Anomalous Ferroelectricity And Local Structure In Nanosized Oxides

Ferroelectric materials possess broad application prospects in the fields of non-volatile storage,capacitance and sensor,due to the multifarious electrical properties.In recent years,nano-ferroelectricity,as the technical basis for the integration and miniaturization of electronic devices,has become a hotspot on theoretical and experimental researches.However,as the physical size of the ferroelectrics decreases,the ferroelectric properties usually attenuate,and even disappears when reaches the critical size.Hence the preservation of well ferroelectric properties in low dimensional ferroelectrics is a big challenge.At the same time,the establishment of ferroelectric order essentially depends on the crystal structure of the materials,so an in-depth understanding of the crystal structure is of significance to study ferroelectric theory and explaining physical properties.Due to the special existential state of nano-ferroelectric materials,the size,surface effects and stress significantly influence the key performance.Therefore,it is of great importance to investigate the coupling relationship between the local structure and the ferroelectric properties of nano-ferroelectrics.This dissertation systematically investigated the local structure and ferroelectric properties of several typical nano-ferroelectric materials;regulated the ferroelectric spontaneous polarization by controlling the size and surface state;at the same time,the coupling mechanism of atomic arrangement,local structure and ferroelectric properties of nano-ferroelectric materials was revealed,followed by the in-depth exploration of the physical mechanism.Firstly,by adjusting the sample size and the state of surface exposure,the ferroelectric properties of zero-dimensional lead titanate nanomaterials have been well controlled.Through synchrotron radiation X-ray diffraction experiments,it was found that along with the size decreases,the tetragonality of the nanoparticles increased.Combined with synchrotron radiation X-ray scattering and Raman spectroscopy,etc.,the local structure as well as the enhanced spontaneous polarization were revealed.Furthermore,X-ray absorption spectroscopy characterized the form of atomic terminations on the surface.Ab initio calculations demonstrated the relationship between the special surface and the enhanced polarization from the electronic structural aspect.The reverse Monte Carlo method(RMC)was employed to analyse the three-dimensional distribution of the ferroelectric spontaneous polarization of zero-dimensional lead titanate nanoparticles.Combining neutron total scattering experiments and electron microscopy experiments,it was found that although the polarization state showed a linear characteristic along the c axis in nanosized lead titanate as a whole,the polarization distribution on the surface and internal was quite different.Further analysis elucidated that this difference was related to the consistency of polarization.Finally,the atomic coordinate parameters are employed as a bridge connecting the short-range local structure and the long-range average structure to clarify the unity of opposites between them in the entire nanoparticle.Via adjusting the size and designing the phase structure,the ferroelectricity of the zirconium dioxide nanomaterials was obtained.X-ray diffraction experiments show that the average crystal structure of zirconium dioxide nanomaterials was cubic phase.Through comprehensive experiments including the X-ray total scattering and X-ray absorption spectrum,the local structural distortion of orthogonal-phase in the material was revealed.Further analysis of the bond angle and coordination number of the reverse Monte Carlo fitting results showed that the dominant distribution of the polar orthogonal-phase local structure is the structure origin of the ferroelectricity in nano-zirconia.This dissertation provides an effective way to control non-traditional nano-ferroelectric oxides based on the knowledge of local structure.Through the accurate characterization of the interface,stress and the local structure,phase structure of the materials,the basis of the structure-activity relationship of optimized ferroelectric-related physical properties is obtained.This is inspiring for the research and development on designing of new functional materials.