Frustrated Materials and Ferroic Glasses by Turab Lookman & Xiaobing Ren

Author:Turab Lookman & Xiaobing Ren
Language: eng
Format: epub
ISBN: 9783319969145
Publisher: Springer International Publishing

At any rate, there is ample evidence of the intrinsic PNR distribution in relaxors. At the Burns temperature T d, the soft TO phonon mode becomes overdamped near the zone center, and starts to condense into PNR [47]. These regions form with local polarizations along <111> directions and are shifted uniformly along their individual polarization direction. Number and size of PNR increase on cooling. At the phase transition, T ≈ T C, a large-scale overall “freezing” of the PNR occurs. Small PNR merge into larger ones and the total volume of PNR in the system keeps increasing. The related ferroelectric soft-mode lifetime increases below T C, and the overdamping near the zone center disappears. A macroscopic ferroelectric polar phase without lattice distortion tends to become established.

Below T C the size of the PNR can grow slowly with further cooling. However, if the coupling between the PNR and the surrounding lattice is not sufficiently strong, as is the case in pure PMN, then the energy barrier created by the uniform phase shift would prevent the PNR from merging further and forming macroscopic lattice distortions. The resulting phase will have a polar lattice of average cubic structure, but with embedded rhombohedrally polarized PNR. In addition to neutron and X-ray diffuse scattering measurements, Raman studies [48] and specific-heat measurements [49] have also provided useful information on PNR in PMN.

Probably the most exciting recent insight is the intimate coupling of the PNR to the ferroelectric polarization and anisotropy in relaxors [50], which raises hopes for tailoring new relaxor systems with unprecedented piezoelectric response. It has become clear that the observed giant electromechanical coupling of relaxor ferroelectrics is controlled by PNR vibrations. Actually the contribution of PNR to the room-temperature dielectric and piezoelectric properties of PMN is in the range of 50–80% [51]. A mesoscale mechanism is proposed to reveal the origin of the high piezoelectricity in relaxor ferroelectrics, where the PNR aligned in a ferroelectric matrix can facilitate polarization rotation. This mechanism emphasizes the critical role of local structure on the macroscopic properties of ferroelectrics.

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