90 research outputs found
Theory of antiferroelectric phase transitions
At variance with structural ferroic phase transitions which give rise to
macroscopic tensors coupled to macroscopic fields, criteria defining
antiferroelectric (AFE) phase transitions are still under discussion due to the
absence of specific symmetry properties characterizing their existence. They
are recognized by the proximity of a ferroelectric (FE) phase induced under
applied electric field, with a double hysteresis loop relating the induced
polarization to the electric field and a typical anomaly of the dielectric
permittivity. Here, we show that there exist indeed symmetry criteria defining
AFE transitions. They relate the local symmetry of the polar crystallographic
sites emerging at an AFE phase transition with the macroscopic symmetry of the
AFE phase. The dielectric properties of AFE transitions are deduced from a
Landau theoretical model in which ferroelectric and ferrielectric phases are
shown to stabilize as the result of specific symmetry-allowed couplings of the
AFE order- parameter with the field-induced polarization.Comment: 7 pages, 5 figures, 1 tabl
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
Optical studies of ferroelectric and ferroelastic domain walls
Recent studies carried out with atomic force microscopy or high-resolution transmission electron microscopy reveal that ferroic domain walls can exhibit different physical properties than the bulk of the domains, such as enhanced conductivity in insulators, or polar properties in non-polar materials. In this review we show that optical techniques, in spite of the diffraction limit, also provide key insights into the structure and physical properties of ferroelectric and ferroelastic domain walls. We give an overview of the uses, specificities and limits of these techniques, and emphasize the properties of the domain walls that they can probe. We then highlight some open questions of the physics of domain walls that could benefit from their use
Phonon-phonon coupling in bismuth vanadate over a large temperature range across the monoclinic phase
In this work we study phonon-phonon coupling in bismuth vanadate (BiVO4),
known for its second-order transition involving a variety of coupling
mechanisms. Using Raman spectroscopy as a probe, we identify two optical
coupled phonon modes of the VO4 tetrahedron and study them by varying light
polarization and temperature. The coupling manifests in non-Lorentzian
line-shapes of Raman peaks and frequency shifts. We use theoretical framework
of coupled damped harmonic oscillators to model the coupling and capture the
phenomena in the temperature evolution of the coupling parameters. The coupling
is negligible at temperatures below 100 K and later increases in magnitude with
temperature until 400 K. The sign of the coupling parameter depends on the
light polarization direction, causing either phonon attraction or repulsion.
After 400 K the phonon-phonon coupling diminishes when approaching phase
transition at which the phonon modes change their symmetry and the coupling is
no longer allowed
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
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