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Seminar Speaker: James LeBeau, MIT

December 2 @ 11:00 am 12:00 pm

“Relaxor materials at the atomic scale: revealing how chemistry and structure govern their unique properties”

James LeBeau

Abstract

Relaxor ferroelectrics are functional oxides characterized by diffuse dielectric and piezoelectric properties that distinguish them from traditional ferroelectrics. This behavior is most commonly attributed to the existence of polar nanoregions—nanoscale domains that are embedded in an otherwise non-polar matrix. Despite its general acceptance, this “plum pudding” model fails to explain relaxor behavior in a variety of materials, including polymer-based systems. Moreover, models of the local polar behavior are largely based on diffuse scattering experiments using X-rays, neutrons, and electrons, which provide a measure of the average global and local structure across a relatively large volume of the material. Consequently, the origin of relaxor properties continues to be extensively debated although these materials have been studied for many decades.
In this talk, I will discuss how aberration-corrected scanning transmission electron microscopy (STEM) can be used to directly separate nanoscale structural and chemical inhomogeneities at the atomic scale in bulk and thin film relaxor ferroelectric materials, complementing diffraction studies. These techniques are applied to the relaxor ferroelectric system—Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). Simultaneous acquisition of images that are sensitive to chemistry (high-angle annular dark-field STEM) and light elements (integrated differential phase contrast STEM) enables direct correlation between nanoscale chemical order, distorted oxygen octahedra, and local polarization. First, we find that contrary to the prevailing model of a binary distribution of chemically ordered regions within a disordered matrix, the degree of chemical order smoothly varies within ordered domains and approaches a minimum at anti-phase boundaries. Second, regions of correlated oxygen octahedral titling are found to be anti-correlated with regions of maximal chemical order. Compared with the projected polarization, we observe that the regions of greatest variation in polarization correspond to the regions of maximum chemical order and maximum octahedral distortion. Based on these results, we show that both structural and chemical inhomogeneities act as a barrier to polarization rotation and thus frustrate long-range polar order. Finally, through in situ heating and/or bias in the sample, we show how the local chemical and atomic structure can provide insight into the regions where polarization rotation first occurs.

Speaker Biography

James earned his B.S. in Materials Science & Engineering from Rensselaer Polytechnic Institute in 2006 and his Ph.D. from the University of California Santa Barbara in 2010. After his graduate work, he joined the faculty of the Department of Materials Science and Engineering at North Carolina State University in January 2011. In 2019, he moved his group to the Department of Materials Science & Engineering at MIT. His research focuses on applying and developing (scanning) transmission electron microscopy techniques to quantify the atomic structure and chemistry of materials to inform our understanding of relaxor/ferroelectric, mechanical, optical, and quantum properties. For his research, he has been honored with numerous awards including the Presidential Early Career Award for Scientists and Engineers (PECASE), NSF CAREER award, an AFOSR Young Investigator grant, the Microanalysis Society K.F.J Heinrich award, and the Microscopy Society of America Burton Medal.

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