425 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
Phase transition in Pr0.5Ca0.5CoO3 and related cobaltites
We present an extensive investigation (magnetic, electric and thermal
measurements and X-ray absorption spectroscopy) of the Pr0.5Ca0.5CoO3 and
(Pr1-yYy)0.7Ca0.3CoO3 (y=0.0625-0.15) perovskites, in which a peculiar
metal-insulator (M-I) transition, accompanied with pronounced structural and
magnetic anomalies, occurs at 76 K and 40-132 K, respectively. The inspection
of the M-I transition using the XANES data of Pr L3-edge and Co K-edge proofs
the presence of Pr4+ ions at low temperatures and indicates simultaneously the
intermediate spin to low spin crossover of Co species on lowering the
temperature. The study thus definitively confirms the synchronicity of the
electron transfer between Pr3+ ions and Co^(3+/4+)O3 subsystem and the
transition to the low-spin, less electrically conducting phase. The large
extent of the transfer is evidenced by the good quantitative agreement of the
determined amount of the Pr4+ species, obtained either from the temperature
dependence of the XANES spectra or via integration of the magnetic entropy
change over the Pr4+ related Schottky peak in the low-temperature specific
heat. These results show that the average valence of Pr3+/Pr4+ ions increases
(in concomitance with the decrease of the formal Co valence) below TMI for
(Pr0.925Y0.075)0.7Ca0.3CoO3 up to 3.16+ (the doping level of the CoO3 subsystem
decreases from 3.30+ to 3.20+), for (Pr0.85Y0.15)0.7Ca0.3CoO3 up to 3.28+ (the
decrease of doping level from 3.30+ to 3.13+) and for Pr0.5Ca0.5CoO3 up to
3.46+ (the decrease of doping level from 3.50+ to 3.27+).Comment: 19 pages, 11 figure
Metal-insulator transition and the Pr/Pr valence shift in (PrY)CaCoO
The magnetic, electric and thermal properties of the
(Y)CaCoO perovskites (~=~Pr, Nd) were
investigated down to very low temperatures. The main attention was given to a
peculiar metal-insulator transition, which is observed in the praseodymium
based samples with and 0.15 at and 132~K, respectively.
The study suggests that the transition, reported originally in
PrCaCoO, is not due to a mere change of cobalt ions from
the intermediate- to the low-spin states, but is associated also with a
significant electron transfer between Pr and Co/Co sites,
so that the praseodymium ions occur below in a mixed
Pr/Pr valence. The presence of Pr ions in the insulating
phase of the yttrium doped samples (PrY)CaCoO
is evidenced by Schottky peak originating in Zeeman splitting of the ground
state Kramers doublet. The peak is absent in pure PrCaCoO
in which metallic phase, based solely on non-Kramers Pr ions, is
retained down to the lowest temperature.Comment: 10 figure
Topological magnetic structures of MnGe: a neutron diffraction and symmetry analysis study
From new neutron powder diffraction experiments on the chiral cubic
() magnet manganese germanide MnGe, we analyse all of the possible
crystal symmetry-allowed magnetic superstructures that are determined
successfully from the data. The incommensurate propagation vectors of the
magnetic structure are found to be aligned with the [100] cubic axes, and
correspond to a magnetic periodicity of about 30 at 1.8 K. Several
maximal crystallographic symmetry magnetic structures are found to fit the data
equally well and are presented. These include topologically non-trivial
magnetic hedgehog and "skyrmion" structures in multi- cubic 3+3 and
orthorhombic 3+2 dimensional magnetic superspace groups respectively, with
either potentially responsible for topological Hall effect [1]. The presence of
microstrain-like peak broadening caused by the transition to the magnetically
ordered state would seem to favour a "skyrmion"-like magnetic structure, though
this does not rule out the cubic magnetic hedgehog structure. We also report on
a new combined mechanochemical and solid-state chemical route to synthesise
MnGe at ambient pressures and moderate temperatures, and compare with samples
obtained by the traditional high pressure synthesis
Pressure and chemical substitution effects in the local atomic structure of BaFe2As2
The effects of K and Co substitutions and quasi-hydrostatic applied pressure
(P<9 GPa) in the local atomic structure of BaFe2As2, Ba(Fe{0.937}Co{0.063})2As2
and Ba{0.85}K{0.15}Fe2As2 superconductors were investigated by extended x-ray
absorption fine structure (EXAFS) measurements in the As K absorption edge. The
As-Fe bond length is found to be slightly reduced (<~ 0.01 Angstroms) by both
Co and K substitutions, without any observable increment in the corresponding
Debye Waller factor. Also, this bond is shown to be compressible (k =
3.3(3)x10^{-3} GPa^{-1}). The observed contractions of As-Fe bond under
pressure and chemical substitutions are likely related with a reduction of the
local Fe magnetic moments, and should be an important tuning parameter in the
phase diagrams of the Fe-based superconductors.Comment: 7 pages, 6 figure
Polarization rotation via a monoclinic phase in the piezoelectric 92%PbZn1/3Nb2/3O3-8%PbTiO3
The origin of ultrahigh piezoelectricity in the relaxor ferroelectric
PbZn1/3Nb2/3O3-PbTiO3 was studied with an electric field applied along the
[001] direction. The zero-field rhombohedral R phase starts to follow the
direct polarization path to tetragonal symmetry via an intermediate monoclinic
M phase, but then jumps irreversibly to an alternate path involving a different
type of monoclinic distortion. Details of the structure and domain
configuration of this novel phase are described. This result suggests that
there is a nearby R-M phase boundary as found in the Pb(Ti,Zr)O3 system.Comment: REVTeX file. 4 pages. New version after referees' comment
Second harmonic generation and X-ray diffraction studies of pretransitional region in relaxor K(1-x) LixTaO3
Optical second-harmonic-generation(SHG) observations and precise X-ray
diffraction experiments have been performed on quantum paraelectrics KTaO3
(KTO) and relaxor K(1-x)LixTaO3 with x=3% (KLT-3) and 7% (KLT-7). It is found
in KLT-3 and KLT-7 that a pretransitional region exists between two
characteristic temperatures Td and Tp (<Td). The average symmetry of the region
is tetragonal with a weak lattice-deformation but non-polar in average. The
temperature interval between Td and Tp is consistent with the interval on which
neutron diffuse scatterings have been previously reported. These facts strongly
suggest that polar micro-regions (PMRs) nucleate around Td and grow toward Tp.
Below Tp, a larger deformation and a field-induced SH intensity start to
develop, while no significant SHG appear in zero-field cooling process. The
temperature dependence of the SH intensity below Tp coincides well with that of
the tetragonality determined from the lattice deformation. The
Landau-Devonshire phenomenological approach suggests that the ferroelectric
phase transition at Tp is of first order and that it approaches the second
order transition with the decrease of Li concentration. A marked increase of
neutron diffraction intensities below Tp indicates that PMRs are transformed to
ferroelectric micro-domains at Tp, and the micro-domains change to macroscopic
ones under the electric field below Tp.Comment: 31 pages, 10 figure
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