Strain relaxation and order-disorder phase transition in irradiated MgAl2O4.

MgAl2O4 spinel is widely studied in many fields of material science because of its variety of interesting properties and potential applications. The influence of cation disorder on the physical properties of MgAl2O4 makes understanding of the effects related to this disorder particularly important. It is known that, upon ion irradiation at low temperature, MgAl2O4 undergoes an order-disorder phase transition followed by amorphization. This paper reports a combined high resolution X-ray diffraction and transmission electron microscopy study elucidating the linkage between this phase transition and irradiation-induced elastic strain. Irradiations were carried out on [110] and [111]-oriented single crystals of MgAl2O4 at T = 77 K with 600 keV Xe ions over a wide range of doses. The data suggests that the beginning of the order-disorder phase transition coincides with the beginning of strain relaxation. This result indicates that the volume of the new phase is slightly smaller than that of the unirradiated spinel. The dose at which the phase transformation occurs is found to be dependent on the crystal orientation, which can be attributed to both elastic and crystallographic properties.