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1.
The full potential linearized augmented plane wave (FLAPW) method is used to study the crystal structure and electronic structure properties of PbFeo.5 Nbo.5O3 (PFN). The optimized crystal structure, density of states, band structure and electron density distribution have been obtained to understand the ferroelectric behaviour of PFN.The analysis result of the density of states shows there is an obvious change of Nbd states in the paraelectric-to-ferroelectric phase transition. The polarization result shows that the contribution to ferroelectricity of Nb atoms is larger than that of Fe atoms. In ferroelectric phase there is a hybridization of Fed-Op and Nbd-Opin ferroelectric PFN. This is consistent with the result of the electronic band structure. This hybridization is responsible for the tendency to its ferroelectricity.  相似文献   

2.
Dense nanocrystalline BaTiO3 ceramics with a homogeneous grain size of 30 nm was obtained by pressure assisted sintering. The ferroelectric behaviour of the ceramics was characterized by the dielectric peak at around 120 ℃, the P-E hysteresis loop and some ferroelectric domains. These experimental results indicate that the critical grain size for the disappearance of ferroelectricity in nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering is below 30 nm. The ferroelectric property decreasing with decreasing grain size can be explained by the lowered tetragonality and the 'dilution' effect of grain boundaries.  相似文献   

3.
We perform first-principle calculations for the study of the orthorhombic Rb2Cd2 (SO4)3 structure. Electronic energy bands, total and partial densities of states are reported and analysed. It is found that oxygen atomic 2p electrons strongly hybridize with Rb/or Cd 4d and S 2p states, resulting in two-type ionic groups with weak couplings. It is shown that macroscopic domain walls originate from such weak-coupling ionic groups, arising at the cell boundaries. The asymmetric cation bonds (Rb-O and Cd-O) and the subsequent rotations of the S04 tetrahedra can lead to the driving force of the ferroelectric behaviour. The predicted pyroelectric current effects are observed experimentally in the ferroelectric phase.  相似文献   

4.
Taking into account surface transition layers (STLs), we study the phase transformation and pyroelectric properties of ferroelectric thin films by employing the transverse Ising model (TIM) in the framework of the mean field approximation. The distribution functions representing the intra-layer and inter-layer couplings between the two nearest neighbour pseudo-spins are introduced to characterize STLs. Compared with the results obtained by the traditional treatments for the thin films using only the single surface transition layer (SSL), it is shown that the STL model reflects a more realistic and comprehensive situation of films. The effects of various parameters on the phase transformation properties have shown that STL can make the Curie temperature of the film higher or lower than that of the corresponding Sulk material, and the thickness of STL is a key factor influencing the film properties. For a film with definite thickness, there exists a critical STL thickness at which ferroelectricity will disappear when the intra-layer and inter-layer interactions are weak.  相似文献   

5.
In the framework of the Skyrme–Hartree–Fock–Bogoliubov approach with the Sk T interaction, the pairing effects on the proton bubble structures of ~(46)Ar and ~(206)Hg are discussed. In calculations, three kinds of pairing forces (volume, surface and mixed pairing interactions) are used. For ~(46)Ar, it is shown that the bubble structure with the volume pairing is almost the same as that with the mixed pairing. The bubble with the surface pairing is less pronounced than those with the volume and mixed pairings. Analyzing the density distributions and occupation probabilities of the proton s states and the quasi-degeneracy between the proton 2s_(1/2) and 1d_(3/2) orbitals, we explain the difference between the bubble structure with the surface pairing and those with the volume and mixed pairings. For ~(206)Hg, it is seen that the proton density distribution with the surface pairing is different from those with the volume and mixed pairings in the whole region of the radial distance. In addition, it is found that the bubbles with the three pairing forces are different from each other and the least pronounced bubble is obtained with the surface pairing. Thus the selection of the pairing force is important for the study of the nuclear bubble structure.  相似文献   

6.
Dielectric and ferroelectric characteristics for Pb1-xBax(Fe1/2Nh/2)O3 (x = 0,0.05,0.1,0.15, and 0.2) ceramics are determined together with their structures. X-ray diffraction (XRD) analysis confirms the solid solutions with the cubic structure. The dielectric nature changes from diffuse ferroelectric to relaxor ferroelectric with increasing x, while the phase transition temperature Tc (or Tm) decreases monotonously. The diffuse ferroelectric phase transition is observed in the solid solutions with 0≤x ≤0.05. For Pbl-xBax(Fe1/2Nb1/2)O3 with 0.1≤ x≤0.2, relaxor ferroelectric behavior is determined, and the Vogel-Fulcher equation is used to describe the relaxor behavior. The 1/ε versus T plots reveal the diffusion dielectric characteristics in both diffuse and relaxor ferroelectrics.  相似文献   

