STRUCTURE OF THE METASTABLE STATE IN ION-IRRADIATED MAGNESIO-ALUMINATE SPINEL

We have studied the structural changes in MgAl2O4 and MgO . 3Al(2)O(3) spinel single crystals following 40DkeV Xe2+ irradiation at 100K to a dose of 1 X 10(16) ions cm(-2). The radiation-damaged layer was observed using cross-sectional transmission electron microscopy. The first-order Bragg reflections decreased in intensity with increasing damage dose and eventually disappeared, indicating a transformation to a metastable crystalline state. Further irradiation resulted in amorphization of the damaged layer. Considering the well known radiation resistance of spinel, these results suggest that the transformation to the metastable state could be an important step in the amorphization process. We have examined various models for the structure of the metastable phase by calculating the diffraction patterns and comparing them to the electron diffraction observations. Our results indicate that a fee oxygen lattice with cation interstitials provides the best explanation for the observed structural changes.