Fyzikální ústav Akademie věd ČR

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.

(More about this group)

Research Activities

• preparation of novel ceramic materials, e.g. EuTiO₃
• single crystal growth, e.g. piezoelectric KIO₃
• soft chemistry technology, e.g. filling nanoporous materials with ferroelectrics,...
• indentification and thermodynamical and structural characterization of novel phase transitions
• developing crystallographic databases
• investigation of ferroelectric heterostructures
• effective medium theory for dielectric and piezoelectric properties
• theoretical studies of ferroelectric phase transitons
• studying of domain walls and domain structures (link to ferrodomains)

Equipment

• Differential scanning calorimeters Perkin-Elmer DSC 7 and Pyris Diamond -180°C – +725°C
  (contact: P. Vaněk, ext. 2909, room 149)
• Apparatus for second harmonic generation, pulsed Nd:YAG laser, wedge or Maker-fringe methods, temperature range 5-800 K
  (contact: J. Kroupa, ext. 2655, room 53)
• Planetary ball micro mill Fritsch Pulverisette 7
  (contact: P. Vaněk, ext. 2909, room 122)
• Ion sputter coater Bal-Tec SCD 050
  (contact: P. Vaněk, ext. 2909, room 122)
• Muffle furnace Linn High Therm, up to 1200°C
  (contact: P. Vaněk, ext. 2909, room 122)

Some recent results

Piezoelectric response of nanotwinned ferroelectric perovskites

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...)

Fig. 1: 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.

Fig. 2: 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.

Sythesis of LaNiO₃ nanocrystalline powder

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 for solid-state synthesis

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 (Eu_0.5Ba_0.5)TiO₃ were prepared by this technique. (more...)