Electron Microscopic Investigations And First-principles Calculations Of Ferroelectric PMN And KNN

The microstructure of typical relaxor ferroelectrics Pb(Mg_1/3Nb_2/3)O₃ (PMN) and (K_1-xNa_x)NbO₃-based lead-free ceramics was investigated to understand the ferroelectric and piezoelectric properties, using electron microscopy and first-principles calculations.Local low energy-loss functions (ELF) of chemical ordered nano-domain in relaxor PMN were studied by combining electron energy-loss spectroscopy and ab initio calculations. The theoretical energy-loss functions are fundamentally in agreement with the experimental ELFs. All of peaks in the low energy-loss function were systematically elucidated based on the ab initio calculated densities of states. It is found that Nb 4 d -O 2p hybridization is enhanced through ferroelectric transition, which makes O 2p collective excitation energy 0.45 eV upper shift in ELF. The compositional fluctuation of Mg/Nb in PMN may play an important role in localizing the polar regions into nanometer scale.Electron diffraction spots splitting in different zone axis was used to distinguish tetragonal (T) 90°, orthorhombic (O) 90°and 60°/120°domain structures in (1-x)K0.5N0.5NbO3-xLiTaO₃ (KNN-LT) system. The structure and width of ferroelectric domains in the component 0.95KNN-0.05LT with the coexistence of O and T phases are similar to that in pure O phase KNN and T phase 0.93KNN-0.07LT. The width of domain ranges from 30 nm to 0.5μm. In 0.95KNN-0.05LT, O and T phases can either coexist or separately exist in one crystalline grain. Those microstructure results comfirm the reason for 0.95KNN-0.05LT ceramics with good properties at room temperature, which is the coexistence of O and T phases due to the descent of polymorphic phase transition (PPT) temperature to room temperature as the LiTaO₃ doping in KNN.First-principles supercell calculations and the modern theory of polarization based on Berry phase method were perfomed to compute the spontaneous polarization P_s and instrinsic piezoelectric coefficient e₃3 of (K_1-xNa_x)NbO₃ system. Born effective charge and the respons of the internal atomic coordinates to a macroscopic strain for different types of atoms in KNbO₃ and (K₀.5Na₀.5)NbO₃ were calculated and compared. The results exhibit that the experimental piezoelectricity of (K_1-xNa_x)NbO₃ ceramics mainly depends on extrinsic contributions.Recently developed spherical-aberration high-resolution transmission electron microscopy (HRTEM) was employed for imaging oxygen atoms. The projections of cation-oxygen dipoles across the 60°/120°domain walls in orthorhombic (K₀.46Na₀.54)NbO₃ ceramics were directly observed. The width across the wall was measured to be ¹.1 nm. This method was further extended to the investigation of surface atomic structure of SnO₂ gas-sensing materials.