We have performed x-ray diffraction and Raman spectroscopy measurements in the temperature range of 300–873 K on a single phase epitaxially oriented BaTiO3 thin film grown by pulsed laser deposition on a single crystal MgO substrate. The θ–2θ room temperature diffraction measurements and asymmetric rocking curves indicate that the film is very weakly tetragonal with the c-axis parallel to the plane of the film. X-ray diffraction measurements up to high temperature reveal only a change in slope in the perpendicular to the plane lattice parameter around 450 K (in bulk Tc=395 K) indicating that a diffuse-like of phase transition is taking place. Room temperature polarized Raman spectra show that the film is indeed tetragonal with C4v symmetry and with the a-axis perpendicular to the film plane. Monitoring of the overdamped soft mode and the 308 cm−1 mode confirms that the phase transition is taking place over a wide temperature range according to the x-ray results. The increase of the phase transition temperature is attributed to the stress effect induced by the substrate.
We have used laser ablation to grow series of PbTiO 3 /BaTiO 3 ͑PTO/BTO͒ multilayers with a modulation wavelength ⌳ that varies between 50 ÅϽ⌳Ͻ360 Å. Modeling the x-ray-diffraction patterns of the multilayers indicates that the PTO layers are a-axis oriented. The Raman measurements reinforce this interpretation. The soft-mode Raman line shifts abruptly in frequency above ⌳ϭ240 Å due to possible strain relaxation of the multilayer, and the ⌳ dependent position of an asymmetric line in the vicinity of 200 cm Ϫ1 is modeled as a confined mode, a signature of a modulated structure.
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The structural evolution of epitaxial monodomain (only 180° domains) ferroelectric PbTiO3 thin film has been investigated, using high-resolution, temperature-dependent, x-ray diffraction. The full set of lattice parameters was obtained from room temperature up to 850K. It allowed the calculation of the different strains stored in the film at room temperature, underlying the difference between the mechanical strain and the misfit strain. The evolution of the misfit strain as a function of temperature was also calculated and was found to be consistent with the theoretical temperature-misfit strain phase diagram. These data strongly suggest that the film remains ferroelectric and tetragonal up to 940K.
The structural evolution of epitaxial mono-oriented (i.e. with the c-axis perpendicular to the interface) ferroelectric Pb(Zr 0.2 ,Ti 0.8 )O 3 thin film has been investigated, using high-resolution, temperature dependent, X-ray diffraction. The full set of lattice parameters was obtained, it allowed to estimate the variation of the polarization as a function of temperature, underlying the difference between the polarization-induced tetragonality and the elastic one. The temperature evolution of the misfit strain has been calculated and found to be in good agreement with the theoretical temperature-misfit strain phase diagram.
We report a temperature-dependent high-resolution x-ray diffraction investigation of 200-nm epitaxial BiFeO 3 thin films grown on ͑001͒ SrTiO 3 . We find that BiFeO 3 undergoes two high-temperature transitions: a first-order ␣- phase transition between 745 and 780°C and a more diffuse transition toward the ␥ phase at 860°C. Reciprocal space maps reveal that thin films remain monoclinic crossing the ␣- phase transition. Linear extrapolation of the in-plane lattice parameters to higher temperatures appears to rule out cubic symmetry for the ␥ phase.
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