Abstract
There is considerable controversy concerning the point and space group of crystals based on the spinel structure. The debated question is whether the space group of spinel crystals is or . has noncentrosymmetric point group and is symmorphic; has centrosymmetric point group and is nonsymmorphic. One characteristic of the space group that distinguishes it from is that reflections hkl such that h+k, k+l, h+l ≠ 4n, for example {200}, are kinematically forbidden.MgAl2O4 spinel was studied by Steeds and Evans whose observations of {200} reflections in {001} ZAPs lead them to conclude that the space group of this crystal was . de Coomen and Carter also investigated MgAl2O4 spinel and came to the same conclusion. Although these authors do emphasize the importance of a HOLZ contribution to double diffraction, they do not show the effect of changes in accelerating voltage on the HOLZ segment responsible for double diffraction. To clearly show that {200} reflections appear only as a result of double diffraction via HOLZ, it is essential to observe both the {200} reflections and the HOLZ segment as a function of accelerating voltage.
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