Structure and Property Evolution Induced by the Phase Transitions in Several Antiferroelectric Materials
Date
2017
Authors
Lu, Teng
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Abstract
Antiferroelectric (AFE) materials are an important group of
functional materials showing unique properties such as double
polarization-electric field (P-E) hysteresis loop, and charge
release under the pressure and temperature. These performances
are strongly connected to the structural phase transitions
induced by external conditions. Many investigations were carried
out to optimize their properties but the structure-property
relationship of such AFE materials still remain unclear. With
this bearing in mind, in this thesis, I firstly investigate the
crystal structure, domain structure and properties evolution
under the external stimuli such as electric-field (E-field),
mechanical force, and temperature in the typical PbZrO3-based AFE
materials. Secondly, a systematic study was conducted on the
doped silver niobate ceramics to understand the impacts of the
chemical composition and E-field cycling on these novel lead-free
AFE materials.
The targeted materials of typical PbZrO3-based samples were
selected as La/Nb doped Pb(Zr, Sn, Ti)O3 ternary systems with the
composition Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST1),
Pb0.99(Nb0.02Zr0.73Sn0.21Ti0.04)O3 (PNZST1) and
Pb0.99(Nb0.02Zr0.65Sn0.28Ti0.05)O3 (PNZST2). These three
compositions are representative, supplying diverse phase
transition behaviours for studying. The in situ neutron powder
diffraction (NPD) of the PLZST1 material reveals that the
pseudo-tetragonal AFE phase is transferred into the rhombohedral
FE phase with an application of the sufficient E-field, and
recovers after withdrawal of the external field. The resultant
average structure change as a function of the E-field is in
accordance with the reversible AFE-FE phase transition. However,
the ω dependent NPD patterns suggest this process is not fully
reversible: in the induced FE state, the strain exhibits an
elliptical distribution, which in turn leads to significant
preferred orientation in the final AFE state. The formation of
this preferred orientation provides an explanation for the
properties variation appearing in AFE materials after exposure to
the sufficiently high E-field.
X-ray diffraction pattern of PNZST1 sample indicates the
orthorhombic AFE phase while the result of NPD contradicts this
conclusion with a rhombohedral FE phase. After careful
characterization of the surface and bulk properties, it is found
that the near surface and bulk regions show different phases.
Additionally, the surface processing such as polishing and
heat-treatment can induce an AFE/FE phase transition within
micrometres of the surface. The in-situ hydrostatic-pressure
neutron diffraction proves that the mechanical force helps
stabilize the AFE phase of this composition. Therefore, the
surface processing induced phase transitions can be attributed to
the change of states of residual stress.
The in-situ NPD studies of PNZST2 material describe its
structural variation as a function of E-field and temperature.
Through the mode decomposition approach, the relationships
between AFE/FE modes and octahedral rotation mode were
systematically investigated. At room temperature, the pristine
AFE phase can be poled into the meta-stable FE phase by applying
the external E-field. At this stage, both AFE and FE phases
consist of modes associated with octahedral rotation and A-site
ionic displacements. The temperature-induced phase transition
indicates that the octahedral rotation and ionic displacements
are weakly coupled in the room-temperature FE phase and decoupled
in the high-temperature FE phase. Furthermore, both temperature
and E-field-induced phase transitions between the AFE and
high-temperature FE phase demonstrate the critical role of
coupling between the octahedral rotation and A-site ionic
displacements in AFE structure stabilization.
The evolution of structure and electrical properties with
composition in (1-x)AgNbO3-xLiTaO3 (ANLT100x) (0 ≤ x ≤ 0.09)
ceramics have been systematically investigated by diffraction
techniques, complemented by dielectric and polarisation
measurements. The symmetry mode decomposition and Rietveld
refinement of distortive modes were firstly used to analyse the
origin of the anti/ferroelectricity observed. The in/out phase
octahedral tilting around the a-axis (H2 mode) and the
antiparallel ionic displacements (Λ3 mode), present large
amplitudes in the pure AgNbO3. These two modes vanish
progressively with increasing x and their amplitudes experience a
sudden drop when x = 0.053. Accompanied by the disappearance of
these two modes, a new phase with R3c symmetry appears and grows
with further increasing LiTaO3 content. The composition dependent
amplitudes of the primary modes, and R3c phase fractions, lead to
a comprehensive understanding of the dielectric and ferroelectric
properties affected by LiTaO3. For the composition located around
the phase boundary, x = 0.045 and 0.06, FE wake-up effects were
detected. The refinement of neutron diffraction patterns after
different electric cyclicity describe an increase of
ferroelectricity associated with the R3c phase fraction
increments i.e., field-cycling-induced phase transition from
Pmc21 to R3c. The local probes such as the electron diffraction
and piezoresponse microscopy (PFM), show that the in/antiphase
octahedral rotation around the <001>p and the local strain state
are the decisive factors for this field-cycling-induced phase
transition. In summary, the wake-up effects can be regarded as
the nucleation and growth of the R3c phase with increasing number
of electric cycles.
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Functional Materials, Ferroelectric, Antiferroelectric, Neutron scattering, Phase transitions
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