College Park, Maryland      June 6 - 10 , 2004

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M3-C5 (2:45 PM): Decoupling of lattice distortion and ferroelectric polarization in the relaxor system PMN-xPT

Guangyong Xu (Physics Department, Brookhaven National Laboratory), D. Viehland, J. F. Li (Department of Materials Science and Engineering, Virginia Tech., Blacksburg, VA), C. Stock (Department of Physics, University of Toronto, Toronto, Ontario, Canada), P.M. Gehring (NIST Center for Neutron Research), Z. Zhong (National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY), G. Shirane (Physics Department, Brookhaven National Laboratory)

We present neutron and x-ray scattering measurements on the structures of lead oxide perovskite relaxor system PMN-xPT (Pb(MgNb)O₃-xPbTiO₃), which has extraordinary piezoelectric properties. The current zero field phase diagrams of PMN-xPT, and its close analog, PZN_xPT, share many common features. The system is in a cubic phase for T above the Curie temperature T_c. When the temperature goes below T_c, it was long believed that a structural phase transition occurs to a rhombohedral phase for small x, and tetragonal phase for large x, separated by a narrow region of monoclinic phase.

Several recent studies have now uncovered evidence of a new phase located on the rhombohedral side of the phase diagram. High energy x-ray (~67 keV) diffraction work by Xu et al. on pure PZN single crystals show that the (111) Bragg peak does not split below T_c = 410 K, and the inside of the crystal does not have any measurable (rhombohedral) lattice distortion, providing conclusive evidence of a new phase (X). While phase X has an undistorted unit cell shape the true symmetry of the phase is still unknown. Similar behavior has also been observed in PMN-xPT series.High q-resolution neutron scattering work by Gehring et al. and Xu et al. show that PMN-10 %PT and PMN-20 %PT also transforms into phase X instead of R below T_c. However, in PMN-27 %PT, the low temperature phase was found to be rhombohedral instead of X. These results indicate that the ground state for the inside of PMN-xPT is similar to that of PZN-xPT system, i.e., phase X for small x values, and with increasing x, rhombohedral distortions develop and the system changes into a true rhombohedral phase.

Another important finding during the x-ray diffraction work on pure PZN was the existence of an “outer-layer” with different structures than the inside of the crystal. Lower energy x-rays (~10.2 keV) were used to study the PZN single crystal, and a rhombohedral distortion was found to exist below T_c for the outer most 10 to 50 μm of the crystal. This naturally explains the discrepancies between earlier x-ray powder diffraction and the recent neutron and high-energy x-ray scattering results. And the fact that x-ray studies of pure PMN have not observed a rhombohedral phase could be understood if PMN represents the limiting case of phase X with a very thin outer-layer.

Despite the absence of a rhombohedral distortion in phase X, ferroelectric polar order is still present in the system. Recently, neutron inelastic scattering measurements by Wakimoto et al. of the ferroelectric soft TO phonon in pure PMN revealed that the soft mode recovers at temperatures below Tc ~ 213 K. Similar measurements on PZN were later performed by Stock et al., where Tc ~ 410 K. The linear relationship between the soft TO phonon energy squared and T is a clear signature of an ordered ferroelectric phase below T_c. Phase X is a very unusual phase where ferroelectric spontaneous polarization and an undistorted lattice coexist in a free-standing crystal.

An important question concerns the origin of phase X and its relationship with the polar nanoregions (PNR). We believe that the uniform phase shift of the PNR plays an essential role in stabilizing this phase (X), by creating an energy barrier thus preventing the PNR from melting into the surrounding ferroelectric phase. The balance between this energy barrier and the coupling between the polar nanoregions and the surrounding environment determines the stable phase of the system below T_c. With higher PT concentrations, the coupling becomes stronger and eventually phase X develops into the rhombohedral phase.

1. G. Xu, Z. Zhong, Y. Bing, Z.-G. Ye, C. Stock, and G. Shirane, Phys. Rev. B 67, 104102 (2003).
2. G. Xu, D. Viehland, J. F. Li, P. M. Gehring, and G. Shirane, Phys. Rev. B 68, 212410 (2003).

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