共查询到20条相似文献,搜索用时 15 毫秒
1.
C.J. Xiao W.W. Zhang S.M. Feng X.H. Wang L.T. Li 《Journal of Physics and Chemistry of Solids》2007,68(2):311-314
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
2.
W. Dmowski M.K. AkbasT. Egami P.K. Davies 《Journal of Physics and Chemistry of Solids》2002,63(1):15-22
The relaxor ferroelectric compound Pb(Mg0.3Ta0.6Zr0.1)O3 (PMT-PZ) was studied by X-ray, neutron and electron diffraction and transmission electron microscopy in the as-sintered and annealed states. The as-sintered sample was comprised of nanometer-sized 1:1 chemically ordered domains dispersed in a disordered matrix. After annealing at 1325°C the domain size increased to ∼30 nm and the degree of order exceeded 95% in terms of the volume fraction of the ordered domains, yet the sample retained its diffuse, frequency dependent relaxor characteristics. Refinements of the chemically ordered structure using the Rietveld analysis revealed that the octahedral (B) site occupancies were in excellent agreement with a “random site” model for the chemical ordering. In this charge-balanced model for the 1:1 ordered Pb(β′1/2β″1/2)O3 structure the Ta cations predominantly occupy the β″ site, while the β′ site is populated by a random distribution of the Mg, Zr and remaining Ta cations. Large temperature factors for Pb and O atoms are observed in both as-sintered and annealed samples, indicating localized displacements of the Pb and O atoms. The mixed occupancy of the β′ position appears to be responsible for the relaxor characteristics in the dielectric response in spite of the growth of the chemical domains. 相似文献
3.
Lead-free (Na0.5K0.5)NbO3-based piezoelectric ceramics were successfully fabricated by substituting with a small amount of BiFeO3 (BF). Difficulty in sintering of pure NKN ceramics can be eased by adding a few molar percent of BF, and the crystalline structure is also changed, leading to a morphotropic phase boundary (MPB) between ferroelectric orthorhombic and rhombohedral phases. The MPB exists near the 1-2 mol% BF-substituted NKN compositions, exhibiting enhanced ferroelectric, piezoelectric, and electromechanical properties of Pr=23.3 μC/cm2, d33=185 pC/N, and kp=46%, compared to an ordinarily sintered pure NKN ceramics. The MPB composition has a Curie temperature of ∼370 °C, comparable to that of some commercial PZT materials. 相似文献
4.
Structural and electronic properties produced by formation of Schottky defects in cubic structure of SrTiO3 crystal are investigated by means of a quantum-chemical simulation based on the Hartree-Fock methodology. The occurrence of Sr partial Schottky defect (VSr+VO) and two types of Ti partial Schottky defects (VTi+2VO) is modeled using a supercell containing 135 atoms. Vacancy-induced changes in the positions of their neighboring atoms are analyzed in light of the computed electron density redistribution in the defective region of supercell. The observed local one-electron energy levels in the gap between the upper valence band and the conduction band can be attributed to the presence of anion and cation vacancies. 相似文献
5.
The electronic properties of the charge carriers at the LaAlO3/SrTiO3 interfaces are investigated by first principles studies. For the n-type interface, the carriers are located only on the SrTiO3 side. For the p-type interface, the carriers are highly localized at the interface. A critical thickness of the LaAlO3 overlayer exists, below which, the interface is insulating. Moreover, we show that the effective masses and mobilities of the carriers are spatially anisotropic and have a strong disparity for the two types of carriers. These results are consistent with experimental observations and are explained by the band structures and alignments of the consisting oxides and their interaction at the interfaces. 相似文献
6.
The effect of a small amount Fe2O3 (0.1-2 mol%) doping on the electrical properties of (Na0.5K0.5)0.96Li0.04Nb0.86Ta0.1Sb0.04O3 (NKLNTS) ceramics was investigated. It was found that the B-site substitution of Fe3+ does not change the crystal structure within the studied doping level and all modified ceramics have a pure tetragonal perovskite structure at room temperature. The addition of Fe2O3 can promote the sintering of NKLNTS ceramics, and simultaneously cause the grain growth so that Fe3+-doped NKLNTS compositions show degraded densification at higher doping level. Furthermore, the dielectric properties of the NKLNTS ceramics do not show a significant change by Fe2O3 doping. However, the addition of Fe2O3 was found to have a significant influence on the electric fatigue resistance and the durability against water. The presence of oxygen vacancies caused by the replacement of Fe3+ for B-site ions makes the NKLNTS ceramics harder. 相似文献
7.
