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Neutron scattering and piezoresponse force microscopy studies of several relaxor ferroelectric materials




Li, Qian

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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 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 [001] 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 [001] and [111] 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 [110] 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 [001] and [111] reciprocal space directions have been measured using triple-axis neutron spectroscopy. Most significantly, PNR-acoustic mode coupling is confirmed for the [110] 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 [001] and [110] 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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