Microstructural Property And In Situ Studies On Relaxor Ferroelectrics

'As one of the most widely used multifunctional materials,perovskite PbTiO3(PT)and doped PMN-PT(solid solution relaxor ferroelectrics)show unique domain/phase evolutions under the external loading e.g.electric,stress and thermal fields,leading to promising ferroelectricity,piezoelectricity,photoelectricity and storage applications.In this work,the microstructure and the domain/phase transition behavior of single crystal PbTiO3(PT)nanorods and(1-x)PbMgi/3Nb2/3O3-xPbTiO3(PMN-PT)single crystal are investigated under loading,by using transmission electron microscope(TEM),Raman spectrometer,X-ray diffraction spectra(XRD),scanning electron microscope(SEM)and X-ray Energy Dispersive Spectrometer(EDS).It provides a further insight on the understanding of the functional properties improvements by size/shape and domain/phase structure engineering,subsequently resulting in some useful experimental finding.Firstly,single crystal PbTiO3 nanorods with diameters of 60-80nm and lengths of several micrometers have been prepared,showing tetragonal phase and growth direction along[100].Its tetragonal-cubic phase transition i.e.the ferroelectric-paraelectric one,accompanying with the disappearance of the soft Raman mode,is seriously influenced by the size effects.The Curie point downshifts 53? than the bulk value.And the soft mode E(1 TO)displays a weaken softening,implying the decreased tetragonality,unit cell and ferroelectricity.Here,the lattice distortions inside the nanorods can be well controlled by using surface active agent during the fabrications.Secondly,we studied the domain and phase structure of PMN-PT through high resolution TEM(HRTEM)and selected area electron diffraction(SAED).With emphasis on the splitting of the diffraction points(i.e the reconstruction of the domain/phase micro-structure)which is closely related to the domain/phase evolutions,we examined the SAED transitions(i.e.the reciprocal lattice structure transitions).We have therefore revealed the domain/phase evolutions' mechanism in the real space,e.g.in PMN-PT single crystal the diffraction points' splitting around the 90° domain boundary corresponds to the miniature and growth of the 90° domains(with the direction of the separation perpendicular to the domain boundary and the length proportional to(c/a-1)and|h +l|,where a,c are lattice constants and h,1 denotes the Miller indexes).The domain evolutions provide strains of c/a-1.It is worth pointing out that the large c/a ratio induced by the domain evolutions can effectively enhance the piezoelectricity,flexibility properties.More,the domain evolutions have been studied with comparisons under the stress and bias loading in TEM.We find that the depinning process by electric field takes much longer than the stress one,owing to high-density nanostructure which acting as pinning centers and the screening effects by the possible flux closure.On the contrast,under stress loading,the domain boundaries display forward growth quickly,afterwards side growth and merge.It implies the advantage of microstructure engineering of PMN-PT by mechanical loading.Finally,we successfully introduced phase structures,reconstructions via high pressure at room temperature.By performing high-pressure in situ Raman scattering investigations and TEM,observations,we found that the phase reconstruction starts as the pressure reaching 11GPa,showing an obvious Raman peaks' merging for the 740cm-1 and 810cm-1 modes.Since the soft mode's Raman peak well survives,we suggest that the phase transition is independent to the classic tetragonal-cubic(i.e.ferroelectric-paraelectric)one.As known,the strongly coupled stress and electric fields can actually lead to dramatically different phase transition sequences under the different mechanical loading.Based on the further SAED studies on the compressed samples,we suggest that the phase transition is towards a MPB,which is of greatly improved functional properties.