749 research outputs found
Universal Phase Diagram for High-Piezoelectric Perovskite Systems
Strong piezoelectricity in the perovskite-type PbZr(1-x)TixO3 (PZT) and
Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) systems is generally associated with the
existence of a morphotropic phase boundary (MPB) separating regions with
rhombohedral and tetragonal symmetry. An x-ray study of PZN-9%PT has revealed
the presence of a new orthorhombic phase at the MPB, and a near-vertical
boundary between the rhombohedral and orthorhombic phases, similar to that
found for PZT between the rhombohedral and monoclinic phases. We discuss the
results in the light of a recent theoretical paper by Vanderbilt and Cohen,
which attributes these low-symmetry phases to the high anharmonicity in these
oxide systems.Comment: REVTeX file. 4 pages,=A0 4 figures embedde
Low-temperature phases in Pb(Zr0.52Ti0.48)O3: A neutron powder diffraction study
A neutron powder diffraction study has been carried out on Pb(Zr0.52Ti0.48)O3
in order to resolve an ongoing controversy about the nature of the
low-temperature structure of this strongly-piezoelectric and
technologically-important material. The results of a detailed and systematic
Rietveld analysis at 20 K are consistent with the coexistence of two monoclinic
phases having space groups Cm and Ic respectively, in the approximate ratio
4:1, and thus support the findings of a recent electron diffraction study by
Noheda et al. [Phys. Rev. B 66, 060103 (2002)]. The results are compared to
those of two recent conflicting neutron powder diffraction studies of materials
of the same nominal composition by Hatch et al. [Phys. Rev. B 65, 212101
(2002)] and Frantti et al. [Phys. Rev. B 66, 064108 (2002)].Comment: RevTex4, 16 pages, 6 color figure
New Features of the Morphotropic Phase Boundary in the PbZr(1-x)TixO3 System
Recently a new monoclinic phase in the PbZr(1-x)TixO3 ceramic system has been
reported by Noheda et al. for the composition x= 0.48. In the present work,
samples with Ti contents of x= 0.47 and 0.50, which are both tetragonal below
their Curie points, have been investigated. In the sample with x= 0.50, the
tetragonal phase was found to transform to a monoclinic phase at about 200 K as
the temperature was lowered. The sample with x= 0.47 showed a complicated
region of phase coexistence between 440-320 K, becoming rhombohedral at around
300 K. No further symmetry change was found down to 20 K. Dielectric
measurements for these two samples are also reported. On the basis of these
results, a preliminary phase diagram is presented. Optimum compositional
homogeneity is needed to properly characterize the new monoclinic region.Comment: 5 pages, 7 PS figures embedded. RevTeX and epsf macros. Presented at
the 9th. European Meeting on Ferroelectricity, Prague, July 1999. To be
published in "Ferroelectrics
The monoclinic phase in PZT: new light on morphotropic phase boundaries
A summary of the work recently carried out on the morphotropic phase boundary
(MPB) of PZT is presented. By means of x-ray powder diffraction on ceramic
samples of excellent quality, the MPB has been successfully characterized by
changing temperature in a series of closely spaced compositions. As a result,
an unexpected monoclinic phase has been found to exist in between the
well-known tetragonal and rhombohedral PZT phases. A detailed structural
analysis, together with the investigation of the field effect in this region of
compositions, have led to an important advance in understanding the mechanisms
responsible for the physical properties of PZT as well as other piezoelectric
materials with similar morphotropic phase boundaries.Comment: 5 pages REVTeX file, 6 figures embedded. Presented at the Workshop on
"Fundamental Physics of Ferroelectrics" held in Aspen, February 00. To appear
in the proceeding
Static and Dynamic Magnetism in Underdoped Superconductor BaFeCoAs
We report neutron scattering measurements on single crystals of
BaFeCoAs. The magnetic Bragg peak intensity is reduced by
6 % upon cooling through T. The spin dynamics exhibit a gap of 8 meV with
anisotropic three-dimensional (3d) interactions. Below T additional
intensity appears at an energy of 4.5(0.5) meV similar to previous
observations of a spin resonance in other Fe-based superconductors. No further
gapping of the spin excitations is observed below T for energies down to 2
meV. These observations suggest the redistribution of spectral weight from the
magnetic Bragg position to a spin resonance demonstrating the direct
competition between static magnetic order and superconductivity.Comment: 4 pages, 4 figure
Spin Waves in the Ferromagnetic Ground State of the Kagome Staircase System Co3V2O8
Inelastic neutron scattering measurements were performed on single crystal
Co3V2O8 wherein magnetic cobalt ions reside on distinct spine and cross-tie
sites within kagome staircase planes. This system displays a rich magnetic
phase diagram which culminates in a ferromagnetic ground state below Tc~6 K. We
have studied the low-lying magnetic excitations in this phase within the kagome
plane. Despite the complexity of the system at higher temperatures, linear
spin-wave theory describes most of the quantitative detail of the inelastic
neutron measurements. Our results show two spin-wave branches, the higher
energy of which displays finite spin-wave lifetimes well below Tc, and
negligible magnetic exchange coupling between Co moments on the spine sites.Comment: 4 pages and 4 figure
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
Lattice dynamics reveals a local symmetry breaking in the emergent dipole phase of PbTe
Local symmetry breaking in complex materials is emerging as an important
contributor to materials properties but is inherently difficult to study. Here
we follow up an earlier structural observation of such a local symmetry broken
phase in the technologically important compound PbTe with a study of the
lattice dynamics using inelastic neutron scattering (INS). We show that the
lattice dynamics are responsive to the local symmetry broken phase, giving key
insights in the behavior of PbTe, but also revealing INS as a powerful tool for
studying local structure. The new result is the observation of the unexpected
appearance on warming of a new zone center phonon branch in PbTe. In a harmonic
solid the number of phonon branches is strictly determined by the contents and
symmetry of the unit cell. The appearance of the new mode indicates a crossover
to a dynamic lower symmetry structure with increasing temperature. No
structural transition is seen crystallographically but the appearance of the
new mode in inelastic neutron scattering coincides with the observation of
local Pb off-centering dipoles observed in the local structure. The observation
resembles relaxor ferroelectricity but since there are no inhomogeneous dopants
in pure PbTe this anomalous behavior is an intrinsic response of the system. We
call such an appearance of dipoles out of a non-dipolar ground-state
"emphanisis" meaning the appearance out of nothing. It cannot be explained
within the framework of conventional phase transition theories such as
soft-mode theory and challenges our basic understanding of the physics of
materials
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