Effect of ternary additions on the structural stability and electronic structure of intermetallic compounds: Al₃Ti + Cu

The effects of substituting Cu for either Ti or Al in Al₃Ti were studied by the first-principles local density self-consistent linear muffin tin orbital method. The Cu atoms are found to strongly favor Al sites and to promote the stability of the L1₂ phase, in agreement with experiment. Surprisingly, the rigid band model is closely followed when Cu substitutes for Al. However, the structure of the density of states undergoes remarkable changes if Cu substitutes for Ti; a much weaker hybridization occurs in both L1₂- and DO₂₂-like structures, giving rise to weaker binding effects. The modification of bond directionality upon Cu addition is determined by comparing the charge density for the structures calculated with those of pure Al₃Ti. The effect of tetragonal distortion is also examined. By comparing with the bonding characteristics of pure Al₃Ti in the L1₂ and DO₂₂ structures, the addition of Cu to Al₃Ti is found to be equivalent to the tetragonal distortion in DO₂₂ Al₃Ti as far as bonding is concerned, resulting in the stabilized L1₂-like structure for (AlCu)₃Ti. The semi-empirical inverse relation between the structural stability and the density of states at Fermi energy is well established.