2, Pohang Accelerator Laboratory, Pohang, , Korea (the Republic of)
Accompanying with doping of Ca, Zr, etc, BaTiO3-based ceramics exhibited excellent dielectric behaviors. Especially, Sn has been found to increase the permittivity by 3 times in comparison with single BaTiO3 and to induce the relaxor-type behavior.[1,2] Recent transmission electron microscopy studies have shown that polar nano region (PNRs) embedded in the matrix is responsible of relaxor behaviors. The lattice constant increases with the doping of Sn, and the PNR region expands due to the stress of the matrix which has a large effect on the ferroelectricity of materials. Through high-resolution scanning transmission electron microscopy and first order calculation, they confirm the unit cells of the PNRs expand by the stress, which indicates that there is more space between Ti atoms and the oxygen octahedra to allow larger off centre displacements. This result shows an extreme improvement in dielectric permittivity. In addition, experimental methods of applying the temperature and electric field to control the stress in the PNR region have been proposed, and simulation has confirmed that these elements increase the piezoelectricity of the relaxor ferroelectric materials.[5,6] However, the relationship between PNR and electric field of the Ba(Ti0.85Sn0.15)O3 (BTS) film was not confirmed experimentally. Therefore, we use a combination of time-resolved micro-X-ray diffraction (TRμXD), which is used to apply biaxial stress to the thin film in real time. As a results, we confirm the relationship between PNR with piezoelectric response through calculation of the correlation length in Ba(Ti0.85Sn0.15)O3 film.
The (111)-oriented Nb:SrTiO3 single crystal substrates with a Nb concentration of 0.5% were used as substrates to grow the epitaxial Ba(Ti0.85Sn0.15)O3 thin films on top. The BaTiO3 film were deposited using pulsed laser deposition (PLD) in an oxygen atmosphere of 10 mTorr. After the deposition, the samples were cooled down in a high vacuum. The dielectric constant was 412, which is twice as high as that of the conventional BTO (about 200). In Pohang Light Source, an X-ray beam with energy of 8 keV was focused to a spot with a size of 11.5 μm onto a circular Pt top electrode with a diameter of 100 μm. With an application of electric field (364 MV/m), (111) reflections of film shifted to the lower qz value from 2.696 to 2.691 Å, which indicates the piezoelectric expansion with piezoelectric coefficient of 6.7 pm/V. We deposited the BTS thin film through PLD for the first time, and experimentally observed the piezoelectric properties as an electric field was applied. The change in the PNR region was confirmed from the results.
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