421 research outputs found
Size effects in multiferroic BiFeO3 nanodots: A first-principles-based study
An effective Hamiltonian scheme is developed to investigate structural and
magnetic properties of BiFeO3 nanodots under short-circuit-like electrical
boundary conditions. Various striking effects are discovered. Examples include
(a) scaling laws involving the inverse of the dots' size for the magnetic and
electric transition temperatures; (b) the washing out of some structural phases
present in the bulk via size effects; (c) the possibility of tailoring the
difference between the Neel and Curie temperatures, by playing with the size
and electrical boundary conditions; and (d) an universal critical thickness of
the order of 1.6 nm below which the dots do not possess any long-range ordering
for the electrical and magnetic dipoles, as well as, for the oxygen octahedral
tiltings.Comment: 3 figure
Soft Phonon Anomalies in the Relaxor Ferroelectric Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3
Neutron inelastic scattering measurements of the polar TO phonon mode
dispersion in the cubic relaxor Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3 at 500K reveal
anomalous behavior in which the optic branch appears to drop precipitously into
the acoustic branch at a finite value of the momentum transfer q=0.2 inverse
Angstroms, measured from the zone center. We speculate this behavior is the
result of nanometer-sized polar regions in the crystal.Comment: 4 pages, 4 figure
Domain evolution of BaTiO3 ultrathin films under electric field: a first-principles study
A first-principles-derived method is used to study the morphology and
electric-field-induced evolution of stripe nanodomains in (001) BaTiO3 (BTO)
ultrathin films, and to compare them with those in (001) Pb(Zr,Ti)O3 (PZT)
ultrathin films. The BaTiO3 systems exhibit 180o periodic stripe domains at
null electric field, as in PZT ultrathin films. However, the stripes alternate
along [1-10] in BTO systems versus [010] in PZT systems, and no in-plane
surface dipoles occur in BTO ultrathin films (unlike in PZT materials).
Moreover, the evolution of the 180o stripe domains in the BaTiO3 systems, when
applying and increasing an electric field along [001], involves four regions:
Region I for which the magnitude of the down dipoles (i.e., those that are
antiparallel to the electric field) is reduced, while the domain walls do not
move; Region II in which some local down dipoles adjacent to domain walls
switch their direction, resulting in zigzagged domain walls - with the overall
stripe periodicity being unchanged; Region III in which nanobubbles are
created, then contract along [110] and finally collapse; and Region IV which is
associated with a single monodomain. Such evolution differs from that of PZT
ultrathin films for which neither Region I nor zigzagged domain walls exist,
and for which the bubbles contract along [100]. Discussion about such
differences is provided.Comment: 19 pages, 4 figures, 27 references, submitted to Phys. Rev.
Monoclinic phase in the relaxor-based piezo-/ ferroelectric Pb(MgNb-PbTiO system
A ferroelectric monoclinic phase of space group ( type) has been
discovered in 0.65Pb(MgNb-0.35PbTiO by means of high
resolution synchrotron X-ray diffraction. It appears at room temperature in a
single crystal previously poled under an electric field of 43 kV/cm applied
along the pseudocubic [001] direction, in the region of the phase diagram
around the morphotropic phase boundary between the rhombohedral (R3m) and the
tetragonal (P4mm) phases. The monoclinic phase has lattice parameters a = 5.692
A, b = 5.679 A, c = 4.050 A and = , with the b-axis
oriented along the pseudo-cubic [110] direction . It is similar to the
monoclinic phase observed in PbZrTiO, but different from that
recently found in Pb(ZnNb-PbTiO, which is of space
group ( type).Comment: Revised version after referees' comments. PDF file. 6 pages, 4
figures embedde
Low temperature superlattice in monoclinic PZT
TEM has shown that the strongly piezoelectric material Pb(Zr0.52Ti0.48)O3
separates into two phases at low temperatures. The majority phase is the
monoclinic phase previously found by x-ray diffraction. The minority phase,
with a nanoscale coherence length, is a slightly distorted variant of the first
resulting from the anti-phase rotation of the oxygen octahedra about [111].
