Defect simulation in MgAl2O4 spinels

Computer modelling methods, based on the lattice energy and the defect energy minimization, have been used to study some properties of the MgAl 2 O 4 spinels. Calculations have been carried out using defect simulation procedures, programmed in the "General Utility Lattice Program - GULP" following the Mott and Littleton strategy. Intrinsic defects which originate from cation antisite disorder due to the partial exchange of the positions of the divalent and trivalent cations have been studied. It was found that the probability of Al 3+ replacing Mg 2+ is higher than the inverse process. The results of the simulation studies indicate that the configuration energy is lower for Al 3+ being replaced by trivalent impurities such as Cr 3+ , Mn 3+ and Co 3+ , than for the replacement of Al 3+ and Mg 2+ by divalent impurities such as Cr 2+ , Mn 2+ and Co 2+ . Another type of defect formed by two Al 3+ replacing two Mg 2+ and one Mg 2+ vacancy has been studied. In this case the two Al 3+ were also substituted by Cr 3+ , Mn 3+ and Co 3+ and the possibility of defect aggregation increased with the atomic number of the replacing impurity ion.