Surface Relaxation and Size Effects in Perovskite Type Ferroelectrics

本研究的目的是探讨钙钛矿型氧化物如钛酸钡（BT）、钛酸铅（PT）、钛酸锶（ST）以及BT和ST的混晶中铁电相变的尺寸效应的起源。通过X射线衍射测量检查的平均尺寸和表面松弛。结果表明，纳米粒子的晶格常数比体相晶体的晶格常数变大，XRD分析表明，纳米粒子表面的晶格向外扩展了0.035 nm，BT团簇为0.015nm，ST团簇为0.001nm，混合晶体为中间晶体，用ab第一性超软赝势计算的电子分布表明，纳米粒子表面的原子键合由共价键变为离子键，说明纳米粒子的尺寸效应是由于纳米粒子表面的键合性质发生了不可避免的变化。

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.