Spectroscopic studies of ferroelectrics and polymers

Dr. Muhetaer Aihaiti (Geophysical Laboratory of the Carnegie Institution of Washington)

This talk will focus on our recent high pressure experimental work on (1) Brillouin scattering from polymers, and (2) Raman and x-ray scattering from ferroelectrics.

(1) The acoustic properties of three polymer elastomers have been measured from ambient pressure to approximately 12 GPa by using Brillouin scattering in a (high pressure) diamond anvil cell. For these elastomers, both transverse and longitudinal acoustic modes were observed, though the transverse modes were observed only at elevated pressures (>0.7 GPa) in all cases. From the Brillouin frequency shifts, longitudinal and transverse sound speeds were calculated, as were the C11 and C12 elastic constants, bulk, shear, and Young's moduli, and Poisson's ratios, and their respective pressure dependencies. P-V isotherms were then constructed, and fit to the equations of state to extract the isothermal bulk modulus and its pressure derivative for each material.

(2) We combined Raman scattering and x-ray diffraction to investigate PbTiO₃ under pressure (at 10 K). The experimental results indicate that lead titanate undergoes successive phase transitions with pressure: from tetragonal to monoclinic at 10 GPa, monoclinic to monoclinic at 16 GPa, and monoclinic to rhombohedral at 20 GPa. These results confirmed that lead titanate displays a morphotropic phase boundary, under pressure, consistent with first-principles theoretical predictions (Wu and Cohen). Our results reveal that a complex microstructure is not necessary to obtain high coupling piezoelectricity in relaxor ferroelectrics with lead titanate as end-members. They also suggest that high performance ferroelectric solid solutions with lead titanate as end-members simply tune the transition down to zero pressure through chemistry. This opens the door to the possible discovery of high performance pure compound electromechanical materials.

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