Hai-Yao Deng 《Solid State Communications》2011,151(6):474-3102
Recently, a clear long-sought Debye mode (D) in barium titanate (BT) was identified [J. Hlinka, T. Ostapchuk, D. Nuzhnyy, J. Petzelt, P. Kuzel, C. Kadlec, P. Vanek, I. Ponomareva, L. Bellaiche, Phys. Rev. Lett. 101 (2008) 167402] and this mode was shown to originate from the polar coordinate that also generates the so-called Slater mode (S). The inter-relations between the D mode and the normal A1-type phonon modes were studied by those authors using a four-mode phenomenological model. The present work is to offer an atomistic support of their work and to better illustrate the nature of and the couplings between these modes. In addition, we extend the as-obtained insights to the investigation of the nature of the low frequency Raman peaks that were found in 18O-substituted strontium titanate (ST) many years ago [M. Itoh, R. Wang, Y. Inaguma, T. Yamaguchi, Y.-J. Shan, T. Nakamura, Phys. Rev. Lett. 82 (1999) 3540]. Thus, our work provides important information on the connections between the lattice dynamics of BT and ST. 相似文献
8.
R. Ranjith 《Journal of Physics and Chemistry of Solids》2010,71(8):1140-1143
BiFeO3 has been studied extensively due to its room temperature multiferroic features and has been proven as a promising candidate for device applications. But BiFeO3 possesses some drawbacks like high leakage current and complicated magnetic ordering, giving rise to a canted antiferromagnetic behavior. Hence, a superlattice approach of BiFeO3 and BaTiO3 with a good lattice matching was fabricated and the room temperature ferroelectric and ferromagnetic properties were studied. The macroscopic and local probe studies reveal a ferroelectric nature at room temperature, and most importantly a weak ferromagnetic like behavior was observed. The ferromagnetic behavior is expected to arise due to the variation introduced in the spin modulation of single BiFeO3 layer due to the superstructure formation. 相似文献
9.
《Journal of Physics and Chemistry of Solids》2002,63(3):471-481
Two sets of ferroelectric (Pb,La)TiO3 thin films have been prepared by a diol-based sol-gel technique by varying the heating rate to the crystallisation temperature of 650°C. Films of increasing thickness were obtained by repeating the solution deposition from 1 to 5 times. The switchable polarisation was evaluated from hysteresis loops and from the integration of the switching current transients during pulse tests, and was found to significantly depend on film thickness. Measurements of the thickness dependence of the reciprocal capacitance and Rutherford backscattering spectrometry results indicate that a layer with different dielectric permittivity, and composition for one set of films, existed next to the bottom electrode. In the one set of films it originated because of interdiffusion between the film and the substrate, while in the other set, it was linked to the tensile stress at the film/substrate interface. These layers had a reduced switchable polarisation, which caused the observed dependence of their properties on thickness. 相似文献
10.
Vacancy-induced magnetism in perovskite SrTiO3 is investigated by ab initio calculations and magnetic measurements. The calculations of the generalized gradient approximation (GGA), the local density approximation (LDA) and the local density approximation with on-site effect U (LDA+U) methods show that stoichiometric SrTiO3 is nonmagnetic. The GGA calculated results indicate that Ti or O vacancy could induce magnetism rather than Sr vacancy. The LDA and LDA+U calculations show that the Ti vacancy could induce magnetism, while Sr and O vacancies couldn't. The experimental results confirm that SrTiO3 nanocrystalline powders exhibit room-temperature ferromagnetism (FM) and the magnetic moment results from cation vacancies. 相似文献
11.
J.L. Sánchez Ll J.T. Elizalde-GalindoJ.A. Matutes-Aquino 《Solid State Communications》2003,127(7):527-530
High coercivity nanostructured YCo5 powders were successfully prepared by mechanical milling of as-cast alloys and subsequent vacuum annealing. Almost single phase YCo5 alloys, obtained by arc melting, were processed by high energy mechanical milling using a SPEX 8000 mill. After 4 h of milling, powders become nearly amorphous. DSC scans revealed the existence of an irreversible broad exothermic transition with a maximum at 516 °C associated with the crystallization process. Annealing in high vacuum at 800 °C during 2.5 min led to the formation of YCo5 nanoparticles with an average particle size of 12 nm. A high intrinsic coercivity of 7.23 kOe together with a σr/σs ratio of 0.75 were obtained. 相似文献
12.
Epitaxial (001)-oriented PbSc0.5Ta0.5O3 (PST) thin films were deposited by pulsed laser deposition. Local piezoelectric investigations performed by piezoelectric force microscopy show a dual slope for the piezoelectric coefficient. A piezoelectric coefficient of 3 pm/V was observed at voltages up to 0.8 V. However, at voltages above 0.8 V, there is a steep increase in piezoelectric coefficient mounting to 23.2 pm/V. This nonlinear piezoelectric response was observed to be irreversible in nature. In order to better understand this nonlinear behavior, voltage dependent dielectric constant measurements were performed. These confirmed that the piezoelectric non-linearity is indeed a manifestation of a dielectric non-linearity. In contrast to classical ferroelectric systems, the observed dielectric non-linearity in this relaxor material cannot be explained by the Rayleigh model. Thus the dielectric non-linearity in the PST films is tentatively explained as a manifestation of a percolation of the polar nano regions. 相似文献
13.
