We investigate materials with globally or locally broken center of inversion, such as piezoelectric, ferroelectric, antiferroelectric, relaxor or incommensurate dielectrics in form of crystals, ceramics, films as well as complex nanostructures. The team focuses on developing preparation methods, by characterization of structural phase transitions by calorimetric, structural and nonlinear optic methods, and by theoretical modeling of their physical properties.
Computer simulations based on Ginzburg-Landau-Devonshire theory was used to study details of a phase-transition between Ising and Bloch type of a domain wall. Such transition was predicted to appear for 180 degree domain wall in the rhombohedral ferroelectric phase of BaTiO3 close to 2GPa compressive stress [V. Stepkova et al., J. Phys.: Condens. Matter 24, 212201 (2012)]. We confirm the hypothesis that the phase transition is of the first-order and predict corresponding divergent increase of the dielectric permittivity in the vicinity of the transition. In case of dense domain-wall pattern it can represent a substantial part of the dielectric response of the material. The results were published in [P. Marton et al., Phase Transitions (2012)].
Dielectric permittivity in dependence on mechanical pressure in the vicinity of the transition from Ising (larger pressure) to Bloch-type of the domain wall.
Bloch-to-Ising phase transition within a rhombohedral 180° domain wall.
LaNiO3 is oxide with perovskite structure and metallic electric conductivity. The agglomerated LaNiO3 nanocrystalline powder was prepared by water-solution sol-gel technique from nitrates with addition of malic acid. (more...)
Mechanochemical activation (intensive ultrafine milling) in a planetary ball micro mill puts deformation energy into components and starts the solid-state reaction. Following calcination (solid-state synthesis at enhanced temperature) is easier, usually only one calcination step is necessary, processing temperature can be reduced and resulting product is more homogeneous. E.g. multiferroic oxides EuTiO3 and (Eu0.5Ba0.5)TiO3 were prepared by this technique. (more...)
Computer simulations based on Ginzburg-Landau-Devonshire theory have been used to investigate piezoelectric properties of tetragonal BaTiO3 crystals. We have shown that piezoelectric response of twinned BaTiO3 increases with increasing density of 90° domain walls. A considerable enhancement of the longitudinal piezoelectric coefficient is predicted for domain sizes below 50 nm. We have also shown that main contribution to the longitudinal piezoelectric coefficient comes from the volume of domains, rather than from the domain wall region [J. Hlinka et al., Nanotechnology 20, 105709 (2009)]. (more...)
Dependence of the effective longitudinal piezoelectric coefficient on domain size calculated from the response of the ideal 2D laminar domain structure to weak uniaxial stresses applied along the [111] crystallographic direction. The inset shows the geometry of the initial 2D laminate domain structure assumed in the simulations.
Nevertheless, the experimentally reported impact of domain wall density is even stronger than in our simulations. Our most recent theoretical investigations [P. Marton et al., Phys. Rev. B 81, 144125 (2010)] suggest that ferroelectric perovskites such as BaTiO3 most likely support also more exotic domain walls, resembling Bloch walls known from ferromagnetism. This kind of domain walls might account for additional enhancement of the electromechanical material response.
Set of mechanically compatible and electrically neutral domain walls in the three ferroelectric phases of BaTiO3. Letter denominating the phase (T, O and R stay for tetragonal, orthorhombic and rhombohedral phase, resp.) is followed by an angle between the polarization in adjacent domains and in some cases also by the domain-wall normal.
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