This work clears up a recent controversy about the origin of superlattice peaks
in these materials, and supports recent theoretical results predicting the
coexistence of ferroelectric and rotational instabilities.Comment: REVTeX4, 4 eps figures embedded. JPG version of figs. 2&4 is also
include
High pressure phases in highly piezoelectric Pb(Zr0.52Ti0.48)O3
Two novel room-temperature phase transitions are observed, via synchrotron
x-ray diffraction and Raman spectroscopy, in the Pb(Zr0.52Ti0.48)O3 alloy under
hydrostatic pressures up to 16 GPa. A monoclinic (M)-to-rhombohedral (R1) phase
transition takes place around 2-3 GPa, while this R1 phase transforms into
another rhombohedral phase, R2, at about 6-7 GPa. First-principles calculations
assign the R3m and R3c symmetry to R1 and R2, respectively, and reveal that R2
acts as a pressure-induced structural bridge between the polar R3m and a
predicted antiferrodistortive R-3c phase.Comment: REVTeX, 4 pages with 3 figures embedded. Figs 1 and 3 in colo
Anomalous transverse acoustic phonon broadening in the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O_3
The intrinsic linewidth of the transverse acoustic (TA) phonon
observed in the relaxor ferroelectric compound
Pb(MgNbTiO (PMN-20%PT) begins to broaden
with decreasing temperature around 650 K, nearly 300 K above the ferroelectric
transition temperature ( K). We speculate that this anomalous
behavior is directly related to the condensation of polarized, nanometer-sized,
regions at the Burns temperature . We also observe the ``waterfall''
anomaly previously seen in pure PMN, in which the transverse optic (TO) branch
appears to drop precipitously into the TA branch at a finite momentum transfer
\AA. The waterfall feature is seen even at
temperatures above . This latter result suggests that the PNR exist as
dynamic entities above .Comment: 6 pages, 4 figure
Stability of the monoclinic phase in the ferroelectric perovskite PbZr(1-x)TixO3
Recent structural studies of ferroelectric PbZr(1-x)TixO3 (PZT) with x= 0.48,
have revealed a new monoclinic phase in the vicinity of the morphotropic phase
boundary (MPB), previously regarded as the the boundary separating the
rhombohedral and tetragonal regions of the PZT phase diagram. In the present
paper, the stability region of all three phases has been established from high
resolution synchrotron x-ray powder diffraction measurements on a series of
highly homogeneous samples with 0.42 <=x<= 0.52. At 20K the monoclinic phase is
stable in the range 0.46 <=x<= 0.51, and this range narrows as the temperature
is increased. A first-order phase transition from tetragonal to rhombohedral
symmetry is observed only for x= 0.45. The MPB, therefore, corresponds not to
the tetragonal-rhombohedral phase boundary, but instead to the boundary between
the tetragonal and monoclinic phases for 0.46 <=x<= 0.51. This result provides
important insight into the close relationship between the monoclinic phase and
the striking piezoelectric properties of PZT; in particular, investigations of
poled samples have shown that the monoclinic distortion is the origin of the
unusually high piezoelectric response of PZT.Comment: REVTeX file, 7 figures embedde
A combined theoretical and experimental study of the low temperature properties of BaZrO3
Low temperature properties of BaZrO3 are revealed by combining experimental
techniques (X-ray diffraction, neutron scattering and dielectric measurements)
with theoretical first-principles-based methods (total energy and linear
response calculations within density functional theory, and effective
Hamiltonian approaches incorporating/neglecting zero-point phonon vibrations).
Unlike most of the perovskite systems, BaZrO3 does not undergo any
(long-range-order) structural phase transition and thus remains cubic and
paraelectric down to 2 K, even when neglecting zero-point phonon vibrations. On
the other hand, these latter pure quantum effects lead to a negligible thermal
dependency of the cubic lattice parameter below ~ 40 K. They also affect the
dielectricity of BaZrO3 by inducing an overall saturation of the real part of
the dielectric response, for temperatures below ~ 40 K. Two fine structures in
the real part, as well as in the imaginary part, of dielectric response are
further observed around 50-65 K and 15 K, respectively. Microscopic origins
(e.g., unavoidable defects and oxygen octahedra rotation occurring at a local
scale) of such anomalies are suggested. Finally, possible reasons for the facts
that some of these dielectric anomalies have not been previously reported in
the better studied KTaO3 and SrTiO3 incipient ferroelectrics are also
discussed.Comment: 8 pages, 5 figures, submitted to Physical Review
Compositional Inversion Symmetry Breaking in Ferroelectric Perovskites
Ternary cubic perovskite compounds of the form A_(1/3)A'_(1/3)A''_(1/3)BO_3
and AB_(1/3)B'_(1/3)B''_(1/3)O_3, in which the differentiated cations form an
alternating series of monolayers, are studied using first-principles methods.
Such compounds are representative of a possible new class of materials in which
ferroelectricity is perturbed by compositional breaking of inversion symmetry.
For isovalent substitution on either sublattice, the ferroelectric double-well
potential is found to persist, but becomes sufficiently asymmetric that
minority domains may no longer survive. The strength of the symmetry breaking
is enormously stronger for heterovalent substitution, so that the double-well
behavior is completely destroyed. Possible means of tuning between these
behaviors may allow for the optimization of resulting materials properties.Comment: 4 pages, two-column style with 3 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#sai_is
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