Relaxor ferroelectrics are an extremely important class of functional
electroceramic materials which shows giant dielectric permittivities and strong
piezoelectric properties. Their structures invariably exhibit a complex hierarchy of
scales, from the very local to the nanoscale to the long range. It is this complexity from
which their functional behavior derives. The current knowledge of the structure-function
relationship of relaxor ferroelectric materials still remains at an...[Show more] unsatisfying
level and more experimental studies are clearly required. With the satisfaction of this
requirement as a general aim for this thesis, two types of relaxor materials,
Pb(In1/2Nb1/2O₃ -Pb(Mg1/3Nb2/3)O₃ -PbTiO₃ (PIN-PMN-PT) single crystals and doped-
(Ba,Sr)TiO₃ ceramics, were investigated using neutron scattering and piezoresponse
force microscopy (PFM).
Single crystal neutron diffraction studies of PIN-PMN-PT single crystals (with
specific compositions) reveal changes in metric symmetry as a function of temperature,
electrical poling as well as applied mechanical stress. It is found that a uniaxial stress
longitudinally applied along the  direction induces a metastable orthorhombic
phase out of the initial pseudo-rhombohedral phase at critical stresses dependent on the
temperature. The calculated lattice strains agree well with the reported static mechanical
properties. The results provide structural evidence for the polarization rotation theory. A
comparative study between the crystals poled along  and  directions show
that, in the former case, an orthorhombic (or monoclinic Pm) phase is stable at room
temperature whereas, in the latter case, the corresponding phase is of rhombohedral
symmetry. Electrical poling is shown to enhance the long range ferroelectric order and
leads to the rhombohedral to tetragonal phase transition becoming clearly apparent in
both the dielectric behavior as well as the diffraction results.
Strictly elastic diffuse scattering confirms the presence of polar nano-regions
(PNR’s), the definitive local structure of relaxors, in the PIN-PMN-PT system. The
PNR’s start to grow in population at the Bums temperature, TB ~630 K and concurrently
the dipole correlation lengths, an indicator of the PNR size, become larger. At 300 K,
characteristic “butterfly” and ellipsoid shaped diffuse scattering patterns are observed
on the HKO reciprocal space plane. Electrical poling along the  direction produces
a marked asymmetry in the diffuse scattering patterns, with the parallel-to-the-field
components enhanced at the cost of the perpendicular-to-the-field components. Several low energy phonon branches along the  and  reciprocal space directions have
been measured using triple-axis neutron spectroscopy. Most significantly, PNR-acoustic
mode coupling is confirmed for the  transverse acoustic phonons polarized along
the [1-10] real space direction. This coupling is anisotropic and correlates with the
distribution of the PNR’s. The famous “waterfall” phenomenon is observed on the 
and  transverse optical branches. The zone-center optical phonon measurements
also indicate a soft mode behavior of the system.
Real space PFM imaging shows a highly disordered, labyrinthine domain pattem
on the (001) and (111) surfaces in contrast to the (110) surface domain pattem that
develops a preferential alignment along the <110> directions. The formation of these
domain patterns is interpreted as a modulation effect associated with the presence of the
PNR’s. PFM switching spectroscopy mapping on an (001) surface reveals the
distribution pattem of near surface defect-related internal charge fields, which
nonetheless is not correlated with the domain pattern thereof. The local polarization
switching behavior is similar for the (001) and (110) surfaces but apparently different
from the (111) surface possibly due to an extrinsic effect of the internal charge field.
Doped-(Ba,Sr)TiO₃ ceramic samples were also studied as a model BaTiO₃ -based
relaxor material. As this relaxor is in the so-called ergodic paraelectric phase at room
temperature, no pristine ferroelectric domains are observed in PFM. However, shortlived/
fast-relaxing surface polar states can be induced by applied tip biases as a result of
the polar precursor present, i.e., the PNR’s, and well-defined piezoresponse hysteresis
loops are thus formed. The relaxation behavior of the induced polar states is related to
the PNR dynamics of the system. Spatially-resolved piezoresponse relaxation mapping
reveals the presence of sub-micron correlation features due to the chemical
inhomogeneity of the A-site Ba/Sr elements, as evidenced by a site-correlated elemental
mapping microanalysis. This work thus demonstrates a novel approach to study the
structure-property relationship at the nano/meso-scales for relaxor materials.
The extrinsic effects of surface charge dynamics in PFM were explored on PIN-PMN-PT single crystals. Abnormal domain switching phenomena and fully-inverted
piezoresponse hysteresis loops are observed, providing evidence for the charge transport
between the tip-sample junction and its coupling with local ferroelectric polarization.
Further insights into this issue are obtained using switching spectroscopy Kelvin probe
force microscopy, a new technique developed in this thesis.
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