Radiation-induced cation disorder in the spinel MgAl2O4

There is some controversy in the literature concerning the nature of radiation-induced cation disorder and phase transformation in the spinel MgAl2O4. Attempts have been made to interpret the experimental data either in terms of a pure inversion involving the exchange between the Mg and Al atoms, or the disordering of the cation sublattice leading to a change in crystallographic symmetry to a defective NaCl-type lattice. We have performed first principles electronic structure calculations in order to examine the nature of this cationic disorder. We find that at low energies an exchange between the Mg and Al atoms is more favourable leading to an inversion in the spinel. With further increase in energy, the cations can be displaced from the tetrahedral sites to the unoccupied octahedral sites in the lattice, both in the normal and inverse spinels. In the case of the inverse spinel, such a displacement leads to a spontaneous change in the value of the u parameter of the oxygen lattice to the ideal value, and thus to an ideal defective NaCl-type lattice. On the other hand, in the case of a normal spinel, the displacement of Mg atoms to the unoccupied octahedral sites leads initially to a pseudo-cubic arrangement which then transforms later with further energy to an ideal defective NaCl-type structure. Thus in both cases a defective NaCl-type structure is obtained as the final structure. We find that the total energy barrier for obtaining this structure is the same in both cases.