Atomistic study of defect, metastable and 'amorphous' structures of MgAl2O4

We have studied the defect and metastable structures of MgAl2O4 using a Born-like description of an ionic lattice and the shell model to represent the polarization of ions. The calculated normal spinel structure of MgAl2O4 has a lower energy than other metastable structures. We have simulated an amorphous phase of MgAl2O4 and found that the volume of the amorphous state is larger and the Young modulus is smaller than those of the normal spinel MgAl2O4. This agrees with experimental observations. In agreement with other studies of spinel materials, the most energetically accessible mode of disorder is via the exchange of cations. The energy necessary for this disorder reaction is far lower than for either the Frenkel or the Schottky reactions. Possible phases intermediate between the normal and amorphous are also considered.