Symmetry Determination of Single and Double Crystals by CBED

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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).