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Artificial superlattices of ferroelectric Bi0.5Na0.5TiO3 and BaTiO3 have been successfully grown on (001) insulator or conductive SrTiO3 substrates by pulsed laser deposition. In these epitaxial layered structures, the BaTiO3 layers were shown to contribute to an improvement of the two dimensional growth of the Bi0.5Na0.5TiO3 layers. The influence of the superlattice period Λ, between 5 and 20 nm, was investigated. We observe an increase in the in-plane tensile strain as Λ is reduced from 20 nm to 10 nm, accompanied by a decrease in the density of dislocations. A concomitant enhancement of the dielectric permittivity was measured, demonstrating the strain tunability of these superlattices. A significant reduction of the dielectric losses is also obtained with decreasing Λ. Furthermore, a minimum value of the coercive field of less than 70 kV/cm, close to that of Bi0.5Na0.5TiO3 bulk ceramics, was reached by decreasing the period. We demonstrate that the dielectric and ferroelectric properties can be explained as the result of the in-plane tensile strain which contributes to improve the structural properties in the superlattices.

International audience; Cubic zirconia single-crystals (yttria-stabilized zirconia (YSZ)) have been irradiated with 4 MeV Au2+ ions in a broad fluence range (namely from 5 × 1012 to 2 × 1016 cm−2) and at five temperatures: 80, 300, 573, 773, and 1073 K. Irradiated samples have been characterized by Rutherford backscattering spectroscopy in channeling mode, X-ray diffraction and transmission electron microscopy techniques in order to determine the disordering kinetics. All experimental results show that, whatever is the irradiation temperature, the damage build-up follows a multi-step process. In addition, the disorder level at high fluence is very similar for all temperatures. Thus, no enhanced dynamic annealing process is observed. On the other hand, transitions in the damage accumulation process occur earlier in fluence with increasing temperature. It is shown that temperature as low as 573 K is sufficient to accelerate the disordering process in ion-irradiated YSZ.ACKNOWLEDGMENTS

International audience; This study reports on the determination of the optical constants of a lead-free ferroelectric material, Na0.5Bi0.5TiO3. The optical transmission measurements were carried out in the 300–3000 nm wavelength range, on a 100-oriented Na0.5Bi0.5TiO3 thin film, epitaxially grown by laser ablation on a 100MgO single crystal substrate. Applying the “method of the envelopes,” developed by Manifacier et al. J. Phys. E 9, 1002 1976 and by Swanepoel J. Phys. E 16, 1214 1983, the analysis of the observed interference fringes allowed extracting some optical data for Na0.5Bi0.5TiO3, namely the linear refractive and extinction indices n and k, the absorption coefficient , and as well the nature direct or indirect transition and value of the optical band gap. It was found that n=2.321 and the transmission data suggested a 3.30 eV indirect transition. Our experimental results are thus in opposition to the few data reported so far for Na0.5Bi0.5TiO3, where a direct transition was evoked. Therefore, we have confronted the optical transmission data to supplementary diffuse reflectance and ellipsometry measurements, and finally, to first principles calculations. The diffuse reflectance data, collected for Na0.5Bi0.5TiO3 powders, indicate a 3.26 eV optical band gap. In addition, the ellipsometry measurements reveal a refractive index of 2.346 at 2.066 m, an energy gap of 3.20 eV, and also confirm the thickness of the layer. These additional data are then in very good agreement with the data derived from the optical transmission measurements. Finally, first principles calculations were carried out in the framework of the density functional theory for the three Na0.5Bi0.5TiO3 polymorphs i.e., rhombohedral, tetragonal, and cubic. For each polymorph, in order to consider the Na/Bi disorder on the perovskite A-site, three different Na/Bi distributions within the unit cell were investigated, leading to nine atomic configurations. The electronic structure of these nine configurations was then calculated after geometry optimization and only one of them did not converge. These calculations complete the above experimental data, as six electronic structures over eight presented an indirect band gap, with a band gap energy Eg falling in the 3.90–4.60 eV energy range. The latter value is higher than the experimental ones but is definitely more compatible with the energy band gap expected for ferroelectric oxide materials.

International audience; This article deals with strain relaxation in SmNiO3 epitaxial films deposited by chemical vapor deposition on SrTiO3 substrates. Thanks to x-ray reciprocal space mapping, we demonstrate that the strain relaxation is driven both “chemically” and “mechanically” by the formation of oxygen vacancies and misfit dislocations, respectively. Besides, a careful interpretation of the resistivity measurements allows us to highlight a correlation between the formation of oxygen vacancies, the stabilization of Ni3+, and the metal-insulator transition in the SmNiO3 films. Furthermore, using coplanar and grazing incidence diffraction, the shape of the strain gradient within the films is retrieved. This latter is calculated using a versatile scattering model involving B-spline functions. Finally, particular planar faults Ruddlesden–Popper faults that give rise to extended diffuse scattering on transverse scans are analyzed using a recent phenomenological model

Thin films of (00l) oriented SrBi2Nb2O9 epitaxially grown on SrTiO3 by sol-gel spin coating have been studied by means of high-resolution x-ray diffraction reciprocal space mapping. It is shown that these materials contain highly localized heterogeneous strain fields due to imperfect stacking faults (i.e., faults that do not propagate throughout the crystallites building up the film). In the film plane, the strain fields are confined to 11 nm wide regions and characterized by a vertical displacement of 0.18c (where c is the cell parameter) showing that the stacking faults are mainly composed of one additional (or missing) perovskite layer. Prolonged thermal annealing at 700 °C strongly reduces the density of stacking faults and yields a more uniform strain distribution within the film volume without inducing significant grain growth.