Symmetry Determination of Single and Double Crystals by CBED

会聚束电子衍射（CBED）是测定晶体点群和空间群的一种十分有效的方法。CBED允许在230个空间群中唯一识别所有32个点群和200个空间群。然而，其他30个空间组仍然无法区分。在本研究中，我们新应用相干CBED方法来区分剩余的空间群。相干CBED技术使用高相干电子源（即，场发射枪），并直接提供晶体结构的相位信息。我们将此方法应用于两对空间群（I23，I2_13）和（I222，I2_12_12_1），这两对空间群由于具有相同的运动学消光而没有动力学消光，因而属于不可区分的集合。我们首次发现相干CBED方法可以解释晶体中关于电子位置的格位对称性。我们成功地证明了相干CBED可以通过研究（I23，I2_13）和（I222，I2_12_12_1）的晶胞位置对称性来区分它们的空间群。本研究是与P.Goodman博士（澳大利亚墨尔本大学）和C.J.Rossouw博士（澳大利亚联邦科学与工业研究组织）密切合作进行的。

It has been established that convergent-beam electron diffraction (CBED) is quite an effective method for determining crystal point groups and space groups. CBED allows the unique identification of all the 32 point groups and 200 space groups among 230 space groups. However, the other 30 space groups have still remained to be indistinguishable. In the present research, we have newly applied the coherent CBED method to distinguish the remaining space groups. The coherent CBED technique observes interference fringes appearing in the overlapping regions of CBED disks using an high coherency electron source (i.e., field-emission gun), and directly provides phase information of crystal structures. We have applied the method to two pairs of space groups (I23, I2_13) and (I222, I2_12_12_1), which belong to indistinguishable sets because of their same kinematical extinction and no dynamical extinction. We have first discovered that the coherent CBED method can elucidate site symmetries of crystals with respect to the electron prove position. We have successfully proved that coherent CBED can distinguish the space groups of (I23, I2_13) and (I222, I2_12_12_1) through investigating their site symmetries in their unit cells. The present research was carried out in close cooperation with Dr.P.Goodman (University of Melbourne, Australia) and Dr.C.J.Rossouw (CSIRO, Australia).