Configurational and energy study of the (100) and (110) surfaces of the MgAl2O4 spinel by means of quantum mechanical and empirical techniques

Abstract

This paper presents a detailed configurational analysis of the {100} and {110} crystallographic forms of the spinel sensu stricto MgAl₂O₄. In order to collect as many structural and energy data as possible about the most stable surface terminations, we have performed accurate calculations, both at the empirical and at the DFT level at 0 K under vacuum, by using a dedicated force field and the hybrid Hartree–Fock/Density Functional B3LYP Hamiltonian, respectively. The configurational analysis performed in this work on MgAl₂O₄ will be useful for studying all of the minerals belonging to the family of normal spinels (i.e., MgCr₂O₄). Indeed, the initial configurations found for the (100) and (110) faces of MgAl₂O₄ are the same for all of the normal spinels. As regards the (100) face, we found that the surface configuration with the lowest surface energy (1.596 J m⁻²) is associated with the Mg-terminated one. Furthermore, we found an Al–O-terminated (100) configuration with a surface energy value of 2.161 J m⁻² that is notably lower than those previously calculated by other authors. This proves that to gain an in-depth knowledge of a crystal surface, it is not sufficient to explore a limited number of terminations (configurations), but it is essential to perform a detailed crystallographic and configurational analysis of the face. In the case of the (110) face and at variance with the (100), the most stable surface configuration (2.752 J m⁻²) was found to be an Al–O-terminated one.