聚偏氟乙烯β相全反式结构链的第一性原理计算

用紧束缚的Hartree-Fock自洽场方法对聚偏氟乙烯(PVDF)β相的全反式结构链进行了第一性原理计算.在结构方面,计算得出,相邻F原子对平均距离为0264nm 相邻H原子对平均距离为0255nm 和F原子相连的C原子的平均距离为0.260nm 和H原子相连的C原子的平均距离为0.257nm.电偶极矩方面,计算得出单体的平均电偶极矩为3.98×10-30C·m(三个单体的链) 4.40×10-30C·m(六个单体的链) 4.44×10-30C·m(十一个单体的链).结构和电偶极矩的计算值与实验值基本符合.最后预测了全反式链的振动模式 关键词： 聚偏氟乙烯β相全反式结构链 Hartree-Fock自洽场方法     

Size Driven Ferroelectric—paraelectric Phase Transition from the Surface Energy Viewpoint

The size driven ferroelectric-paraelectric phase transition in a ferroelectric of small size is studied within the framework of Landau phase transition theory.The transition is a consequence of the competition between decreasing the volume free energy by polarization and increasing the surface energy of the ferroelectric phase,which has a surface energy density higher than that in the paraelectric phase.A simple expression for the ferroelectric critical size as a function of the Landau free energy coefficients and the surface energy density is derived.     

K(Ta0.56Nb0.44)O3四方铁电结构的第一性原理研究

在广义梯度近似(GGA)下利用全电势线性化的缀加平面波法(FLAPW)的总能量计算,确定了K(Ta_0.56Nb_0.44)O₃固溶体的四方精细结构,即B位离子(Ta和Nb)在铁电相的平衡构型.结果表明,Ta相对于氧八面体沿[001]方向有约为0005nm的偏心位移,而Nb的偏心位移约为0016nm,铁电非稳的发生主要应归因于后者. 关键词：     

四方铁电体PbFe0.5Nb0.5O3精细结构的第一性原理研究

采用全电势线性缀加平面波法(FPLAPW),计算了PbFe0.5Nb0.5O3的总能量,从而确定了它的四方铁电稳态结构,即B位离子(Fe和Nb)在四方铁电相的平衡构型.计算结果表明,Fe相对于氧八面体沿[001]方向的位移为0.022nm,而Nb的偏心位移为0.0020nm,其中铁电非稳性主要归因于前者 关键词： 铁电体 密度泛函理论 平面波法     

Li0.025Na0.975NbO3低温铁电相变及晶体结构

Li_(0.025)Na_(0.975)NbO_3 undergoes a ferroelectric-ferroelectric phase transition at 180K (on cooling) or 260 K (on heating). The structure determinations of both phases have been carried out by single crystal diffraction analysis. The ferroelectric room-temperature phase is orthorhombic, with space group Pc2_1b; the cell parameters are a=0.5508(3), b=1.5521(6), c=0.5568(2) nm. The structure is same with Li_(0.02)Na_(0.98)NbO_3. The ferroelectric low-temperature phase is also orthorhomhie, a=0.5497(7), b=0.7913(18), c=0.5565(10) nm, V=0.2421 nm, Z=4. There is disordernal position distribution of O(2) atomes in the low-temperature phase. Its “apparent space group” is P2/n2/n2/n. The relation of the ferroelectric properties to the low-temperature phase structure is discussed.     

关于铁电相变的一个唯象讨论

从不可逆过程热力学的角度研究了铁电相变中的不可逆性.一级铁电相变中的热滞及铁电体的多畴结构,可以在最小熵产生原理的基础上得到说明.并得出结论,热滞并不是一级铁电相变体系的内禀性质,体系表面的有限性与热滞是有关的. 关键词： 热滞 畴构型 不可逆性 最小熵产生     

铁电-铁电相变与热电性

研究铁电－铁电相变附近的热电性,可以获得关于相变前后自发极化的大小和取向,相变的温度分布和时间过程以及新相对称性的信息．因此,热电性测量是研究铁电－铁电相变的一个重要手段．     

La3Ga5SiO14晶体的介电性质、弹性与压电性质

研究了La3Ga5SiO14晶体的介电性质、弹性和压电性质，其中弹性和压电性质是利用高频振动模式通过谐振法测定的. 关键词： 介电性质 弹性 压电性质     

钛酸锶钡的铁电相变与晶胞体积的关联

考察了Ba_μSr_1-μTiO₃(BST)的T_C对晶格 参数γ的依赖关系.并对BST的依赖关系与BaTiO₃和SrTiO₃的相应依 赖关系进行了比较.得到了一个观点：BST的T_C的变化可归因于晶胞体积效应.还 从体积效应的观点对BST相变的弥散性和级进行了讨论.BST的晶胞体积效应可归因于BST中Ba 和Sr的离子化现象. 关键词： 铁电性 钛酸锶钡 晶胞体积效应     

Ab Initio Calculations for the BaTiO3 （001） Surface Structure

The ab initio method within the local density approximation is applied to calculate cubic BaTiO3 (001) surface relaxation and rumpling for two different terminations (BaO and TiO2 ). Our calculations demonstrate that cubic perovskite BaTiO3 crystals possess surface polarization, accompanied by the presence of the relevant electric field. We analyse their electronic structures (band structure, density of states and the electronic density redistribution with emphasis on the covalency effects). The results are also compared with that of the previous ab initio calculations. Considerable increases of Ti-O chemical bond covalency nearby the surface have been observed. The band gap reduces especially for the TiO2 termination.