No abstract

Artificial superlattices (SLs) consisting of ferroelectric BaTiO3 (BT) and paraelectric BaZrO3 (BZ) have been growth by a pulsed laser deposition technique. The epitaxial BT and BZ layers with a periodicity from 16 Å to 1056 Å were sequentially deposited on (001) MgO substrate buffered with an oxide conducting layer of La1/2Sr1/2CoO3. The out-of-plane lattice parameters of the SLs constituents were determined by modeling of the x-ray diffractograms. The results indicate that the polar c-axis of the BT layers lies in the plane of the substrate and BZ layers exhibit enhanced tetragonal distortion which is induced by the mismatch between the alternating BZ and BT layers. The Raman data reinforces this interpretation and suggest a monoclinic phase in BT layers and polar phase in BZ layers. The Raman spectra give evidence of coupling between the constituent layers and a narrowing of the Raman peaks is attributed to a reduction in the disorder of the Ti4+ ions due to the epitaxial strain. This strain is responsible for the upward frequency shift in the soft modes, especially, the E(1TO) mode, which is markedly altered with respect to its analogs in BT-bulk crystal and BT thin film. Ferroelectric hysteresis loops measurements, confirmed the x-ray diffraction and Raman results, and revealed a clear ferroelectric behavior for the constrained SLs (32 Å≤Λ≤256 Å) while a weak ferroelectricity is observed for the large periods (Λ=500 Å and 1056 Å). This is attributed to the strain-induced ferroelectricity in BZ layers for the constrained SLs.

Metallic nanoparticles dispersed in a cholesteric liquid crystal can order in accordance with the helical structure of the chiral phase. Since the liquid crystals we used have a glassy state, the nanostructures may be examined by transmission electron microscopy.The platinum nanoparticles form periodic ribbons which mimic the well-known fingerprint cholesteric texture. The particles do not decorate the pristine texture but create a novel structure with a larger periodicity. The distance between the ribbons is directly correlated to the helical pitch which therefore becomes a simple control parameter to tune the structuring of nanoparticles. Investigations of cross-sections show how the particles are arranged in the volume; a selective segregation proceeds at the periphery of the film and the particle ordering is localized close to the film-air interface. On the fingerprint patterning of nanoparticles, we do an analogy with the positive staining of polymer films with heavy-metal-containing compounds for transmission electron microscopy investigations and we discuss the accumulation of particles in the sites with the highest energy of director distortions.

We used pulsed laser deposition to grow a series of ͓BaTiO 3 ͔ ⌳/2 / ͓BaTi 0.68 Zr 0.32 O 3 ͔ ⌳/2 ͑BTZ/BT͒ superlattices ͑SL's͒ with a modulation period ⌳ that varies between 16 Å ഛ⌳ഛ1008 Å; the total thickness is kept constant at about 4000 Å. We determine the out-of-plane lattice parameters of the SL constituents by modeling the x-ray diffractograms of the SL's. The results indicate that the polar c axis of the BT layers lies in the plane of the film. The Raman data reinforces this interpretation. The Raman spectra of SL's give evidence of coupling between BT and BTZ layers and a narrowing of the Raman peaks suggest a reduction of the disorder of the Ti 4+ ions due to the strain. This strain resulting from the lattice mismatch between the constituent layers is responsible for the upward frequency shift of the soft modes, especially the E͑1TO͒ mode, which is markedly altered with respect to its analogs in BT-bulk crystals and BT film. This soft-mode behavior as a function of ⌳ indicates that the crystal structure of all SL's is more rigid than in BTZ and BT single thin films.