采用会聚束电子衍射 (CBED) 对Bi--2212相和Bi--2223相中具有非公度调制结构的超胞结构进行了测定. 测定结果表明: 具有 A型调制结构Bi--2212相超胞的点群是2mm, 空间群是I2mm; 具有B型调制结构Bi--2223相超胞的点群是mmm, 空间群是Ibmm. 结合实验结果讨论了摇摆曲线对CBED图对称性的影响, 并指出: 采用CBED测定晶体结构的对称性时, 应以高阶Laue线为依据, 而不应依 靠0层盘内摇摆曲线的对称性. CBED图模拟结果表明: Bi--2212相CBED图显示的是调制结构超胞的对称性, 而不是亚胞的对称性.
The investigation of microstructures of the Bi--2212 and Bi--2223 phases in Bi high temperature superconductors is very significant for improving technology of production and researching superconducting properties and mechanisms. The determination of space groups for the Bi--2212 and Bi--2223 phases is a key problem, which is very complicated because of their incommensurate modulation structures. The supercell structures of the Bi--2212 and Bi--2223 phases with incommensurate modulation structures were determined by convergent beam electron diffraction (CBED). The point group of the supercell of the Bi--2212 phase with type A modulation structure is 2mm, its space group is I2mm. The point group of the supercell of the Bi--2223 phase with type B modulation structure is mmm, its space group is Ibmm. The effects of the rocking curve for the symmetries of CBED patterns were discussed according to experimental results. The simulation results of CBED patterns for the Bi--2212 phase confirmed the symmetries of CBED patterns to be the symmetries of the supercell with modulation structure, but not to be the symmetries of the subcell structure.
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