Origin of morphotropic phase boundaries in ferroelectrics
Ahart, M., M. Somayazulu, R.E. Cohen, P. Ganesh, P. Dera, H.k. mao, R.J. Hemley, Y. Ren, P. Liermann, and Z. Wu. Nature, 451, 545, (2008)
Under pressure, even a pure compound can display a morphotropic phase boundary under pressure. The results are consistent with first-principles theoretical predictions, but show a richer phase diagram than anticipated. Our results show that the high electromechanical coupling in solid solutions with lead titanate is due to tuning of the high-pressure morphotropic phase boundary in pure lead titanate to ambient pressure. It suggests that complex microstructures or compositions are not necessary to obtain strong piezoelectricity. This opens the door to the possible discovery of high-performance, pure-compound electromechanical materials, which could greatly decrease costs and expand the utility of piezoelectric materials.
High resolution x-ray diffraction data clearly display peak splittings that result from symmetry-lowering transitions, proving strong evidence against the nanotwin theory for the monoclinic phases at morphotropic phase boundaries, which asserts that observed monoclinic phases are actually nanotwinned rhombohedral and tetragonal nanodomains.