Search results for "ferroelectric"
showing 10 items of 374 documents
In situ study of the phase transition in Bi2Ti4O11
1995
Abstract The paraelectric ⇌ antiferroelectric phase transition of the compound Bi 2 Ti 4 O 11 is studied in situ by electron diffraction and electron microscopy. The transition is reversible and clearly second order. Above T c faint streaking at the superlattice positions persists. In the low-temperature phase, antiphase boundaries with a displacement vector R = 1/2[101] are revealed; they show a finite width, suggesting a gradual displacement of the Bi atoms at the transition. A model for the domain wall configuration is proposed, based on the influence of the lone pairs of Bi 3+ ions at the transition.
Critical effects in optical response due to charge transfer vibronic excitons and their structure in perovskite-like systems
1999
Abstract A mechanism for bilinear interaction between high-frequency light-induced electronic polarization and low-frequency soft lattice polarization is proposed. It is based on the fluctuations of the charge transfer connected with charge transfer vibronic excitons (CTVE). This bilinear mechanism leads to the appearance of the critical peculiarities of the absolute diffraction efficiency of transient gratings near the ferroelectric phase transition. A semi-empirical Hartree–Fock INDO method was used for the evaluation of the energy parameters and the equilibrium displacements for the CTVE in KTaO 3 . This numerical study did confirm the proposed CTVE-model. It was shown that the CTVE-phas…
Phase transitions in modified Na 1/2 Bi 1/2 TiO 3 ‐SrTiO 3 solid solutions
2009
Continuous transfer from relaxor to normal ferroelectric phase transition is found in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3 if concentration of PbTiO3 increases. The relaxor properties in NBT solid solutions are described by the power law and Vogel-Fulcher relationship, and concentration dependence of the relevant parameters is discussed. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Self-Ordered Second-Component Nb Clusters in KNbxTa1?xO3 Solid Solutions and Their Physical Properties
1999
Semi-empirical Hartree-Fock calculations using the intermediate neglect of the differential overlap (INDO) method, for self-ordered cubic symmetry clusters of seven Nb ions in KTaO 3 are performed with the aim of verifying the cluster model [1] of second component-induced phase transitions in ferroelectric perovskite matrices. It is shown that a seven-particle cluster in KTaO 3 :Nb has two types of states with different nature. Namely, a state with a dilatation combined with an off-center displacement of the central Nb ion in [111] directions in the cluster, and a state with a full-symmetric compression without any off-center effect. The consequences of such cluster structures on the multi-…
Crystal structure and dielectric properties of the [(CH3)2NH2]3Sb2(1-x)Bi2xCl9(DMACAB) mixed crystals
2001
Phase transitions in [(CH3)2NH2]3Sb2(1-x)Bi2xCl9 (DMACAB) mixed salts in the composition range 0≤x≤0.41 have been investigated by the pyroelectric method and dielectric measurements over the frequency range from 75 kHz to 900 MHz. The phase situation is additionally confirmed by the differential scanning calorimetry (DSC) and dilatometric techniques. A transition from the paraelectric (PE) to the ferroelectric (FE) phase is observed for crystals with 0≤x≤0.14. Pyroelectric measurements support the presence of polar phases. The dynamic dielectric behaviour of ferroelectric systems is found to be determined by the existence of two independent relaxators. The low-frequency relaxator reveals a …
The Ferroelectric Photo-Groundstate of SrTiO$_3$: Cavity Materials Engineering
2021
Significance Controlling collective phenomena in quantum materials is a promising route toward engineering material properties on demand. Strong THz lasers have been successful at inducing ferroelectricity in S r T i O 3 . Here we demonstrate, from atomistic calculations, that cavity quantum vacuum fluctuations induce a change in the collective phase of S r T i O 3 in the strong light–matter coupling regime. Under these conditions, the ferroelectric phase is stabilized as the ground state, instead of the quantum paraelectric one. We conceptualize this light–matter hybrid state as a material photo ground state: Fundamental properties such as crystal structure, phonon frequencies, and the col…
Crystal structure transformations induced by surface stresses in BaTiO3and BaTiO3@SiO2nanoparticles and ceramics
2015
International audience; Lattice structure transformations in nanopowders of ferroelectric BaTiO3 and BaTiO3@SiO2 core-shell nanostructured ceramics were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) at the temperatures 120–450 K and particle size of 300 and 500 nm. NMR spectra of all studied samples in the paraelectric phase are identical to the spectra in bulk material indicating their perfect perovskite structure without visible influence of particle surface. However, we have found that surface of particles essentially influence the ferroelectric phase transitions detected by both NMR and EPR techniques. The strongest changes as compared to bulk mat…
DIELECTRIC PROPERTIES OF LEAD-FREE [(1-x)(Na0.5Bi0.5)-xBa]Zr1−yTiyO3CERAMICS (x = 0.01, 0.06, 0.085, 0.09, 0.1 AND y = 0.96, 0.97)
2008
ABSTRACT Lead-free ceramics based on (Na0.5Bi0.5TiO3, NBT)-(Ba(Ti, Zr)O3, BTZ) were prepared by solid phase synthesis and hot pressing sintering process and their dielectric properties have been studied. The obtained results were compared with these for pure NBT. The obtained samples show pure perovskite structure. Low frequency (100 Hz-100 kHz) investigations reveal the diffuse phase transitions and relaxorlike behaviour. The obtained results are discussed in terms of inhomogeneity of ions distribution, their ferro- and antiferroactivity and internal stresses. The NBT-BTZ system is expected to be a new promising candidate for lead-free electronic ceramics.
Phase Transitions in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3Solid Solutions
2010
All sequential steps of transfer from relaxor to normal ferroelectric phase transition are found in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3 if concentration of PbTiO3 increases. The relaxor properties in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3 solid solutions are described by the power law and Vogel-Fulcher relationship. Concentration dependence of the relevant parameters is discussed. Stability of relaxor and ferroelectric phases is discussed by analyzing behaviour of critical electric fields.
Dielectric and Ultrasonic Investigation of Phase Transitions in PbFe1/2Nb1/2O3Ceramics
2012
We have investigated dielectric and acoustic properties of PbFe1/2Nb1/2O3 ceramics, obtained by conventional ceramic technology, in 120–450 K temperature interval. Dielectric response showed diffused dielectric permittivity maximum caused by ferroelectric phase transition around 380 K, but no dielectric anomalies associated with magnetic phase transition at low temperatures were observed. Formation of the polar phase was confirmed by ultrasonically detected piezoelectric sensitivity. Ultrasonic investigations using pulse-echo method showed two anomalous regions in temperature dependencies of ultrasonic velocity and attenuation, associated with magnetic and ferroelectric phase transitions.