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Department
of Chemistry
Institute
of Material Science
2:00
pm, Tuesday, November 15, 2005
Academic
Center, 7th Floor, Conference Room #736
Origin Of Large
Electromechanical Coupling In Relaxor Ferroelectrics
Ronald Cohen
Carnegie Institution of
Washington
Relaxor ferroelectrics
such as -x (PMN-xPT) will revolutionize transducer
applications such as medical ultrasound, sonar, etc. These are complex
materials, with much interesting physics arising from mesoscopic
structure and relaxation phenomena, but based on our studies the origin
of the large strain coupling is more straightforward, and not due
directly to these complications. We have used first-principles linear
response calculations and molecular dynamics to study PT, PMN, and
PMN-PT. Morphotropic phase transitions to lower symmetry rhombohedral
and monoclinic structures is predicted for the first time in a pure
material, PT, at high pressures, showing predicted piezoelectric
response as high as is seen in the giant coupling relaxor
ferroelectrics. It seems that the main role of the relaxor endmember,
for example PMN, is to bring these transitions down to zero pressure.
We have developed a transferable shell-model potential for PMN-xPT by
fitting to first-principles data. The potential qualitatively
reproduces the compositional phase diagram for PMN-xPT and is being
used to further understand these complex materials. A new
exchange-correlation potential, which gives excellent predictions for
ferroelectric materials, will also be discussed.
Biographical Sketch
Dr. Ronald E. Cohen
obtained his BS in Geology from Indiana University and AM, Ph.D. in
Geology from Harvard University. He has joined the Geophysical
Laboratory, Carnegie Institution of Washington as research geophysicist
since 1990. Dr. Cohen is one of the pioneers and authorities in the
field of ferroelectrics, has conducted research in theoretical mineral
physics, material science and high pressure physics, published over 170
papers, given over 140 invited talks and edited 6 books. He is a Fellow
of American Physical Society, American Geophysical Union, and
Mineralogical Society of America.