Atomistic simulation methods based on pair-wise interatomic potentials and energy minimization have been applied to elucidate the energetics of cation vacancies and the incorporation of 13 trivalent M3+ cations (Cr3+, Ga3+, Fe3+, Lu3+, Yb3+, Er3+, Y3+, Tb3+, Gd3+, Eu3+, Sm3+, Nd3+, La3+) in -Al2O3. Calculations have been carried out using Al64O96 defect spinel supercells containing eight aluminum vacancies. The lowest energy configurations correspond to a random distribution of tetrahedral and octahedral vacancies. The energy gain in comparison with exclusive tetrahedral or octahedral vacancies is rather small (0.03 and 0.09 eV/Al2O3, respectively). Unit cell volume, density, and lattice properties of optimized structures are in good agreement with the experimental values or the results of high-quality density functional theory calculations. The trends observed for the solution energy of the M2O3 oxides in the supercell with minimum energy indicate the preferential incorporation of the foreign ions at the tetrahedral site and an increase of the solubility of M2O3 in the defect spinel in comparison with α-Al2O3. Configurations with the lowest energy have negative solution energies and, consequently, incorporation of trivalent ions can improve the thermodynamic stability of -Al2O3 in comparison with α-Al2O3 and increase the →α transition temperature.
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Document Type: Research Article
INSTM, IENI/CNR, and Department of Physical Chemistry, University of Pavia, I 27100 Pavia, Italy
Institute for Energetics and Interphases, National Research Council, I-16149 Genova, Italy
Publication date: January 1, 2008