7.
First-principle calculations are performed to study the infrared optical and electronic properties of the tetragonal CaTiO3. We find that dielectric function and reflectivity exhibit a new single-peak infrared spectrum. It is shown that strong orbital hybridization can form the dipole distribution of covalent bonds (Ti-O1 and Ti-O2)and can result in anomalous optical behaviour. The tetragonal structure that determines the dipole configuration and leads to the optical dipole-dipole transition forbidden is therefore realized to be vital to cause such optical phenomena with the insulator feature.  相似文献   

8.
Dense nanocrystalline BaTiO3 ceramics with a grain size of 5Onto are prepared under 6 GPa at 1273K using a high pressure sintering method. The sintered bulk is uniform and the relative density is above 97%. We anneal the ceramic samples in oxygen with various temperatures and for the annealing, several broadened peaks can be observed at different times without apparent grain growth. After about 378K( by dielectric measurements. However, these peaks are very different from those of coarser-grained ceramics. It is indicated that both the elimination of oxygen vacancies and the release of residual stresses caused by high pressure greatly improve the overall ferroelectric properties of BaTiO3 ceramics. The observation of nearly linear polarization hysteresis loop after anneal provides the solid evidence of ferroelectricity in these nano-sized BaTiO3 ceramics. It is believed that the absence of 90° domains and the existence of poor-permittivity nonferroelectric grain boundaries contribute to the slim loop.  相似文献   

9.
孙桂全  靳祯  刘权兴  李莉 《中国物理 B》2008,17(11):3936-3941
This paper considers the Holling-Tanner model for predator-prey with self and cross-diffusion. From the Turing theory, it is believed that there is no Turing pattern formation for the equal self-diffusion coefficients. However, combined with cross-diffusion, it shows that the system will exhibit spotted pattern by both mathematical analysis and numerical simulations. Furthermore, asynchrony of the predator and the prey in the space. The obtained results show that cross-diffusion plays an important role on the pattern formation of the predator-prey system.  相似文献   

10.
The period length is usually much larger than the cavity effective length in a low energy superconducting linac.The long drifts between cavities will not only decrease the acceptance of the linac, but also lead to possible instability. The linac will be more sensitive to mismatch and other perturbations. From the longitudinal motion equation, the function which describes the parametric resonance is deduced and the relation between the instability region and the cavity filling factor is discussed. It indicates that if the zero current phase advance per period is kept below 90°, instability driven by parametric resonance will never occur. The space charge effect will enhance the instability, so that a stricter limitation on the phase advance per cell is required. From the numerical simulation results for two different schemes of Injector-Ⅰ of the C-ADS driver linac, one can find that even with just three cells in the unstable region, significant emittance growth can be observed. Further investigations show that it is apt to produce halo particles under resonance, and the machine becomes more sensitive to errors and mismatches. Therefore, it is important to keep all cells in the stable region throughout the linac of very high beam power to minimize beam losses.  相似文献   

11.
《Current Applied Physics》2014,14(5):757-760
CaTiO3 is a well-known incipient ferroelectric material that does not undergo a ferroelectric phase transition in spite of the intriguing dielectric constant behavior. Especially, unlike a prototypical incipient ferroelectric SrTiO3, the paraelectric state of CaTiO3 cannot be easily destroyed by small perturbations, including cation doping and epitaxial strain. We present that a nearly strain-free epitaxial CaTiO3 film grown at a low oxygen partial pressure exhibits polarization–voltage hysteresis loops and the distinct difference of piezoresponse force microscopy phase signals, implying that a ferroelectric phase is induced. Such results are shown even at room temperature. We suggest that the observed ferroelectric behavior in CaTiO3 film comes from the defect dipoles composed of vacancies inside the film. Using electron-probe microanalysis and optical absorption spectra measurements, we found that CaTiO3 film has considerable Ca and O vacancies, forming the localized defect state in electronic structure. This work highlights the importance of vacancies and their clusters, such as defect dipoles, in understanding the electronic properties of perovskite oxide thin films, including ferroelectricity.  相似文献   

