MgAl2O4 spinel crystal structure. An ab initio perturbed ion study

Abstract

An ab initio perturbed ion (aiPI) study has been carried out for pure and doped MgAl2O4 normal and inverse spinel crystal structures. Clusters containing 136 ions have been built up, using large Slater-type orbitals to represent each atomic center. Basis sets and geometry optimizations have been performed with the aim of determining the relative stability, cell parameters, bulk modulus, force constants, and vibrational frequencies of radial displacements associated with the local relaxation for pure and doped structures. Numerical results are confronted against experimental data and previous theoretical calculations. The bulk modulus of the pure structures has been calculated by means of the Birch--Murnaghan equation of state, the normal structure being less compressible than the inverse one. The optimized geometrical cell parameters of the structures obtained are compared with experimental results. This comparison allows us to analyze the validity of the aiPI methodology for the theoretical characterization of the local properties of complex ionic systems. The energy changes associated with the substitution of Co2+, Mn2+, Ni2+, and Fe2+ for Mg2+ and Cr3+ and Fe3+ for Al3+ in normal and inverse MgAl2O4 structures are evaluated from a direct solid state reaction. All substitutions are favorable, except the replacements of Fe3+ for Al3+ in the normal structure and Fe2+ for Mg2+ in the inverse one. However, defect reaction energies for the normal structure produce large positive values for the substitutions at the octahedral site, and only the replacement of Mg2+ for Co2+, Mn2+, and Ni2+ at the octahedral site given negative defect reaction energies for the inverse structure. The doping process produces a decrease of force constant (K) values associated with the breathing fundamental vibrational mode at tetrahedral site for the normal structure while an opposite effect appears in the inverse structure. © 1995 John Wiley & Sons, Inc.