Crystal distortion in spinels containing Mn<SUP>3+</SUP> ions

The effect of chemical composition on tetragonal-cubic spinel transformations in the systems Mn3O4-MgAl2O4 and MgMn2O4-MgAl2O4 has been studied by X-ray diffraction. The unit cell dimensions of the various compounds have been determined at room temperature. The c' and a' values do not change linearly with composition but show an abrupt change at a critical composition, beyond which c'=a', and the systems have the cubic spinel structure. The cation distribution has been worked out from an analysis of the X-ray diffraction intensities, and thus a correlation between the actual number of distorting Mn3+ ions at the octahedral sites, and the degree of distortion, has been obtained. The critical number of Mn3+ ions required at these sites to maintain the tetragonal spinel structure at room temperature has been accurately determined