Formation of a metastable crystalline phase during ion irradiation of spinel

We have examined the radiation resistance of magnesio-aluminate spinel by irradiating single crystals of MgAl2O4 with 400 keV Xe++ions at 100 K. At low irradiation doses, the material transformed into a metastable crystalline phase with half the lattice spacing of the original crystal. Electron diffraction analysis revealed that this structural change can be explained in terms of the redistribution of cations among octahedral, tetrahedral, and three- fold coordinated interstitial sites of the close-packed anion lattice. Corresponding to this transformation, the hardness and elastic modulus increased with dose to values about 10% greater than those of unirradiated spinel. We believe that the formation of this metastable phase plays an important role in determining the radiation resistance of spinel