109 research outputs found
Order-Parameter Symmetries of Domain Walls in Ferroelectrics and Ferroelastics
The symmetry of boundaries between ferroelectric, ferroelastic and antiphase
domains is a key element for a theoretical understanding of their properties.
Here, we derive this symmetry from their organic relation to the symmetry of
the primary transition order parameters. The domain wall symmetries are shown
to coincide with directions of the order-parameter n-dimensional vector space,
corresponding to sum of the vectors associated with adjacent domain states.
This property is illustrated by the determination of the domain wall maximal
symmetries in BaTiO3, LaAlO3, SrTiO3 and Gd2(MoO4)3. Besides, the domain
pattern in YMnO3 is interpreted as resulting from an annihilation-creation
process, the annihilation of the antiphase domain walls creating six
ferroelectric domain walls merging at a single point.Comment: 5 pages, 3 figure
First-principles study of PbTiO under uniaxial strains and stresses
The behavior of PbTiO under uniaxial strains and stresses is investigated
from first-principles calculations within density functional theory. We show
that irrespectively of the uniaxial mechanical constraint applied, the system
keeps a purely ferroelectric ground-state, with the polarization aligned either
along the constraint direction ( phase) or along one of the pseudo-cubic
axis perpendicular to it ( phase). This contrasts with the cases of
isotropic or biaxial mechanical constraints for which novel phases combining
ferroelectic and antiferrodistortive motions have been previously reported.
Under uniaxial strain, PbTiO switched from a ground state under
compressive strain to ground-state under tensile strain, beyond a
critical strain \%. Under uniaxial stress, PbTiO
exhibits either a ground state under compression () or
a ground state under tension (). Here, however, an
abrupt jump of the structural parameters is also predicted under both
compressive and tensile stresses at critical values
GPa and GPa. This behavior appears similar to that predicted under
negative isotropic pressure and might reveal practically useful to enhance the
piezoelectric response in nanodevices.Comment: Submitted, 9 pages, 9 figure
High-pressure phase transitions in BiFeO3: hydrostatic vs. non-hydrostatic conditions
We report high-pressure x-ray diffraction experiments on BiFeO3 (BFO) single
crystals in diamond-anvil cells up to 14 GPa. Two data sets are compared, one
in hydrostatic conditions, with helium used as pressure-transmitting medium,
and the other in non-hydrostatic conditions, with silicon oil as
pressure-transmitting medium. It is shown that the crystal undergoes different
phase transitions in the two cases, highlighting the high sensitivity of BFO to
non-hydrostatic stress. Consequences for the interpretation of high-pressure
structural studies are discussed.Comment: 6 pages, 4 figure
Rules and mechanisms governing octahedral tilts in perovskites under pressure
The rotation of octahedra (octahedral tilting) is common in ABO3 perovskites
and relevant to many physical phenomena, ranging from electronic and magnetic
properties, metal-insulator transitions to improper ferroelectricity.
Hydrostatic pressure is an efficient way to tune and control octahedral
tiltings. However, the pressure behavior of such tiltings can dramatically
differ from one material to another, with the origins of such differences
remaining controversial. In this work, we discover several new mechanisms and
formulate a set of simple rules that allow to understand how pressure affects
oxygen octahedral tiltings, via the use and analysis of first-principles
results for a variety of compounds. Besides the known A-O interactions, we
reveal that the interactions between specific B-ions and oxygen ions contribute
to the tilting instability. We explain the previously reported trend that the
derivative of the oxygen octahedral tilting with respect to pressure (dR/dP)
usually decreases with both the tolerance factor and the ionization state of
the A-ion, by illustrating the key role of A-O interactions and their change
under pressure. Furthermore, three new mechanisms/rules are discovered. We
further predict that the polarization associated with the so-called hybrid
improper ferroelectricity could be manipulated by hydrostatic pressure, by
indirectly controlling the amplitude of octahedral rotations.Comment: Submitted to Phys. Re
Multiple high-pressure phase transitions in BiFeO3
We investigate the high-pressure phase transitions in BiFeO3 by single
crystal and powder x-ray diffraction, as well as single crystal Raman
spectroscopy. Six phase transitions are reported in the 0-60 GPa range. At low
pressures, up to 15 GPa, 4 transitions are evidenced at 4, 5, 7 and 11 GPa. In
this range, the crystals display large unit cells and complex domain
structures, which suggests a competition between complex tilt systems and
possibly off-center cation displacements. The non polar Pnma phase remains
stable over a large pressure range between 11 and 38 GPa, where the distortion
(tilt angles) changes only little with pressure. The two high-pressure phase
transitions at 38 and 48 GPa are marked by the occurence of larger unit cells
and an increase of the distorsion away from the cubic parent perovskite cell.
