"Joint Theoretical and Experimental Studies for Developing New Electromechanical Materials"

R.E. Cohen, P. Ganesh, Muhtar Ahart, M. Somayazulu, R. Caracas, and R.J. Hemley, Carnegie Institution of Washington

Advances in theory and experiment are now being used to help design new piezoelectrics for transducer, non-linear optic and other applications. Morphotropic phase boundaries were thought to appear only in solid solutions, but computations and experiments show the same behavior in the classic ferroelectric lead titanate (PbTiO₃) under pressure. We are now using the concept of chemical pressure to design new useful polar pure compounds. Pure compounds have the advantage that compositional variation is not a problem, and production of high-quality homogeneous crystals and oriented ceramics should be much easier than for complex solid solutions that melt incongruently. We have searched for new pure compounds with high electromechanical coupling by substituting the ‘A’ site of PT in the perovskite ABO₃ structure by atoms having smaller ionic radii than Pb. Our in-situ cryogenic high-pressure single crystal high-resolution X-ray diffraction and Raman experiments show a MPB in pure PbTiO₃ under pressure, consistent with theoretical computations, but showing a richer phase diagram with two monoclinic phases. Multiple monoclinic or orthorhombic phases may appear in other MPBs, but may be obscured by compositional heterogeneity. Our experimental and theoretical results show incontrovertibly that the ground state in MPBs has monoclinic symmetry, and are consistent with complex nanotwinning observed experimentally. Our results are not consistent with the claim that monoclinic symmetry diffraction is due to nanotwinning of tetragonal and rhombohedral domains.

This work was sponsored by the Office of Naval Research under Grants No. N00014-02-1-0506, N00014-97-1-0052 and N00014-99-1-0738.