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Origin of morphotropic phase boundaries in ferroelectrics


A piezoelectric material is one that generates a voltage in response to a mechanical strain (and vice versa). The most useful piezoelectric materials display a transition region in their composition phase diagrams, known as a morphotropic phase boundary1,2, where the crystal structure changes abruptly and the electromechanical properties are maximal. As a result, modern piezoelectric materials for technological applications are usually complex, engineered, solid solutions, which complicates their manufacture as well as introducing complexity in the study of the microscopic origins of their properties. Here we show that even a pure compound, in this case lead titanate, can display a morphotropic phase boundary under pressure. The results are consistent with first-principles theoretical predictions3, but show a richer phase diagram than anticipated; moreover, the predicted electromechanical coupling at the transition is larger than any known. 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. We also find 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.

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Figure 1: Pressure dependence of energy dispersive and high-resolution angle-dispersive X-ray diffraction spectra at selected pressures at 10 K.
Figure 2: Raman spectra.
Figure 3: Lattice strain and monoclinic angle.
Figure 4: Phase diagram for lead titanate.


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We thank D. Rytz for the PbTiO3 crystals. We thank B. Noheda and E. Salje for discussions. We also thank our GL colleagues R. Caracas, K. P. Esler Jr. and S. Gramsch for discussions. This work was sponsored by the Office of Naval Research. Support was also received from the Carnegie/Department of Energy Alliance Center (CDAC). High-pressure X-ray diffraction at the HPCAT facility of Advanced Photon Source was supported by DOE-BES, DOE-NNSA (CDAC), and the W. M. Keck Foundation. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.

Author Contributions M.A., M.S., H.-k.M., R.E.C. and R.J.H. conceived the project as a part of previous work3. M.A., M.S., H.-k.M. and R.J.H. executed the sample loading, Raman scattering and X-ray diffraction studies. P.D., Y.R. and P.L. helped in synchorotron X-ray diffraction experiments. P.G., Z.W. and R.E.C. carried out first-principles simulations.

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Correspondence to R. E. Cohen.

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This file contains Supplementary Methods, Supplementary Table 1 and Supplementary Figures 5-9 with Legends. (PDF 555 kb)

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Ahart, M., Somayazulu, M., Cohen, R. et al. Origin of morphotropic phase boundaries in ferroelectrics. Nature 451, 545–548 (2008).

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