Change from 3D-Ising to random field-Ising-model criticality in a uniaxial relaxor ferroelectric

Theory of relaxor-ferroelectricity

Relaxor-ferroelectrics are fascinating and useful materials, but the mechanism of relaxor-ferroelectricity has been puzzling the scientific community for more than 65 years. Here, a theory of relaxor-ferroelectricity is presented based on 3-dimensional-extended-random-site-Ising-model along with Glauber-dynamics of pseudospins. We propose a new mean-field of pseudospin-strings to solve this kinetic model. The theoretical results show that, with decreasing pseudospin concentration, there are evolutions from normal-ferroelectrics to relaxor-ferroelectrics to paraelectrics, especially indicating by the crossovers from, (a) the sharp to diffuse change at the phase-transition temperature to disappearance in the whole temperature range of order-parameter, and (b) the power-law to Vogel-Fulcher-law to Arrhenius-relation of the average relaxation time. Particularly, the calculated local-order-parameter of the relaxor-ferroelectrics gives the polar-nano-regions appearing far above the diffuse-phase-transition and shows the quasi-fractal characteristic near and below the transition temperature. We also provide a new mechanism of Burns-transformation which stems from not only the polar-nano-regions but also the correlation-function between pseudospins, and put forward a definition of the canonical relaxor-ferroelectrics. The theory accounts for the main facts of relaxor-ferroelectricity, and in addition gives a good quantitative agreement with the experimental results of the order-parameter, specific-heat, high-frequency permittivity, and Burns-transformation of lead magnesium niobate, the canonical relaxor-ferroelectric.

3D Imaging of Ferroelectric Kinetics during Electrically Driven Switching

Čerenkov-type second-harmonic generation microscopy is used for in situ 3D imaging of the switching process of ferroelectric domains in a relaxor-type ferroelectric crystal. The lateral and longitudinal domain wall motion is optically measured and compared with the hysteresis loop to reveal the connection between the domain kinetics and the ferroelectric aging effect in strontium barium niobate.

Phase transition of chemically doped uniaxial relaxor ferroelectric

We report a neutron scattering study of the ferroelectric phase transition in Sr0.585Ce0.025Ba0.39Nb2O6 (SBN-61:Ce). We find no evidence for a soft transverse optic phonon. We do, however, observe anisotropic diffuse scattering. This scattering has inelastic and elastic contributions. In the paraelectric phase the susceptibility associated with the elastic diffuse scattering from SBN-61:Ce increases on approaching the transition temperature. In the ferroelectric phase the lineshape of the elastic scattering is consistent with the form expected for the ferroelectric domain walls. In contrast to the macroscopic observations, the scattering properties of Ce-doped crystal do not exhibit important changes with respect to those of pure Sr0.61Ba0.39Nb2O6.

Reducing the positional modulation of NbO6-octahedra in Sr x Ba1–x Nb2O6 by increasing the barium content: A single crystal neutron diffraction study at ambient temperature for x = 0.61 and x = 0.34

We report on the influence of the barium content on the modulation amplitude in Sr x Ba1–x Nb2O6 compounds by comparing Sr0.61Ba0.39Nb2O6 (SBN61) and Sr0.34Ba0.66Nb2O6 (SBN34). Our single crystal neutron diffraction results demonstrate that the amplitude of the positional modulation of the NbO6 octahedra is reduced with increasing barium content, indicating that the origin of the modulation is the partial occupation of the pentagonal channels by Sr and Ba atoms. By increasing the Sr content the bigger Ba atoms are replaced by the smaller Sr atoms, which leads to a larger deformation of the surrounding lattice and hence to a larger modulation amplitude. The more homogeneous the filling of these channels with one atomic type (Ba) the lower the modulation amplitude. Our results also show that the structure can be described with a two-dimensional incommensurate harmonic modulation. No second order modulation has been observed, both by single crystal diffraction measurements and q-scans. The positional modulation of the Nb atoms is much smaller than that of the oxygen atoms, such that the modulation can be seen as a rotational modulation of almost rigid NbO6-octahedra.

Thermo-optic properties of epitaxial Sr0.6Ba0.4Nb2O6 waveguides and their application as optical modulator

A prism-coupler technique was introduced to determine the refractive indices and thermo-optic coefficients of epitaxial Sr(0.6)Ba(0.4)Nb(2)O(6) (SBN) waveguides, in a temperature range covering the ferroelectric-paraelectric phase transition. A strong enhancement in the TO coefficient is observed near T(c). This strong enhancement is related to the critical change of the polarization. The values of dn(e)/dT are significantly larger than dn(o)/dT due to the larger quadratic electro-optic coefficient in TM polarization. In TM mode, the refractive index of SBN is increased by 1.3% as the temperature is increased to 160 degrees C. Our results suggest that SBN waveguide is a potential candidate for thermo-optic modulators and switches.

Electric-field-induced redistribution of polar nano-regions in a relaxor ferroelectric

Relaxor ferroelectrics, with their strong dependence of polarization on the applied electric field, are of considerable technological importance. On a microscopic scale, however, there exists competition as well as coexistence between short-range and long-range polar order. The conventional picture is that the polar nano-regions (PNRs) that appear at high temperatures beyond the Curie transition, form nuclei for the field-induced long-range order at low temperatures. Here, we report high-energy X-ray diffuse-scattering measurements on the relaxor Pb(Zn(1/3)Nb(2/3))O(3) (PZN) to study the short-range polar order under an electric field applied along the [111] direction. In contrast to conventional expectations, the overall diffuse-scattering intensity is not suppressed. On the other hand, the field induces a marked change on the shape of the three-dimensional diffuse-scattering intensity pattern, corresponding to a redistribution of PNRs in real space. We show that these surprising results are consistent with a model in which the PNRs with [110]-type polarizations, orthogonal to that of the surrounding environment, are embedded and persist in the [111]-polarized ferroelectric order of the bulk.