约化胞与电子衍射谱的标定

对应于14种布喇菲点阵有44种约化胞,不但约化矢的选法是唯一的,而且可以根据约化矢标量积矩阵(a·a b·b c·c b·c c·a a·b)的不同形式,确定布喇菲点阵类型。在晶体点阵已知的情况下,从点阵参数计算出倒易点阵中的平面约化胞,并与试验得到的电子衍射谱对比以标定衍射斑点的指数。在晶体点阵未知的情况下,从已知转角的两个电子衍射谱中选三个接近中心的衍射斑点,相当于三个低指数倒易矢量。由它们构成倒易点阵的一个初基胞,变换成正点阵的初基胞后,经过约化得出约化胞和相应的布喇菲单胞的类型和参数,同时标定衍射斑点的指数。这些标定计算已编成电子计算机程序,可以用来直接确定晶体点阵类型和进行物相分析。

REDUCED CELLS AND THE INDEXING OF ELECTRON DIFFRACTION PATTERNS

Corresponding to the 14 Bravais lattices there are 44 different types of reduced cells whose base vectors are unique, therefore it is possible to deduce the Bravaislattice type from the scalar products of reduced cell vectors(a·a b·b c·c b·c c·a a·b). If the crystal lattice is already known, the plane reduced cells in the reciprocal space are first calculated from the lattice parameters and then the electron diffraction patterns can be indexed by comparing them with the calculated plane reduced cells. If the crystal lattice is unkown, three diffraction spots are selected from two electron diffraction patterns taken one before and the other after tilting the crystal through an angle ψ, the spots being chosen so that they lie close to the center, corresponding to three reciprocal lattice vectors with low indices. These vectors constitute a primitive cell in reciprocal space which is first transformed to a primitive cell then further to a reduced cell in the crystal space. The lattice type and parameters of the Bravais lattice are then deduced and the indices of diffraction spots fixed. Computer pro-grams of the above indexing procedures have been written in Fortran for the purpose of phase analysis and the direct determination of the Bravais lattice of unknown crystals.