C.B. SamantarayAnushree Roy M. RoyM.L. Mukherjee S.K. Ray 《Journal of Physics and Chemistry of Solids》2002,63(1):65-69
Thin films of Ba0.8Sr0.2TiO3 have been deposited on p-type Si substrate by radio frequency magnetron sputtering. Polycrystalline bulk Ba0.8Sr0.2TiO3 sample has also been studied for comparison. X-ray diffraction patterns reveal that both the bulk sample and thin films are polycrystalline without any preferential orientation and belong to paraelectric cubic phase. We have compared the room temperature Raman and IR spectra of powder and thin films (both annealed and as-deposited) of Ba0.8Sr0.2TiO3. The extra feature in the Raman spectrum for the annealed film has been explained as due to the presence of intergrain stresses from the submicron size grains in it. 相似文献
14.
D.V. Azamat A. Dejneka J. Lančok L. Jastrabik V.A. Trepakov Z. Bryknar E.V. Neverova A.G. Badalyan 《Journal of Physics and Chemistry of Solids》2014
The trivalent chromium centers were investigated by means of electron paramagnetic resonance (EPR) in SrTiO3 single crystals grown using the Verneuil technique. It was shown that the charge compensation of the Cr3+-VO dominant centers in octahedral environment is due to the remote oxygen vacancy located on the axial axis of the center. In order to provide insight into spin-phonon relaxation processes the studies of axial distortion of Cr3+-VO centers have been performed as function of temperature. The analysis of the trigonal Cr3+ centers found in SrTiO3 indicates the presence of the nearest-neighbor strontium vacancy. The next-nearest-neighbor exchange-coupled pairs of Cr3+ in SrTiO3 has been analyzed from the angular variation of the total electron spin of S=2 resonance lines. 相似文献
15.
Yoshiki Ozaki Mizuhiko Ichikawa Torbjörn Gustafsson 《Journal of Physics and Chemistry of Solids》2004,65(6):1095-1102
Crystal structure of Rb3D(SeO4)2 has been investigated at 25 K (below the transition temperature Tc=95.4 K) by single-crystal neutron diffraction. Accompanying the transition, the SeO4 groups, which are all equivalent in the phase above the transition (space group A2/a), split into eight nonequivalent groups in a superlattice (a×2b×2c, space group A2) in the low-temperature phase. Based on the D atom positions obtained, each of the SeO4 groups was identified to be in the state closer to a HSeO4− ion or to a SeO42− ion and the dipole arrangement of SeO4-D-SeO4 dimer was revealed. This dipole arrangement has ‘ferri’ structure along the polar b-axis, but ‘antiferro’ structure in the plane perpendicular to the b-axis. These results are consistent with the characteristics found in the earlier dielectric measurements. 相似文献
16.
Donor doped positive temperature coefficient of resistivity barium titanate is highly sensitive to minor changes in processing conditions, Ba/Ti ratio, and dopant concentration. This leads to a lack of reproducibility of properties and microstructure. This study was performed in an effort to obtain a more microstructurally stable PTCR material. Barium titanate ceramics were prepared by blending Ba-excess BaTiO3 powder with Ti-excess powder, in different ratios. Such donor modified blended systems display uniform, medium grain size (4-6 μm), high-density microstructures which are more stable to changes in processing parameters. The microstructures are characterized by flat grain edges, large grain-to-grain contact area and high degree of domain coherence across grain boundaries. The PTCR effect was, however, measured to be nominal in these samples. This has been attributed to the presence of a smaller barrier potential, and such microstructural features as strong domain coherence across grain boundaries, large grain-to-grain contact area, and high density. It was found that the simple act of blending donor doped BaTiO3 powders of different Ba/Ti ratios drastically changes both microstructure and electrical properties. Blending results in the suppression of liquid-phase induced anomalous grain growth, suppressing grain growth processes and allowing sintering processes to go to a greater degree of completion. The proposed mechanism whereby this happens is that the presence of the donor in blended systems either changes the kinetics of liquid-phase formation and/or the wettability of grains, affecting liquid-phase distribution. 相似文献
17.
