Magnesium aluminate spinel is considered to be a remarkably radiation resistant material. Single crystal spinel exhibits very little damage when irradiated with fast neutrons or energetic ions. Recently, amorphization of single-crystal MgAl204 was observed, for the first time, following irradiation with 400 keV Xe2+ at about 100 K to a peak dose of 25 dpa. Electron diffraction patterns from spinel subjected to different doses show the extinction of first-order fundamental reflections prior to the occurence of amorphization. This suggests that, enroute to becoming amorphous, ion-irradiated spinel transforms to a metastable crystalline state with a lattice constant that is half that of spinel. In an effort to understand this phase transformation, we have analyzed the structure of the metastable state using electron diffraction.
Spinel belongs to the spacegroup Fd3m (fee Bravais lattice). The unit cell has a lattice constant of about 0.8 nm and is made of two sets of four octants.