Transition metal AB3 intermetallics: Structure maps based on quantum mechanical stability

We study a two-dimensional structure map for AB3 binary transition metal compounds with variables appropriate for direct quantum-mechanical energy calculations. The variables are the electron count and ΔHii, the difference in d-orbital Coulombic integrals. The experimental structure map differentiates between the six known AB3 transition metal structure types: Cr3Si,AuCu3,SnNi3,TiAl3,TiCu3 and TiNi3. The theoretical quantum mechanical map (based on μ2-Hückel calculations) gives good agreement with the experimental map.The numerical accuracy of the μ2-Hückel energies is assessed by direct comparison to LDA-DFT calculations carried out on TaIr3. For this system, both the μ2-Hückel and LDA-DFT calculations place the six structure types in the same energetic order. The μ2-Hückel theory, in addition, allows further analysis on the structural origins of these differences in energy. The chief structural features responsible for differences in energy prove to be the varying number of three- and four-member rings of bonded atoms. These results help delineate the principal factors responsible for transition metal icosahedral, Frank-Kasper vs. closest-packed structures.