Qing Xu Yu-Heng Huang Wen Chen Byung-Kuk Ahn 《Journal of Physics and Chemistry of Solids》2008,69(8):1996-2003
(Na0.5Bix)0.93Ba0.07TiO3 (x=0.500-0.492) ceramics were prepared by a citrate method, and the structure and electrical properties of the ceramics were investigated with respect to the amount of Bi deficiency. It was detected that the Bi deficiency had a considerable impact on the crystal structure and microstructure. The inspection of both the temperature dependence of the dielectric properties (free permittivity ε33T/ε0 and dielectric loss tan δ) and the evolution of the polarization-electrical field (P-E) hysteresis loops with measuring temperature suggests that the Bi deficiency served to increase the depolarization temperature (Td). The Bi deficiency led to an increase in the coercive field (Ec) and mechanical quality factor (Qm) together with a decrease in the remanent polarization (Pr) and piezoelectric constants (d33). The variation of the structure and electrical properties with Bi deficiency amount was qualitatively interpreted in terms of the formation of Bi and oxygen vacancies in the Bi-deficient specimens. This research indicates the importance of adequately controlling Bi stoichiometry of (Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics in obtaining the desired ferroelectric and piezoelectric properties. 相似文献
18.
Nanocrystalline Cr2O3 and NiO are prepared using high-energy ball milling. Average sizes of the particles obtained from Scanning Electron Microscopy and crystallite sizes obtained from X-ray diffraction are larger for Cr2O3 than NiO particles. At low temperature, large high-field magnetization and small coercivity lead to a weak exchange bias for Cr2O3, whereas small high-field magnetization and large coercivity lead to a considerable exchange bias for NiO. The training effect is observed for NiO at 4 K which could be described with a recursive formula constructed in the framework of the spin configurational relaxation model. The results suggest that the pinning mechanism at the interface between the antiferromagnetic and the weak ferromagnetic component ascribed to uncompensated spins leads to the exchange bias effect. 相似文献
19.
Kenji Yoshii Yasuhiro Yoneda Ignace Jarrige Tatsuo Fukuda Yasuo Nishihata Chikashi Suzuki Yoshiaki Ito Takahito Terashima Shinzo Yoshikado Sei Fukushima 《Journal of Physics and Chemistry of Solids》2014
We have studied the electronic properties of the ferroelectric barium titanate BaTiO3 using two complementary 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 and Ti-K absorption edges. Contrary to a previous study, we found no fine structure in the pre-edge area of the PFY-XAS spectrum at the Ba-L3 edge, and no temperature-induced spectral change was observed between room temperature and 150 °C. This result is not supportive of the possible presence of the displacement around Ba2+ at the Curie temperature. RXES spectra were measured at the Ti-K edge for BaTiO3, along with SrTiO3 and La-doped metallic SrTiO3. The photon energy of the emission peak is found to be nearly constant throughout the absorption edge for all three compounds. We deduce the Ti 3d states to have a delocalized character, in contrast with the Ba 5d states, a property which is consistent with the proposed scenario of the formation of electric dipoles in BaTiO3. 相似文献
20.
Vesna Đorđević Željka AntićMarko G. Nikolić Miroslav D. Dramićanin 《Journal of Physics and Chemistry of Solids》2014
Eu3+-doped La2O3 nanocrystalline powder was prepared by polymer complex solution method and further used for preparation of Eu3+-doped La(OH)3. Structural and optical characterization was carried out by powder X-ray diffraction and photoluminescent spectroscopy. XRD measurements confirmed the formation of hexagonal La2O3 and its recrystallization into La(OH)3 in a humid atmosphere. Excitation spectra show redshift of host lattice and charge transfer emission bands in La(OH)3 while bands that correspond to Eu3+f–f transitions are placed at same wavelengths in both samples. Photoluminescence spectra recorded over the temperature range from 10 K to 300 K show that intensities of emission lines in Eu3+-doped La2O3 do not depend on temperature as much as in La(OH)3 sample. Observed dominant 5D0→7F2 and markedly visible 5D0→7F0 emissions in doped La2O3 indicate that Eu3+ ion is located in a structural site without an inversion center. On the other hand, in Eu3+-doped La(OH)35D0→7F0 transition is barely visible while 5D0→7F2 is not prominent, and with temperature drop three 5D0→7FJ (J=1, 2, 4) transitions become almost of the same intensity. In both La2O3 and La(OH)3 structures Eu3+ ion replaces La3+ in non-centrosymmetric C3v and C3h crystallographic sites, respectively, and difference in symmetry of the crystal field around europium ion is explained by comparing shape and volume of these sites. Decay times of the 5D0- level recorded over the temperature range 10−300 K revealed that emission lifetime values in La2O3 (~0.7 ms) are almost two times higher than in La(OH)3 (~0.4 ms), and unlike in La2O3, lifetime in La(OH)3 is temperature dependent. 相似文献