摘要:

We have measured coercive field, remanent polarization, built-in bias field, and the dielectric constant and loss in Mn-doped lead scandium tantalatePbSc1/2Ta1/2O3thin films from 15–400 K. Evidence for four low-temperature anomalies near 50 K, 100 K, 160 K, and 233 K is summarized, and a discussion is presented of whether these are phase transitions. For comparison, ceramic samples were studied via X-ray techniques down to 20 K, and via DSC (differential scanning calorimetry) techniques near 233 K. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399684

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We have computed the elastic constant tensor forPbZrTiO6,PZT 50/50, in the rhombohedral structure. We find it to be very soft elastically, and the Young’s modulus to be very anisotropic. Our results for PZT are compared with literature values on other oxide perovskites, and the new experimental results on PMN-PT and PZN-PT. The elastic response is an important part of the electromechanical response, and should be studied experimentally and theoretically to help build up a systemic understanding of electromechanical materials. We find that anisotropy in Young’s modulus and elastic softness may be key discriminators in evaluating or improving electromechanical properties. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399685

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The lattice total energy surface and the structural instabilities that characterize rhombohedralPbZrxTi1−xO3in the morphotropic phase boundary region nearx=0.5are investigated using first principles density functional supercell calculations. As expected, we find a strong ferroelectric instability. However, we also find a substantial R-point rotational instability, close to but not as deep as the ferroelectric one. This is similar to the situation in purePbZrO3.These two instabilities are both strongly pressure dependent, but in opposite directions so that lattice compression of less than 1&percent; is sufficient to change their ordering. Because of this, local stress fields due to B-site cation disorder may lead to coexistence of both types of instability are likely present in the alloy near the morphotropic phase boundary. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399686

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The atomic structure ofPb(Zr,&hthinsp;Ti)O3solid solutions near the morphotropic phase boundary (MPB) was examined using time-of-flight neutron diffraction. In addition to the conventional crystallographic refinement the atomic pair distribution function (PDF) analysis was used. Crystallographic analysis suggests that the average structure changes significantly through the MPB. However, the PDF analysis, which is more accurate in describing the local structure, shows that changes are more gradual. In particular the PDF suggests that the local environment of each element remains relatively invariant of composition. Ti is always ferroelectrically active, while Zr is not. Pb is always displaced against oxygen atoms and forms short bonds at about 2.45 Å. What changes most with the Ti/Zr ratio is the distribution in the direction of Pb displacement. It is suggested that the population of local Pb displacements changes between ⟨100⟩ and ⟨110⟩ in the pseudocubic notation with the Ti/Zr ratio, and the MPB is a crossover point with maximum disorder. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399687

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The temperature dependence of the superlattice reflections: a) F spots and b) &agr; spots in a lead magnesium niobate (PMN) single crystals containing 0&percent; and 6&percent; ofPbTiO3(PT) has been studied using synchrotron x-ray scattering techniques. (No superlattice reflections were found in PMN doped with 32&percent; PT). Analysis of the temperature dependence of the &agr; spots suggests the existence of the correlated anti-parallel atomic displacements that form nanoregions different from the chemical nanodomains. While the correlation length is temperature independent, the magnitude of these displacements increases on cooling below the freezing temperatureTf.Intensities of the &agr; spots above this temperature become indistinguishable from the background. Our results show that value ofTffor each composition is very close to the one obtained from a Vogel-Fulcher fit to the frequency dependence of the dielectric constant maximumTm.The relation of these correlated anti-ferrodistortive fluctuations to polar ferroelectric nanodomains and relaxor behavior needs further study. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399688

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The dynamical properties of relaxor ferroelectrics have been studied by broadband Brillouin scattering. In 0.65PMN-PT an intense central peak (CP) was observed in a wide temperature range below about 250°C. The relaxation time determined from CP shows slowing down above the cubic to tetragonal transition temperatureTct=158°C.Under the assumption that the origin of CP is the dynamical behavior of polar micro regions, this behavior was well reproduced by the extended superparaelectric model aboveTct.The intense CP of uniaxial relaxor 0.61SBN with the dielectric maximum temperatureTm=70°Cwas also observed in a wide temperature range below aboutTc+400°C.The anisotropy of CP intensity indicates that CP of 0.61SBN is originated mainly from fluctuating polar clusters along the c-axis. However, the slowing down of the relaxation time determined from CP stopped at aboutTm+100°C.This fact may indicate growing interaction between PMRs in 0.61SBN. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399689

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

A first-principles-derived approach is used to study finite-temperature properties of aPb(Sc,&hthinsp;Nb)O3alloy that is compositionally sinusoidally modulated along the [001] crystallographic direction. Thelocalelectrical polarizations—centered in the planes perpendicular to the modulation direction—are found to be spatially-dependent for any temperature. This unusual structural feature can be viewed as a direct consequence of an internal composition-modulation-induced electric-field. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399690

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

This paper examines the domain patterns and its relation to the macroscopic electromechanical behavior of ferroelectric solids using a theory based on homogenization and energy minimization. The domain patterns in different crystalline systems are classified, the spontaneous strain and polarization for single crystals and polycrystals are characterized, and the optimal texture of polycrystals for high-strain actuation is identified. The results also reveal why it is easy to pole PZT at compositions close to the ‘morphotropic phase boundary.’ ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399691

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The Vienna Ab-initio Simulation Package (VASP) was used to perform fully relaxed, planewave pseudopotential calculations of formation energies(&Dgr;EV&hthinsp;ASP)for a large number of ordered supercells in the perovskite based systemNaTiO3−BiTiO3,including 38 supercells with the[Na1/2Bi1/2]TiO3(NBT) composition. The&Dgr;EV&hthinsp;ASPwere used to fit a cluster expansion Hamiltonian to verify that the no other superstructure is predicted to have lower energy than the lowest&Dgr;EV&hthinsp;ASP,which is a 40 atom supercell with space group symmetryP11m.Its chemical ordering is characterized by pseudocubic doubling of the cell constants, with alternating [100] rows of Na and Bi atoms in (hk0) planes plus alternating [010] rows in (hk1/2) planes. Chemical ordering alone reduces space group symmetry from cubic,Pm3¯m,to tetragonal,P42/mmc,and octahedral tilting (a−a−c+system) further reduces it to monoclinic,P11m.©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399692

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We have recently developed a geometric formulation of the stress field for an interacting quantum system within the local density approximation (LDA) of density functional theory (DFT). We obtain a stress field which is invariant with respect to choice of energy density. In this paper, we explicitly demonstrate this uniqueness by deriving the stress field for different energy densities. We also explain why particular energy densities give expressions for the stress field that are more tractable than others, thereby lending themselves more easily to first-principles calculations. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399693

出版商:AIP    

年代:1901

数据来源: AIP