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A structural study of Sm- and Gd-doped ceria was performed with the aim to clarify some unexplained structural features. (Ce1-xREx)O2-x/2 samples (RE ≡ Sm, Gd; x = 0, 0.1, ..., 1) were prepared by coprecipitation of mixed oxalates and subsequent thermal treatment at 1473, 1173, or 1073 K in air; they were then analyzed at room temperature both by synchrotron X-ray diffraction and μ-Raman spectroscopy. Two structural models were adopted to fit the experimental data, namely, a fluoritic one, resembling the CeO2 structure at low RE content, and a hybrid one at higher RE content, intermediate between the CeO2 and the RE2O3 structures. Two main transitions were detected along the compositional range: (a) an RE-dependent transition at the boundary between the fluoritic and the hybrid regions, of a chemical nature; (b) an RE-independent transition within the hybrid region at ∼0.5, having a purely geometrical nature. The presence of two finely interlaced F- and C-based structures within the hybrid region was confirmed, and hints of their composition were obtained by μ-Raman spectroscopy. The obtained results indicate a possible explanation for the non-Vegard behavioral trend of the cell parameters.

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Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO 2 (70 nm) with BaCO 3 of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO 3 in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO 3 already leads to formation of the single-phase BaTiO 3 after calcination for 8 h at 8001C. The final powder consists of primary particles of % 100 nm, has a narrow particle size distribution with d 50 5 270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 10001C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO 3 and TiO 2 represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.

The results of an atomistic simulation study on the incorporation of ions of the first series of transition metals (Cr 3؉ , Cr 4؉ , Fe 2؉ , Fe 3؉ , Co 2؉ , Co 3؉ , Ni 2؉ , and Ni 3؉ ), Y 3؉ , and ions of the lanthanide series (Er 3؉ , Gd 3؉ , Tb 3؉ , Pr 3؉ , Pr 4؉ , and La 3؉ ) in the BaTiO 3 lattice are presented and discussed. The ions of the transition metals prefer to substitute at the titanium site with oxygen-vacancy compensation. For iron and cobalt, oxidation from the divalent to the trivalent state during incorporation is favored. Nickel and chromium are preferentially incorporated in the valence state 2؉ and 3؉, respectively. Formation of stable defect pairs with different types of lattice defects is predicted for the transition-metal impurities. For La 3؉ and Pr 3؉ , substitution occurs at the barium site, whereas Y 3؉ , Tb 3؉ , Gd 3؉ , and Er 3؉ tend to simultaneous substitution on both cation sites. Formation of dopant-titanium-vacancy pairs is predicted for the rare-earth ions and Y 3؉ . The effect of doping on the lattice parameter of c-BaTiO 3 has been studied by a mean-field calculation. Comparison with experimental data confirms the dependence of the preferred substitution site on the ionic radius of the impurity. For dopants with intermediate size (Y 3؉ , Er 3؉ , Tb 3؉ , and Gd 3؉ ), the Ba/Ti ratio is important in the incorporation mechanism.

A systematic kinetic investigation on the chemical synthesis of BaTiO 3 particles from aqueous solutions of BaCl 2 and TiCl 4 at T < 100°C and at pH 14 has been performed. Initially, a viscous suspension of a Ti-rich gel phase is obtained at room temperature. Later, formation of BaTiO 3 is induced by heating above 70°C and the gel phase is gradually converted to the crystalline perovskite. The isothermal formation kinetics of BaTiO 3 and the evolution of crystal size and particle size during the course of reaction are significantly influenced by temperature, concentration, and barium-to-titanium ratio of the solution. The early stages of reaction (yield < 1%) are dominated by primary nucleation, and slow formation of single nanocrystals of BaTiO 3 was observed by HRTEM. At a later stage, formation of polycrystalline particles occurs by secondary nucleation of BaTiO 3 on the surface of already existing crystals. During this stage, the reaction rate increases by 1 order of magnitude. When the yield exceeds 50%, nucleation becomes less important and the reaction is dominated by growth. Final particles have a diameter in the range 0.3-1.6 µm, depending on the processing parameters.

We investigate the second-harmonic generation (SHG) signal from single BaTiO3 nanoparticles of diameters varying from 70 nm down to 22 nm with a far-field optical microscope coupled to an infrared femtosecond laser. An atomic force microscope is first used to localize the individual particles and to accurately determine their sizes. Power and polarization-dependent measurements on the individual nanoparticles reveal a diameter range between 30 and 20 nm, where deviations from bulk nonlinear optical properties occur. For 22 nm diameter particles, the tetragonal crystal structure is not applicable anymore and competing effects due to the surface to volume ratio or crystallographic modifications are taking place. The demonstration of SHG from such small nanoparticles opens up the possibilities of using them as bright coherent biomarkers. Moreover, our work shows that measuring the SHG of individual nanoparticles reveals critical material properties, opening up new possibilities to investigate ferroelectricity at the nanoscale.