Why ion irradiation does not lead to the same structural changes in normal spinels ZnAl2O4, MgAl2O4 and MgCr2O4?

Ion irradiation induced phase transformations in three normal spinel compounds MgAl2O4, MgCr2O4 and ZnAl2O4 were investigated by X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The structural and microstructural changes occurring in these three systems are described. Irradiation acts at the atomic and mesoscopic scales in different ways. At the atomic scale it produces the inversion of the cations in the structure. At the mesoscopic scale it produces a change of the size of the diffracting domains, responsible for the colossal changes in the observed X-ray diffraction patterns.