On the space group of aluminium oxynitride spinel

In view of the controversy surrounding the crystal symmetry of spinels, an aluminium oxynitride phase reported to be based on the spinel structure has been investigated using convergent beam electron diffraction (CBED) and computer simulation. A variety of CBED point and space group determination methods have been utilized in search of possible non-centrosymmetry which reduces the space group symmetry from Fd 3 m to F 4 3m. The occurrence of Gjønnes-Moodie (GM) lines in the debated {2 0 0} reflections is also explored through careful voltage variation experiments. The detailed analyses clearly show that the point group of this spinel is the centrosymmetric m 3 m and that the space group is Fd 3 m.