MGAL2O4 SPINEL CRYSTAL-STRUCTURE - AN AB-INITIO PERTURBED ION STUDY

An ab initio perturbed ion (aiPI) study has been carried out for pure and doped MgAl2O4 normal and inverse spinel crystal structures. Cluste rs containing 136 ions have been built up, using large Slater-type orb itals to represent each atomic center. Basis sets and geometry optimiz ations have been performed with the aim of determining the relative st ability, cell parameters, bulk modulus, force constants, and vibration al frequencies of radial displacements associated with the local relax ation for pure and doped structures. Numerical results are confronted against experimental data and previous theoretical calculations. The b ulk modulus of the pure structures has been calculated by means of the Birch-Murnaghan equation of state, the normal structure being less co mpressible than the inverse one. The optimized geometrical cell parame ters of the structures obtained are compared with experimental results . This comparison allows us to analyze the validity of the aiPI method ology for the theoretical characterization of the local properties of complex ionic systems. The energy changes associated with the substitu tion 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 repl acements of Fe3+ for Al3+ in the normal structure and Fe2+ for Mg2+ in the inverse one. However, defect reaction energies for the normal str ucture produce large positive values for the substitutions at the octa hedral site, and only the replacement of Mg2+ for Co2+, Mn2+, and Ni2 at the octahedral site given negative defect reaction energies for th e inverse structure. The doping process produces a decrease of force c onstant (k) values associated with the breathing fundamental vibration al mode at tetrahedral site for the normal structure while an opposite effect appears in the inverse structure. (C) 1995 John Wiley & Sons, Inc.