Convergent-Beam Electron Diffraction Symmetry from a Disordered Structure (Ce, Ta) Ta₆0₁₉

The space group of (Ce,Ta)Ta₆0₁₉ is shown by convergent-beam diffraction to be P6c2 and not P6₃/mcm as previously reported by X-ray diffraction. The X-ray structure, however, is essentially correct, the major change being the ordering of the Ce and Ta atoms in the special positions 1/3, 2/3 and 2/3, 1/3 in strings parallel to the c axis with Ce at 1/3, 2/3 and Ta at 2/3, 1/3. Occasional interchanging of these strings would explain the space group observed in the X-ray determination.