Electronic structure, phase stability and elastic moduli of AB transition metal aluminides

Abstract

Binding energy curves for the 4d transition metal TM aluminides TMAl with respect to 12 different AB-structure types have been computed using the full-potential linear muffin-tin orbitals (FP-LMTO) method within local-density-functional approximation (LDA). Combining with our previous calculations for the 3d TMAl, we find that the observed ground state structures are predicted correctly for all the AB transition metal aluminides including the low-temperature monoclinic C2/m phase of CuAl. Moreover, the B32 phase is predicted to be the most stable amongst competing metastable phases for CrAl, MoAl, TcAl, whereas for VAl and NbAl the most stable phases are L1 0 and Ω, respectively. The calculated heats of formation are in good agreement with available experimental data. The critical roles played by the average number of valence electrons per atom and the angular character of the valence orbitals are emphasized in explaining the structural phase stability across the TMAl series. In particular, it is shown that the structural trend going from B2 (bcc-like) → L1 0 (fcc-like) → B19 (hcp-like) → B2 as a function of electron concentration, can be understood from a band structure energy analysis. The calculated electronic structure for all the stable phases of both the 3d and 4d TMAl demonstrates a correlation between structural stability and the shape of the density of states due to the strong directional bonding between the sp(Al) and d(TM) orbitals. Finally, the elastic moduli have been computed for all observed ground states of TMAl with new results for ScAl and RhAl (in the B2 structure), YAl and ZrAl (in the B33 structure) and for PdAl (in the B20 structure).