Surface Relaxation and Size Effects in Perovskite Type Ferroelectrics

点击翻译按钮获取中文摘要

The purpose of this research is to investigate the origin of the size effect on the ferroelectric phase transition in perovskite type oxides such as barium titanate (BT), lead titanate (PT), strontium titanate (ST) and mixed crystals of BT and ST.The samples of several to several hundred nanometer in diameter were prepared through an alkoxide route. The average size and surface relaxation were examined by X-ray diffraction measurements. The results show that the lattice parameter of the nanoparticle becomes large compared to that of bulk crystal.Detailed analysis of the XRD patterns shows that crystal lattice on the surface expands towards the outside by 0.035 nm in PT, 0.015 nm in BT and 0.001 nm in ST. Mixed crystals show intermediate character.Computer simulation was performed for BT clusters of 25 to 100 atoms using ab initio ultra soft pseudopotentials. The electron distribution calculated shows the atomic binding changes from covalent to an ionic one.We conclude that the size effect originates from the unavoidable change in the bond nature near the surface of the nanoparticle.