Search results for "Ferroelectric"
showing 10 items of 374 documents
Transient and stable color centers in pure and Cu-doped LiNbO3
2003
The present work is devoted to investigation of stable and transient color centers that are induced by radiation and temperature in pure and Cu-doped LiNbO 3 single crystals. The transient changes of absorption of the crystals induced by pulsed electron beam (E=0.25 MeV) are studied in 0-5000 ns time range, as well as stable changes of absorption induced by reducing annealing and irradiation with γ-quanta, high energy electrons or fast reactor neutrons.
Fourier-transform infrared study of the switching process in a ferroelectric liquid crystalline polymer
1995
The implementation of the step-scan technique on our spectrometer enabled us to follow the electric field induced reorientation dynamics of different molecular segments of the ferroelectric liquid crystalline polymer [Si(Me)(R)O/Si(Me)(R')O/Si(Me) 2 O] n (R= (CH 2 ) 11 OPhCOOPhOCO * CHCl * CH(Me)(Et); R'= (CH 2 ) 11 OPhN=NPhO * CH(Me)(Hex)) on a sub-millisecond time scale. It was detected that not only the mesogen but also the spacer and at least part of the backbone take part in the reorientation process
Influence of uniaxial pressure and aging on dielectric and ferroelectric properties of BaTiO3ceramics
2013
The effect of uniaxial pressure (0–3000 bar) applied parallel to the AC electric field and aging on dielectric and ferroelectric behavior of BaTiO3 ceramics were investigated. Applying uniaxial pressure leads to a reduction of the peak intensity of the electric permittivity (e) of the polarization as well as of the dielectric hysteresis. The peak intensity of e becomes diffused and shifts to a higher temperatures with increasing the pressure. The electrostrictive coefficient Q 11 and differential permittivity were evaluated from the obtained data. We interpreted our results based on the hardening of the soft-mode and domain switching processes under the action of pressure. The aging effect …
Ferroelectric polysiloxane liquid crystals with ‘de Vries’-type smectic A*–smectic C* transitions
2004
We report preliminary results of optical and small angle X-ray scattering (SAXS) experiments on the smectic A*−smectic C* transition in two ferroelectric liquid crystalline polysiloxanes. Although the optical tilt angle in the SmC* phases reaches values up to 30°, temperature-dependent SAXS measurements clearly reveal that the smectic layer spacing is basically conserved during the A*–C* transition as well as in the subsequent C* phase. Connected with the A*–C* transition we further observed a significant increase in birefringence, hence reflecting an increase of orientational order. The practical absence of layer shrinkage and the enhanced orientational ordering are consistent with the de …
Semi-empirical Hartree-Fock calculations for KNbO 3 and KTaO 3
1997
As a first step in modeling the electronic structure of Perovskite-type ferroelectric mixed crystals K(Nb,Ta)O3, semiempirical calculations for pure KNbO3 and KTaO3 are performed with the intermediate neglect of the differential overlap (INDO) quantum chemical method. The calculations are mostly done for 40-atom supercells. The choice of the INDO parameters based on the comparison of results with ab initio and experimental data is discussed. INDO results for the equilibrium geometry and the (Gamma) -TO phonon frequencies are given. The results show that the accuracy of the INDO method is sufficient for reliably reproducing the energy differences on the order of 1 mRy (per formula unit) scal…
Ab initio modelling of the effects of varying Zr (Ti) concentrations on the atomic and electronic properties of stoichiometric PZT solid solutions
2017
Abstract Lead zirconate titanate Pb(ZrxTi1−x)O3 solid solution is considered as one of the most advanced ferroelectric and piezoelectric materials. Consequent variation of Zr (Ti) concentrations significantly affects the atomic and electronic properties of PZT structures. To perform ab initio modelling of different morphologies for lead zirconate titanate, we are using approach of hybrid density functional B3PW as implemented in CRYSTAL14 computer code. In this study, we are performing large-scale calculations of such PZT parameters as optimized lattice constants, atomic charges and bond populations, as well as band structure (e.g., band gap) and density of states.
The phase situation and ferroelectric properties in the mixed crystals [4-NH2PyH][SbCl4(1−x)Br4x]
2008
Abstract The group of mixed crystals of general formula: [4-APyH][SbCl4(1−x) Br4x] with x ranging from 0 to 1 was studied by means of differential scanning calorimetry, dilatometry and dielectric spectroscopy. The ferroelectric properties are preserved for mixed crystals with replacement fraction, x, less than 0.30. Moreover, such a substitution of the chlorine atoms by the bromine ones significantly lowers the temperature of the structural phase transition shifting it from 240 K for pure [4-APyH][SbCl4] to about 185 K for the mixed crystals richer in bromine. The dielectric relaxation process exhibited by the mixed crystal with x = 0.90 in the radio-frequency region was analyzed and the ac…
Electromechanical Properties of Ba(1–x)SrxTiO3 Perovskite Solid Solutions from First-Principles Calculations
2017
Many thanks to M. Maček-Kržmanc, R. A. Evarestov, D. Gryaznov and D. Fuks for fruitful discussions. This study was supported by the ERA-NET HarvEnPiez project.
Calculations of F centers in KNbO 3 ferroelectric crystals
1997
Semi-empirical method of the intermediate neglect of the differential overlap (INDO) combined with the supercell model is applied to the calculations of the F center optical properties in ferroelectric KNbO 3 perovskite crystals. It is shown that two electrons of the defect are weakly localized inside the O vacancy, unlike similar defects in ionic alkali halides, but are considerably spread over two nearest Nb atoms. For the orthorhombic phase stable at room temperatures three absorption bands are predicted to be at 2.72 eV, 3.04 eV and 3.11 eV, respectively. The first energy is close to the band at 2.7 eV observed in electron- irradiated crystals. In the high temperature, cubic phase only …
Tetragonal tungsten bronze compounds: relaxor versus mixed ferroelectric-dipole glass behavior.
2011
We demonstrate that recent experimental data (E. Castel et al J.Phys. Cond. Mat. {\bf 21} (2009), 452201) on tungsten bronze compound (TBC) Ba$_2$Pr$_x$Nd$_{1-x}$FeNb$_4$O$_{15}$ can be well explained in our model predicting a crossover from ferroelectric ($x=0$) to orientational (dipole) glass ($x=1$), rather then relaxor, behavior. We show, that since a "classical" perovskite relaxor like Pb(Mn$_{1/3}$ Nb$_{2/3}$)O$_3$ is never a ferroelectric, the presence of ferroelectric hysteresis loops in TBC shows that this substance actually transits from ferroelectric to orientational glass phase with $x$ growth. To describe the above crossover theoretically, we use the simple replica-symmetric so…