Krystian Roleder

Electrostrictive strain and pyroeffect in the region of phase coexistence in Na0.5Bi0.5TiO3

Abstract Local maxima and other singularities distinguish the temperature range 200-320°C in the electrostrictive versus temperature graph of Na0.5Bi0.5TiO3, known from other experiments as a region of coexistence of phases. Investigation of the pyroeffect in virgin Na0.5Bi0.5TiO3 single crystals reveals the existence of polar microregions.

Infrared, Raman and high-frequency dielectric spectroscopy and the phase transitions in Na1/2Bi1/2TiO3

Infrared reflectivity, time-domain terahertz transmission, high-frequency dielectric measurements and Raman spectroscopy of Na0.5Bi0.5TiO3 complex ferroelectric perovskite were performed in a broad temperature range. The results were analysed considering the macroscopic symmetry as well as symmetry resulting from local Na–Bi ordering in all three known phases. An overdamped infrared soft mode was revealed in the THz range which, together with central-mode type dispersion in the GHz range, contribute to the strong and broad dielectric permittivity maximum around 600 K. Anharmonic Bi and/or Na vibrations and local hopping, respectively, are suggested to be the main origins of these excitations. The broad phonon spectra and the frequency independent dielectric losses at low temperatures are compatible with the existence of nanoscopic polar regions and disorder to liquid He temperatures similar to relaxor ferroelectrics.

Time dependence of electric permittivity in na0.5Bi0.5TiO3 single crystals

Abstract Performed measurements showed that in Na0.5Bi0.5TiO3 single crystals the long-time variation of the electric permittivity is connected with the existence of polar regions. The size and dynamics of the polar regions change with temperature. These regions may be ordered in an external electric field.

The piezoelectric effect in Na0.5Bi0.5TiO3 ceramics

Piezoelectric properties of Na0.5Bi0.5TiO3 ceramics have been investigated over a broad temperature range using a resonance technique. The thermal behaviours of the piezoelectric tensor coefficients d31, d33 and d15 confirm that there is a distinct phase transformation at 200 °C. For the first time it has been found that a piezoelectric signal connected with the tensor component d33 is observed up to 280 °C. This corresponds to earlier neutron and x-ray diffraction data on the point of transition from rhombohedral to tetragonal symmetry.

Ferroelectric translational antiphase boundaries in nonpolar materials

Ferroelectric materials are heavily used in electro-mechanics and electronics. Inside the ferroelectric, domain walls separate regions in which the spontaneous polarization is differently oriented. Properties of ferroelectric domain walls can differ from those of the domains themselves, leading to new exploitable phenomena. Even more exciting is that a non-ferroelectric material may have domain boundaries that are ferroelectric. Many materials possess translational antiphase boundaries. Such boundaries could be interesting entities to carry information if they were ferroelectric. Here we show first that antiphase boundaries in antiferroelectrics may possess ferroelectricity. We then identify these boundaries in the classical antiferroelectric lead zirconate and evidence their polarity by electron microscopy using negative spherical-aberration imaging technique. Ab initio modelling confirms the polar bi-stable nature of the walls. Ferroelectric antiphase boundaries could make high-density non-volatile memory; in comparison with the magnetic domain wall memory, they do not require current for operation and are an order of magnitude thinner.

Birefringence above Tc in single crystals of barium titanate

The temperature dependence of the birefringence above Tc in high quality BaTiO3 single crystals has been measured with very high accuracy. On the basis of literature data, the behaviour of the anomalous birefringence can be explained by the existence of polar clusters (microregions/nanoregions) connected with correlated jumps between the off-centre positions of Ti ions inside the oxygen octahedra. The underlying origin of the polar clusters is an intrinsic effect connected with the interdependence of the structural and polar soft modes (Bussmann-Holder et al 2007 Phys. Rev. Lett. 99 167603). These clusters locally break the intrinsic cubic symmetry and the phase above Tc becomes birefringent. A birefringence Δn of the order of 10−5 has been found up to about 30° above Tc. In this temperature range Tc<T<Tc+30 K, the function Δn(T) exhibits a thermal hysteresis: the values of Δn were higher on heating than on cooling. A characteristic feature is the existence of a local Δn(T) maximum at a temperature close to Tc on the high temperature side that appears due to a specific cluster–cluster interaction. Since the values of Δn(T) runs were not dependent on the crystal thickness, one can conclude that in this limited temperature range above Tc the birefringence is an inherent feature of the crystal.

