On the space group of spinel, MgAl₂O₄

The controversial space-group problem of spinel, MgAl2O4, whether it is Fd3m or F{ 4}3m, was studied by electron diffraction. It was confirmed that the appearance of 'forbidden reflections' such as {200} was caused by the double reflection process of reflections with high indices on the non-zero-order Laue zone. Consequently, the space group of spinel is Fd3m, and the assignment of the space group to F{ 4}3m is ruled out. The change of space group from F{ 4}3m to Fd3m associated with a phase transition, proposed by Mishra & Thomas [Acta Cryst. (1977), A33, 678], can also be explained by double diffraction: the magnitudes of primary reflections with high indices rapidly decrease due to the increase of the Debye-Waller factors at elevated temperatures.