Experimental Study For The Dynamics Of Polarization Clusters In Ferroelectircs By Coherent Light

Ferroelectric materials(ceramics, crystals, membranes) exhibit excellentferroelectric, dielectric, piezoelectric and pyroelectric properties etc., which make themplay an irreplaceable role in the key functional electronic devices such as ferroelectricmemory, high-performance capacitor, precision actuator and electroacoustic transducer,etc., and the various ferroelectric-material-based functional electronic devices arewidely used in the fields of information, manufacture, environment, medical treatment,ocean, aerospace and so on. A lot of theoretical and experimental studies show that theexcellent properties and phase transition of ferroelectric materials are closely related tothe internal polarization clusters. But for a long time, experiments have most dominatedby measurement of lattice structural displacement through X-ray scattering, ormeasurement of lattice vibration through neutron scattering, Brillouin scattering andRaman scattering. Piezoelectric force microscope (PFM) provides a straightforwardexperimental method to research the polarization clusters on the surface of ferroelectricmaterials, however, this method could not be used to research the internal polarizationclusters. Hence, searching an experimental method which can be used to directlyresearch the temporal-spatial structure is of great significance for understanding theorigin of excellent properties and the mechanism of phase transition in ferroelectricmaterials.Based on He-Ne laser photon correlation spectroscopy(PCS) method, therelaxation for the long-range fluctuation of polarization clusters in BaTiO3near Curietemperature(TC) and0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3as well as0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3at the temperature range from room temperature tothe temperature corresponding to dielectric peak(Tm) were researched. Based on thetheory of random fluctuation of polarization clusters and the model of Wiener randomprocess, the relaxation law of long-range fluctuation of polarization and possible forms of light intensity autocorrelation function g2() measured from PCS experiments havebeen derived. For BaTiO3, dual relaxation processes for polarization clusters have beenobserved in a straightforward way by using PCS method. The measured intensityautocorrelation function was fitted well by the proposed theoretical model, and thecharacteristic relaxation times were extracted. The long-time relaxation process occursat4K above TCand exhibits the relaxation time of30–100μs. The short-time relaxationprocess covers a broad temperature region and exhibits the relaxation time of7–40μs.Based on the temperature dependence of relaxation process and the value of itscharacteristic relaxation time, the long and short relaxation processes were inferred tooriginate from the flipping and boundary fluctuation of polarization clusters.Polarization cluster was originated from the correlation movement of order-disorder Tiions, the existence of flipping process for polarization clusters indicate the existence oforder-disorder component of Ti ions during phase transition in BaTiO3. The longrelaxation time ldecrease with temperature increasing, which has a decreasing slopethat is almost the same as that of the dielectric permittivity. This similarity indicatesthat the polarization flipping may contribute to the dielectric performance. The shortrelaxation decrease with temperature increasing except for some abnormal values at thetemperature region from399–402K. Based on the characteristics of intensityautocorrelation function, the temperature evolution of polarization clusters near TCwas proposed.For0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3and0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3, thedual relaxation processes were directly observed in the both sides of Tm. Thetemperature dependence of long relaxation time is consistent with that of dielectricconstant in both two samples. PCS provide a new approach to research the origin ofexcellent properties and the mechanism of phase transition in relaxor ferroelectricswhere the polarization cluster is treated as medium.By taking advantage of small angle X-ray scattering (SAXS), the spatial structureof polarization clusters in0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3single crystal near Tmwaspreliminarily researched. The symmetric petal shaped scattering pattern from polarization clusters was observed, which shrinks gradually to center with thetemperature increasing. The spatial period of polarization clusters approximatelycomplies with exponential decay with temperature increasing.The relaxation process of polarization clusters was researched by PCS method, andthe spatial structure of polarization clusters was preliminary researched by SAXS. Forthe studies of temporal-spatial structure, which could conducted by using the3ndgeneration synchrotron radiation and free electron laser (FEL), by using full coherentX-ray, more smaller polarization cluster can be studied.