Relative stability of LI₂, DO₂₂, and DO₂₃ structures in MAl₃ compounds

The structural energy differences between cubic LI₂ and tetragonal DO₂₂ crystal structures are calculated for MAl₃ compounds, where M is a group III, IV, or V transition metal. The stability of the DO₂₂ structure relative to L1₂ increases rapidly as the transition-metal d-electron count increases. Typical values of E(DO₂₂) – E(L1₂) are 0.1–0.15 eV/atom (9600–14500 J/g-atom) for group III,  0.05 eV/atom ( 4800 J/g-atom) for group IV, and ∼ –0.2 eV/atom (∼ –19000 J/g-atom) for group V trialuminides. Similar trends are calculated for the DO₂₃/L1₂ energy difference. The calculated electronic densities of states (DOS) show that each structure has a minimum in the DOS distribution at a characteristic d-electron count. The preferred crystal structure for a given compound is the one in which the Fermi level lies in the minimum.