The modulated structure of Ba0.39Sr0.61Nb2O6. I. Harmonic solution

The structure of a crystal of Sr0.61Ba0.39Nb2O6 has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(pp1/2; p-p1/2), unit-cell parameters a = 12.4566 (9), c = 7.8698 (6) Angstrom, modulation vectors q(1) = 0.3075 (6) (a* +b*), q(2) = 0.3075 (6) (a* - b*). The data collection was performed on a KUMA-CCD diffractometer and allowed the integration of weak first-order satellite reflections. The structure was refined from 2569 reflections to a final value of R = 0.0479. The modulation affects mainly the positions of the O atoms, which are displaced by as much as 0.5 Angstrom, and the site 4c that is occupied by Sr and Ba atoms. Only a simplified model, in which this atomic position is occupied by an effective atom Sr/Ba, could be refined from the data set. The modulation of displacement parameters has been used to account for the modulated distribution of Sr and Ba. The whole refinement uses only first-order modulation waves, but there are strong indications that for a complete solution the use of higher-order satellites and a more complicated model is necessary

Phase transitions in Sr

The linear birefringence (LB) of $\mathrm{Sr_{0.61}Ba_{0.39}Nb_{2}O_{6}{:}Ce^{3+}}$ (SBN61:Ce) has been measured as a function of temperature within the range of $78 \leq T \leq 850 \mathrm{K}$. Large tails have been observed above the ferroelectric phase transition temperatures $T_{\mathrm{c}} = 350$, 328, 320 and 291 K for the concentrations x = 0, 0.0066, 0.0113 and 0.0207, respectively. Within an Ornstein-Zernike analysis the critical exponents γ, ν and β are determined. It suggests that pure SBN61 belongs to the 3D Ising universality class. Doping with $\mathrm{Ce}^{3+}$ ions, which seem to act as random fields, enhances the relaxor properties. The critical exponents γ and ν of SBN61:Ce shift against those of the three-dimensional random-field Ising model

Uniaxial relaxor ferroelectrics: The ferroic random-field Ising model materialized at last

Owing to their intrinsic charge disorder ferroelectric crystals of strontium-barium-niobate doped with \chem{Ce^{3+}} materialize the three-dimensional ferroic random-field Ising model (RFIM) as evidenced by order paramenter and susceptibility criticalities with \chem{{}^{93}Nb} NMR and dielectric spectroscopy, respectively. Upon cooling towards T_\ab{c}, extreme critical slowing-down due to activated dynamic scaling gives rise to relaxor-like dispersion of the susceptibility and to a metastable ferroelectric nanodomain state with fractal size distribution as imaged by piezoelectric force microscopy

TILTING OF HOLOGRAMS IN PHOTOREFRACTIVE SR0.61BA0.39NB2O6 CRYSTALS BY SELF-DIFFRACTION

Holographic gratings are written in photorefractive Sr0.61Ba0.39Nb2O6 crystals by two interfering light beams. Angular-dependent readout of the holograms shows that applying an external electric field to the crystal produces a pronounced tilting of the holograms. The results are in good agreement with theoretical predictions considering self-diffraction of the recording beams. (C) 1995 Optical Society of America20212249225

Intensity and wavelength dependence of the photoconductivity in Cr-doped Sr

We examine the light-induced charge transport properties of a series of chromium-doped $\rm{Sr}_{0.61}\rm{Ba}_{0.39}\rm{Nb}_2\rm{O}_6$ single crystals by measurements of the optical absorption and the electric conductivity. By comparing the wavelength dependence of the specific photoconductivity and the optical absorption we show that both effects stem from the same center. The intensity dependence of the photoconductivity shows the applicability of a one-center charge transport model for high doping concentrations, while for low doping concentrations a more sophisticated model is needed. The validity of a one-center model is exemplarily verified for a crystal doped with 0.51 mol % Cr over a wide intensity range using a holographic method. The product of mobility and recombination time of photoexcited electrons is deduced from the specific photoconductivity

Random-field Ising-type transition of pure and doped SBN from the relaxor into the ferroelectric state

Activated critical dynamics and vanishing of the susceptibility ratio $a_3 =\chi_3/\chi_1^4\rightarrow 0$ in the zero-frequency limit unequivocally evidence the phase transitions of the uniaxial relaxor crystals SBN, SBN:\chem{Ce} and SBN:\chem{Cr} to refer to the three-dimensional random-field Ising universality class. The relaxor-like freezing of polar nanoregions within the precursor regime is successfully described within the framework of the mesoscopic uniaxial spherical random-bond random-field model

Evidence of random electric fields in the relaxor-ferroelectric

We present strong evidence of the existence of quenched random electric fields in relaxor ferroelectrics by investigating the aging in ferroelectric hysteresis loops under light illumination. The temporal behaviour of ferroelectric domains under external electric fields and light illumination is measured in cerium-doped strontium-barium-niobate crystals using quasistatic surface charge detection. The aging, caused by electric pinning centres which hinder domain wall motion, leads to a decrease in domain wall mobility. Raising the sample conductivity by illumination eliminates the pinning centres and fully restores the polarization dynamics