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Soft Mode Dynamics Above and Below the Burns Temperature in the Relaxor Pb(Mg_1/3Nb_2/3)O_3
We report neutron inelastic scattering measurements of the lowest-energy
transverse optic (TO) phonon branch in the relaxor Pb(Mg_1/3Nb_2/3)O_3 from 400
to 1100 K. Far above the Burns temperature T_d ~ 620 K we observe well-defined
propagating TO modes at all wave vectors q, and a zone center TO mode that
softens in a manner consistent with that of a ferroelectric soft mode. Below
T_d the zone center TO mode is overdamped. This damping extends up to, but not
above, the waterfall wave vector q_wf, which is a measure of the average size
of the PNR.Comment: 4 pages, 4 figures; modified discussion of Fig. 3, shortened
captions, added reference, corrected typos, accepted by Phys. Rev. Let
Soft Mode Anomalies in the Perovskite Relaxor Pb(Mg1/3Nb2/3)O3
Neutron inelastic scattering measurements of the polar TO phonon mode in the
cubic relaxor Pb(Mg1/3Nb2/3)O3, at room temperature, reveal anomalous behavior
similar to that recently observed in the Pb(Zn1/3Nb2/3)_{0.92}Ti_{0.08}O3
system in which the optic branch appears to drop precipitously into the
acoustic branch at a finite value of the momentum transfer q = 0.20
1/Angstroms, measured from the zone center. By contrast, a recent neutron study
showed that PMN exhibits a normal TO phonon dispersion at 800 K. We speculate
this behavior is common to all relaxor materials and is the result of the
presence of nanometer-scale polarized domains in the crystal that form below a
temperature Td, which effectively prevent the propagation of long wavelength (q
= 0) phonons.Comment: 5 pages, 4 figures To appear as an AIP Conference Proceedings Volume
for the Aspen 2000 Winter Conference on the Fundamental Physics of
Ferroelectric
Ferroelectric Dynamics in the Perovskite Relaxor PMN
We review results obtained from recent neutron scattering studies of the
lead-oxide class of perovskite relaxors PMN and PZN. A ferroelectric soft mode
has been identified in PMN at 1100 K that becomes overdamped near 620 K. This
is the same temperature at which polar nanoregions (PNR) begin to form, denoted
by Td, and suggests that a direct connection exists between the soft mode and
the PNR. The appearance of diffuse scattering intensity at Td reported by
Naberezhnov et al. lends further support to this picture. At lower temperature
the soft mode in PMN reappears close to Tc = 213 K (defined only for E > Ec).
These results are provocative because the dynamics below Tc are characteristic
of an ordered ferroelectric state, yet they occur in a system that remains
cubic on average at all temperatures. We discuss a coupled-mode model that
successfully describes these data as well as those from earlier lattice
dynamical studies of other perovskites such as BaTiO3.Comment: 10 pages, 7 figures, Conference - Fundamental Physics of
Ferroelectrics 200
Neutron Diffuse Scattering from Polar Nanoregions in the Relaxor Pb(Mg1/3Nb2/3)O3
We have studied the neutron diffuse scattering in the relaxor PMN. The
diffuse scattering appears around the Burns temperature (~620K), indicating its
origin from the polar nanoregions (PNR). While the relative diffuse intensities
are consistent with previous reports, they are entirely different from those of
the lowest-energy TO phonon. Because of that, it has been considered that this
TO mode could not be the ferroelectric soft mode. Recently, a neutron
scattering study has unambiguously shown that the TO mode does soften on
cooling. If the diffuse scattering in PMN originates from the soft mode
condensation, then the atomic displacements must satisfy the center of mass
condition. But, the atomic displacements determined from diffuse scattering
intensities do not fulfill this condition. To resolve this contradiction, we
propose a simple model in which the total atomic displacement consists of two
components: is created by the soft mode condensation, satisfying
the center of mass condition, and, represents a uniform
displacement of the PNR along their polar direction relative to the surrounding
(unpolarized) cubic matrix. Within this framework, we can successfully describe
the neutron diffuse scattering intensities observed in PMN.Comment: 7 pages, 7 figures (Revised: 11-16-2001
α-Arylation of cyclopentanone : palladium-catalyzed coupling of enamines and aryliodides & n-acyloxazines as novel bicyclic Weinreb amides
The introduction of an aryl group a to a carbonyl is a useful step in the preparation of a variety of biologically interesting compounds. The palladium-catalyzed a-arylation of ketones and esters, through their enolates, has received a great deal of attention lately. Despite these efforts, cyclopentanones remain one of the most difficult classes ofketones to arylate. Many of these methods suffer from the limitation of the relatively basic reaction conditions needed in order to prepare the enolate nucleopbiles inÂsitu. The direct palladium-catalyzed α-arylation of enamines as an alternative route to αÂaryl ketones bas received very little attention and might be a way to circumvent the strong base problem. In an effort to overcome this problem a method for the direct palladium-cawyzed a-arylation of unactivated enamines of cyclopentanone through the use of high-throughput screening was developed. When applied to reactions with 1- pyrollidino-1-cylcopentane and various iodobenzene derivatives we have achieved yields of above 90% for the arylated cyclopentanone in all successful cases at a one mmol scale
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