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 MgAl2O4 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 Al3+ replacing Mg2+ is higher than the inverse process. The results of the simulation studies indicate that the configuration energy is lower for Al3+ being replaced by trivalent impurities such as Cr3+, Mn3+ and Co3+, than for the replacement of Al3+ and Mg2+ by divalent impurities such as Cr2+, Mn2+ and Co2+. Another type of defect formed by two Al3+ replacing two Mg2+ and one Mg2+ vacancy has been studied. In this case the two Al3+ were also substituted by Cr3+, Mn3+ and Co3+ and the possibility of defect aggregation increased with the atomic number of the replacing impurity ion.