorescence spectra were studied. Time-resolved fluorescence spectra enabled to separate different kinds of Eu 3+ centers in BaTiO 3 samples. Fluorescence spectroscopy of probe Eu 3+ ions was used to detect the structural O h -D 4h phase transition in nanocrystalline SrTiO 3 . The Eu 3+ excited states lifetimes were studied in a wide temperature range and the effects of modified refractive index of the media on radiative transitions were observed. The study of ferroelectric C 4v -O h phase transition in nanocrystalline BaTiO 3 with optical second harmonic generation technique revealed remarkable hysteresis in the temperature dependence of the second harmonic intensity.

Effect of Eu3+Concentration on the Luminescent Properties of SrTiO3Phosphors Prepared by Pressure-Assisted Combustion Synthesis

“…The substitution of Sr 2+ ions by Eu 3+ can induce a charge decompensation as has been reported [14]; this result can distort the crystalline lattice which could enhance the luminescence intensity of SrTiO 3 :Eu 3+ because the interatomic 4f-4f Eu 3+ forbidden transitions are more probable in lattices with no inversion of symmetry [17]. Thus, the crystalline lattice is distorted by the charge decompensation and by the difference between the atoms sizes of the Eu 3+ (1.087Å) and Sr 2+ (1.44Å); this has been observed in several perovskite systems [14][15][16][22][23][24]. A measure of the distortion of a perovskite-type ABO 3 lattice can be determined by using the Goldsmith tolerance factor ( ) that depends on the ionic radii , , and of the perovskite ABO 3 structure [24] and is given by the following equation:…”

“…The Sr 2+ or Eu 3+ ions are surrounded by eight TiO 6 octahedrons in the SrTiO 3 :Eu 3+ matrix; thus, the TiO 6 anions should be distorted in the perovskite structure SrTiO 3 :Eu 3+ by the doping effect of Eu 3+ ; in consequence, the sites where the Eu 3+ ions are located within the crystal lattice of STO lack symmetry. This is beneficial for our powders because the photoluminescence properties are enhanced [14][15][16]. The imbalance of the crystalline structure can also be observed by a shift of the diffraction peaks in the XRD patterns.…”

“…This phosphor is very efficient for the generation of light with a quantum efficiency of nearly 100% [11], but scientists are focusing energies currently to find new materials which combine magnetic, ferroelectric, low toxicity, and luminescent properties (multifunctional materials) and those additional properties cannot be provided by Y 2 O 3 host. Some previous works have found that SrTiO 3 :Pr 3+ and SrTiO 3 :Eu 3+ phosphors are potential candidates as red phosphors with good paramagnetic properties [12][13][14][15][16]. In particular, studies have shown that SrTiO 3 :Eu 3+ is a good absorber in the long ultraviolet region and it is an oxide material with high chemical stability [14].…”

“…Besides, strontium titanate doped with 2 Advances in Materials Science and Engineering rare earths (REs) is a compound widely studied due to its high dielectric constant (∼300) and ferroelectric properties, which make SrTiO 3 :RE a multifunctional material [14]. Several techniques to produce SrTiO 3 :Eu 3+ powders include the solid state reaction method, synthesis from molten chloride complex salts, and the sol-gel technique [14][15][16]. However, the pressure-assisted combustion synthesis (PACS) technique has not been used to obtain Eu 3+ doped SrTiO 3 to the best of our knowledge and this technique has been used successfully to produce highly luminescent materials by changing their size and morphology [17][18][19].…”

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

“…[6], see [7] for the spectra of our samples) in the temperature range, which includes the C 4v -O h phase transition temperature and where the SHG hysteresis was observed [3]. This SHG hysteresis (which is not related to the presence of impurity ions) is due to ordering in the ensemble of dipole ferroelectric nanoparticles [3].…”