Ph.D. (Doctor of Philosophy)
Department of Physics
Physics; Condensed matter
Relaxor ferroelectrics are characterized by their dielectric permittivity, and some of these materials display outstanding electromechanical coupling, a property that makes them useful in applications from sonar and ultrasound to precision actuators. However, there is a surprising lack of consensus regarding the local structure of these materials and how it relates to their useful bulk properties. A common feature of many proposed mechanisms is the importance of short-range order differing from the long-range symmetry of the material. Such a difference between short-range and long-range order makes these materials ideal candidates for study via diffuse scattering, a technique sensitive to differences between local and average structure. Using modern techniques that allow a survey of a large volume of reciprocal space comprising dozens of Brillouin zones, diffuse neutron scattering data from across the entire phase diagram of the canonical relaxor ferroelectric system PMN-xPT has been collected, allowing for the identification of relevant diffuse scattering features. Complementary x-ray diffuse scattering measurements confirm the presence of these features, and the difference between scattering lengths of the two techniques provides contrast that shows how the different atoms in the material behave. By characterizing how these features evolve with temperature and composition, they provide a connection between the bulk properties of the system and local atomic displacements and correlations underlying these bulk properties. Four main components of diffuse scattering were found. Ferroic diffuse scattering, centered on the Bragg peak, is found to correlate in intensity to the strength of piezoelectric coupling and not directly to relaxational behavior. This study finds new structure to this component that indicates a significant role for correlated oxygen displacements in determining local polar structures. Broad, temperature-dependent diffuse peaks centered on M points in the Brillouin zone are indicative of antiferroelectric behavior that become a clear proxy for bulk relaxor behavior. Temperature-independent peaks centered on R points indicate chemical short-range order for pure PMN and are suppressed with the addition of titanium, suggesting that chemical order may seed relaxational character but is not responsible for piezoelectricity. Finally, size-effect scattering is identified and separated from other components, indicating highly anisotropic local environments. These findings are extended by studies of the electric field dependence of diffuse scattering from PMN-30PT which confirm the ferroic nature of diffuse scattering close to the Bragg peak and indicate that the local distortions revealed through size-effect diffuse scattering remain locally dominant even at high electric fields. The identification of similar features in diffuse scattering from the related material PZT indicates the applicability of these results to other lead-based perovskites with competing ferroic orders.
Krogstad, Matthew J., "Diffuse scattering and local order in lead-based relaxor ferroelectrics" (2018). Graduate Research Theses & Dissertations. 2237.
Northern Illinois University
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