We find no evidence for a cubic phase at high pressure, nor indications that
the structure tends to become cubic. The previously reported insulator-to-metal
transition at 50 GPa appears to be symmetry breaking.Comment: 11 pages, 8 figure
Jahn-Teller, polarity and insulator-to-metal transition in BiMnO3 at high pressure
The interaction of coexisting structural instabilities in multiferroic
materials gives rise to intriguing coupling phenomena and extraordinarily rich
phase diagrams, both in bulk materials and strained thin films. Here we
investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four
interacting mechanisms: electric polarity, octahedra tilts, magnetism, and
cooperative Jahn-Teller distortion. We have probed structural transitions under
high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60
GPa. We show that BiMnO3 displays under pressure a rich sequence of five phases
with a great variety of structures and properties, including a metallic phase
above 53 GPa and, between 37 and 53 GPa, a strongly elongated monoclinic phase
that allows ferroelectricity, which contradicts the traditional expectation
that ferroelectricity vanishes under pressure. Between 7 and 37 GPa, the Pnma
structure remains remarkably stable but shows a reduction of the Jahn-Teller
distortion in a way that differs from the behavior observed in the archetypal
orthorhombic Jahn-Teller distorted perovskite LaMnO3.Comment: 5 pages, 3 figures + supplemental material included (3 pages, 1
figure, 3 tables
Revised structural phase diagram of (Ba0.7Ca0.3TiO3)-(BaZr0.2Ti0.8O3)
The temperature-composition phase diagram of barium calcium titanate zirconate (x(Ba0.7Ca0.3TiO3)(1-x)(BaZr0.2Ti0.8O3); BCTZ) has been reinvestigated using high-resolution synchrotron x-ray powder diffraction. Contrary to previous reports of an unusual rhombohedral-tetragonal phase transition in this system, we have observed an intermediate orthorhombic phase, isostructural to that present in the parent phase, BaTiO3, and we identify the previously assigned T-R transition as a T-O transition. We also observe the O-R transition coalescing with the previously observed triple point, forming a phase convergence region. The implication of the orthorhombic phase in reconciling the exceptional piezoelectric properties with the surrounding phase diagram is discussed
Bifurcated polarization rotation in bismuth-based piezoelectrics
ABO3 perovskite-type solid solutions display a large variety of structural and physical properties, which can be tuned by chemical composition or external parameters such as temperature, pressure, strain, electric, or magnetic fields. Some solid solutions show remarkably enhanced physical properties including colossal magnetoresistance or giant piezoelectricity. It has been recognized that structural distortions, competing on the local level, are key to understanding and tuning these remarkable properties, yet, it remains a challenge to experimentally observe such local structural details. Here, from neutron pair-distribution analysis, a temperature-dependent 3D atomic-level model of the lead-free piezoelectric perovskite Na0.5Bi0.5TiO3 (NBT) is reported. The statistical analysis of this model shows how local distortions compete, how this competition develops with temperature, and, in particular, how different polar displacements of Bi3+ cations coexist as a bifurcated polarization, highlighting the interest of Bi-based materials in the search for new lead-free piezoelectrics
Increasing bulk photovoltaic current by strain tuning
Photovoltaic phenomena are widely exploited not only for primary energy generation but also in photocatalytic, photoelectrochemistry, or optoelectronic applications. In contrast to the interface-based photovoltaic effect of semiconductors, the anomalous or bulk photovoltaic effect in ferroelectrics is not bound by the Shockley-Queisser limit and, thus, can potentially reach high efficiencies. Here, we observe in the example of an Fe-doped LiNbO3 bulk single crystal the existence of a purely intrinsic ``piezophotovoltaic'' effect that leads to a linear increase in photovoltaic current density. The increase reaches 75 under a low uniaxial compressive stress of 10 MPa, corresponding to a strain of only 0.005\%. The physical origin and symmetry properties of the effect are investigated, and its potential for strain-tuned efficiency increase in nonconventional photovoltaic materials is presented
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