Finite Electric-Field Study of Pressure Effects on Polarization Rotation in PbTiO 3
- PDF / 75,616 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 69 Downloads / 181 Views
1199-F11-06
Finite Electric-Field Study of Pressure Effects on Polarization Rotation in PbTiO3 P. Ganesh and R. E. Cohen Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015
Abstract: We perform first-principles finite-electric field simulations of PbTiO3 both at zero and high pressures to investigate the effect of pressure on polarization rotation. We find that whereas a large electric field is required at zero pressure to induce a phase transition from the tetragonal (P4mm) phase to the rhombohedral (R3m) phase, at 8GPa a relatively small electric field is required indicating the greater ease of polarization rotation at high pressure. Pressure reduces the relative well depth between the two phases leading to a softer free-energy surface. This explains the increased electro-mechanical coupling obtained in PbTiO3 with pressure.
Introduction: Piezoelectric single crystals such as PMN-PT (PbMg2/3Nb1/3O3-PbTiO3) [1] with huge electromechanical coupling have great potential as a new generation of transducer materials. The best piezoelectric materials are ferroelectric solid solutions, characterized by a morphotropic phase boundary (MPB) separating tetragonal (T) and rhombohedral (R) regions. In PZT the electromechanical properties peak at the MPB, but in single crystals a wide range of compositions on the rhombohedral side of the boundary have strong coupling. The boundary region is actually not a single phase transition, but contains one or more monoclinic (M) and possibly orthorhombic (O) phases, varying among different materials. [2-5] The high electromechanical coupling in single crystal piezoelectrics and the presence of monoclinic phases in the MPB regions has been explained as being due to polarization rotation, where the polarization is rotated away from the [111] direction from an obliquely applied electric field, or in the phase transition region. [2, 7] There are also competing zone boundary instabilities, which are particularly important in PbTiO3 [8]. First-principles calculations [10,11], and cryogenic high pressure in situ Raman and synchrotron powder x-ray diffraction experiments [12] show formation of an MPB in pure PbTiO3 (PT) with pressure, with electromechanical coupling greater than any known material. This suggests that all of the high-coupling materials, including PZT, can be considered to be engineered PT with a transition under ambient conditions. Presence of the MPB region allows for polarization rotation from the tetragonal to the rhombohedral phase resulting in huge electromechanical coupling [2,3,4,5]. Complex solid solutions with PbTiO3 as one of the end members only tune the MPB region to ambient pressures by applying chemical pressure, and do not have an intrinsic role in the high coupling. One can drive a phase transition either by changing composition or applying external pressure or electric field. This alters the energy surface allowing easy rotation of polarization. Depending on how the anisotropy of the energy surface changes with
external field, one
Data Loading...