12.
The crystal structure of the ground state of ten free-standing ferroelectric superlattices based on crystals with the perovskite structure (BaTiO3/SrTiO3, PbTiO3/SrTiO3, PbTiO3/PbZrO3, SrZrO3/SrTiO3, PbZrO3/BaZrO3, BaTiO3/BaZrO3, PbTiO3/BaTiO3, BaTiO3/CaTiO3, KNbO3/KTaO3, and KNbO3/NaNbO3) was calculated from first principles within the density functional theory taking into account criteria for stability of the structures with respect to acoustic and optical distortions. It was shown that the ground state in all the considered superlattices corresponds to the ferroelectric phase. It was found that the polarization vector has a tendency toward a tilt to the plane of the superlattice layers, which makes it possible to decrease the electrostatic and elastic energy in the superlattices consisting of materials with different ferroelectric properties. The importance of the inclusion of structural distortions due to unstable phonons at the Brillouin zone boundary, which, in a number of cases, lead to significant changes in ferroelectric and dielectric properties of the superlattices, was demonstrated.  相似文献   

13.
SrTiO3 and CaTiO3 conventional bulk materials are incipient ferroelectrics. In this note, we report for the first time that ferroelectricity could occur in SrTiO3 nanocrystalline disks even at room temperature. The peak in the temperature dependence of permittivity for a CaTiO3 nanocrystalline disk at a low temperature is also observed. The observed ferroelectricity (or permittivity peak) in SrTiO3 (or CaTiO3) nanocrystalline disks could be attributed to the strain effect.  相似文献   

14.
The local environment of Pb impurity atoms in BaTiO3, SrTiO3, and CaTiO3 crystals was studied by XAFS technique. It is shown that, in both polar and nonpolar phases of BaTiO3 and in SrTiO3, the Pb atoms are displaced from the A lattice sites by ~0.15 Å; in CaTiO3 this displacement is absent. Large values of Debye-Waller factors (0.05–0.10 Å2) for the atoms in the first shell of Pb observed in all the three crystals indicate the distortion of the oxygen environment of Pb atoms. The appearance of these features was explained by the fact that the Pb-O chemical bond has a noticeable covalent component and a Pb atom can form strong bonds only with four of the 12 surrounding oxygen atoms. The obtained data were used to determine the main factors responsible for the occurrence of ferroelectric phase transition in SrTiO3 and CaTiO3 and for the increase of the Curie temperature of BaTiO3 when it is doped with Pb.  相似文献   

15.
We have directly probed the Ba 5d states in the ferroelectric barium titanate BaTiO3 using two bulk-sensitive spectroscopic probes, resonant x-ray emission spectroscopy (RXES) and x-ray absorption spectroscopy in the partial fluorescence mode (PFY-XAS) at the Ba-L3 edge. The results are compared with those of the non-ferroelectric barium sulfate BaSO4. While the RXES spectra point to a localized character for the Ba 5d states in both compounds, the main peak of the PFY-XAS spectrum, corresponding to the dipolar transitions from 2p to 5d, is found to be significantly broader for BaTiO3 than for BaSO4. On the basis of band structure calculations, this broadening is ascribed to strong hybridization between the unoccupied Ba 5d and O 2p states in the ferroelectric. This suggests that the hybridization between the conduction states of the Ba2+ and O2? ions, and not only Ti4+ and O2?, plays a central role in determining the electronic structure of BaTiO3, and is therefore likely to be indirectly correlated with the occurrence of ferroelectricity in this material.  相似文献   

16.
The effect of an external magnetic field on ferroelectric characteristics of BaTiO3-type crystals is discussed and estimated in the framework of the vibronic theory of ferroelectricity.  相似文献   

17.
We report on exotic dielectric properties of composite materials consisting of traditional ferroelectric BaTiO3 and electronic ferroelectric LuFe2O4. The composites were synthesized by a simple ceramic processing method with properties that can be tailored by adjusting LuFe2O4 concentration. A percolative effect was observed at the volume fraction of ~0.26, which led to an abrupt increase in the dielectric permittivity. For composites with high LuFe2O4 fractions, a giant dielectric tunability was observed over a broad temperature interval, which is in strong contrast to traditional ferroelectrics.  相似文献   

18.
Dense nanocrystalline BaTiO3 ceramics with uniform grain sizes of 30 nm was obtained by pressure assisted sintering. The phase transitions were investigated by Raman scattering at temperatures ranging from −190 to 200 °C. With increasing temperature, similar to 3 μm BaTiO3 normal ceramics, the successive phase transitions from rhombohedral to orthorhombic, orthorhombic to tetragonal, tetragonal to cubic were also observed in 30 nm BaTiO3 ceramics. Especially, the coexistence of ferroelectric tetragonal and orthorhombic phases was found at room temperature. The ferroelectric behavior was further characterized by P-E hysteresis loop. The experimental results indicate that the critical grain size of the disappearance of ferroelectricity in nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering is below 30 nm.  相似文献   

19.
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