Petr Ondrejkovic

The piezoelectric response of nanotwinned BaTiO3.

The piezoelectric properties of tetragonal BaTiO(3) crystals with a very high density of 90 degrees twin domain boundaries are analyzed in the framework of the Ginzburg-Landau-Devonshire theory. Computer simulations predict a considerable enhancement of piezoelectric coefficients for domain thicknesses below 50 nm. This enhancement is much larger than the effect of the domain wall broadening mechanism of Rao and Wang (2007 Appl. Phys. Lett. 90 041915), but it is still a too weak effect to explain the domain density enhancement observed in the experiments of Wada and Tsurumi (2004 Br. Ceram. Trans. 103 93). The phenomenon observed here should nevertheless manifest in materials with nanoscopic domains, such as relaxor ferroelectrics and artificial ferroelectric nanostructures.

Anisotropic elasticity of DyScO3 substrates

The full elastic tensor of orthorhombic dysprosium scandate (DyScO(3)) at room temperature was determined by resonant ultrasound spectroscopy (RUS). Measurements were performed on three 500 μm thick substrates with orientations (110), (100) and (001) in the Pbnm (a < b < c) setting. For this purpose, a modification of the RUS method was developed, enabling simultaneous processing of the resonant spectra of several platelet-shaped samples with different crystallographic orientations. The obtained results are compared with ab initio calculations and with elastic constants of other rare-earth scandates, and are used for discussion of the in-plane elasticity of the (110)-oriented substrate.

Phase–field modelling of 180° “Bloch walls” in rhombohedral BaTiO3

This article describes investigations of 180° walls in rhombohedral BaTiO3 in the framework of generalized Ginzburg–Landau–Devonshire model, using phase-field simulation approach. It is demonstrated that the 180° wall with the normal parallel to [1−10] direction has a Bloch-like character. Its domain profile can be understood as a pair of very close 71° and 109° walls, which can be continuously separated by a strong transverse electric field. The intermediate region can be considered as a few nanometers thick monoclinic domain.

An x-ray scattering study of Sn2P2S6: absence of incommensurate phase up to 1 GPa

The ferroelectric phase transition in a semiconductor Sn(2)P(2)S(6) single crystal has been studied by means of high-resolution synchrotron x-ray diffraction in the pressure-temperature range where an incommensurate modulated phase has been anticipated for many years. In contrast with the predictions, the present measurements reveal only a direct ferroelectric-paraelectric phase transition close to T = 100 K, p = 1.1 GPa. In the vicinity of this phase transition, a characteristic critical diffuse scattering was observed, but no satellite peaks could be resolved there. It is concluded that the earlier hypothesis about the presence of an incommensurate phase and associated Lifshitz point in the temperature-pressure phase diagram of Sn(2)P(2)S(6) is incorrect.

Electric-field influence on the neutron diffuse scattering near the ferroelectric transition of Sr0.61Ba0.39Nb2O6

ABSTRACT Uniaxial relaxor ferroelectric Sr0.61Ba0.39Nb2O6 single crystal has been investigated in the vicinity of its phase transition using neutron scattering and dielectric spectroscopy. A global-type thermal hysteresis is evidenced by both techniques in the ferroelectric phase and up to about 15 K above Tc. In addition, a part of the transverse neutron diffuse scattering in the 001 Brillouin zone, presumably related to static nanodomain structure, can be suppressed by prior poling the crystal in electric field of 3 kV/cm. The remaining part of the transverse neutron diffuse scattering and the real part of permittivity show a similar temperature dependence. The temperature position of the maximal scattering intensity Tmax depends significantly on the scattering wave vector. Tmax shifts monotonically to higher temperature with the increasing wave vector in all investigated cooling and heating regimes. It is concluded that the critical fluctuations have space correlations which depend on frequency and wave vector.

Terahertz Kerr Effect in an Organic Ferroelectric

Using the terahertz (THz) Kerr effect, we record nonlinear time-domain vibrational responses in tris-sarcosine calcium chloride (TSCC) through two interactions with THz fields. The results reveal two Raman-active modes excited by THz fields.

Inelastic neutron scattering study of lead-free relaxor ferroelectric (Na0.5Bi0.5)0.96Ba0.04TiO3 single crystal

Low frequency phonons and diffuse scattering have been investigated in a single crystal of (Na0.5Bi0.5)1−x Ba x TiO3 solid solution with composition (x = 4%) close to the morphotropic phase boundary by means of inelastic neutron scattering. Constant-energy scans for momentum transfer Q = (2, 2, q) reveal a typical waterfall-type dispersion at room temperature. The anomalous steep dispersion occurs at q wf ≈ 0.15 r.l.u., similar to lead-based relaxors. Since these measurements are done well below the temperature interval of the relaxor-like dielectric dispersion, our findings support a generic, anharmonic phonon-coupling nature of the phenomenon.

Acoustic phonons in unfilled tetragonal tungsten-bronze crystals

ABSTRACT We have studied several unfilled tetragonal tungsten-bronze crystals (SBN-35, CBN-28, CBN-30, CBN-32) by differential scanning calorimetry, Brillouin spectroscopy and piezoresponse-force microscopy. The Brillouin backscattering configuration in the c-plates revealed the longitudinal acoustic phonon, which has a frequency near 50 GHz and displays softening when approaching the phase transition in all crystals. The ferroelectric domain structure and the domain size are dependent on the Sr/Ba or Ca/Ba ratios, and on the occupation rate in the channels as shown by the piezoresponse-force microscopy images. A splitting of the longitudinal acoustic phonon was found in SBN-35 in the multidomain sample, but poling of the crystal removed this splitting and just left the phonon with higher frequency.

X-ray diffuse scattering observations for SrxBa1−xNb2O6 single crystals with x=0.35 and 0.81

ABSTRACT In an attempt to contribute to understanding of structure–property relationship in ferroelectric relaxor SrBaNbO, we studied X-ray diffuse scattering for compositions with x = 0.35 and x = 0.81 at room temperature. Within signal-rich reciprocal-space images, features related to (i) local deformation due to chemical inhomogeneity, (ii) incommensurate modulation and (iii) fluctuation of polarization have been identified. There are substantial differences between the two compositions, some of which can be straightforwardly attributed to different states of ferroelectric ordering in these materials. In the Sr-rich composition, we have observed additional features in the form of -oriented streaks on planes. They indicate that transverse correlation of uniaxial polarization is anisotropic. At the same time, the fact that streaks appear also on the superlattice planes raises the question whether the modulation and polarization order parameter are interrelated.

Far-infrared reflectivity spectra of nanotwinned GaV4Se8

ABSTRACT GaVSe is one of the lacunar spinel materials able to host magnetic skyrmionic textures. Earlier investigations at temperatures below its Jahn–Teller ferroelectric phase transition disclosed a nanoscale ferroelectric domain separated by 109 domain walls. Here, we have investigated the impact of this nanoscale twinning on polar phonon modes. We report dielectric and reflectivity spectra of GaVSe crystal in the phonon frequency region, derived from the ab initio predicted phonon frequencies, eigenvectors and Born-effective charges. Reflectivity spectra of the nanotwinned crystal, calculated using exact expressions for the effective macroscopic dielectric response of a perfect regular twin, are compared with the predicted reflectivity spectra of single-domain crystal. A characteristic spectral feature caused by the nanoscale twinning has been identified. While this feature is not a well-defined geometric resonance, it may still be accessible in experiments.