Dielectric and structural relaxation phenomena in Na0.5Bi0.5TiO3 single crystal

Abstract Dielectric permittivity studies of Na0.5Bi0.5TiO3 single crystals in a broad range of frequency up to 10 MHz and temperature 300—823 K are reported. In this temperature range dielectric dispersion below 1 MHz has been found. The obtained data were fitted to the Cole-Cole relation. The mean relaxation time τ is strongly temperature dependent (0.04 − 2.6 × 10−5 s). A remarkable hysteresis effect in the values of τ on cooling and heating took place. The Δe(T) dependence (the maximal value of Δe ∼ 400) is similar to the global e′(T) response at low frequency. An isothermal structural transformation in Na0.5Bi0.5TiO3 was observed by X-ray measurements. The order of the time in which the transformation takes place (∼300 minutes) corresponds to the time in which the strongest time evolution of electric permittivity and time changes of dielectric dispersion were detected.

Raman scattering in PbZr1-xTixO3 single crystals with low Ti content and a study of the Ti influence

Investigations of the Raman scattering spectra of PbZr0.99Ti0.01O3 single crystals were performed over a large temperature range from 10 to 650 K and compared with those of pure PbZrO3. The main modifications of the Raman spectra have been found to appear near the transitions between the antiferroelectric-intermediate and the intermediate-paraelectric phases. In particular, a polarisation effect of the light scattering in the para-electric phase has been detected and attributed to the special role of Ti ions introduced in the host lattice of lead zirconate. These ions can be regarded as defects breaking the local symmetry and leading to the creation of polar clusters in the para-electric phase. Below a defined temperature in this phase, a long-range interaction appears and causes cluster ordering. The change in the Raman spectra and in the light scattering intensity and polarisation is analysed and connected with the dielectric and pyroelectric properties investigated on the same sample.

Precursor dynamics, incipient ferroelectricity and huge anharmonicity in antiferroelectric lead zirconate PbZrO3.

To better understand the phase transition mechanism of PbZrO3 (PZO), the lattice dynamics of this antiferroelectric compound are investigated within the polarizability model, with emphasis on the cubic to orthorhombic phase transition. Similarly to ferroelectric phase transitions in ABO3 perovskites, polar dynamical clusters develop and grow in size upon approaching T(C) from the high temperature side and never form a homogeneous state. Simultaneously, elastic anomalies set in and compete with polar cluster dynamics. These unusual dynamics are responsible for precursor effects that drive the PZO lattice towards an incipient ferroelectric state. Comparison of the model calculations with the temperature dependences of elastic coefficients measured on PZO single crystals reveals a striking similarity.

Lattice dynamics and antiferroelectricity in PbZrO3 tested by x-ray and Brillouin light scattering

We report the results of a comprehensive study of the critical dynamics of the classical perovskite antiferroelectric PbZrO3. The combination of inelastic x-ray and diffuse x-ray scattering techniques and Brillouin light scattering was used. It is found that the dispersion of the transverse acoustic (TA) phonons is strongly anisotropic. The dispersion curve of the in-plane polarized TA phonons propagating in [1 1 0] direction demonstrates pronounced softening. Slowing down of the excitations at R point is found, it is manifested in growing of the central peak. This slowing down is too weak to be considered as a primary origin of the corresponding order parameter. Obtained results are treated in terms of TA-transverse optic (TO) flexoelectric mode coupling. It is demonstrated that the structural phase transformation in PbZrO3 can be considered as the result of the only intrinsic instability associated with the ferroelectric soft mode.