Ground state structural stability of ordered fcc- and bcc-based Li-Al compound under first and second nearest neighbour pair approximation

Self-consistent local density electronic structure calculations have been performed on a series of ground state ordered superstructures of lithium-aluminium alloys spanning the entire concentration range. These structures are based on both fee and bcc lattices under the first and second nearest neighbour pair approximation which is adequate to stabilize all the stable and metastable phases of the Li-Al system. Using the efficient tight-binding linear muffin-tin orbital (TB-LMTO) method, we have calculated the volume dependent total ground state energies and the systematic trends in various cohesive and electronic properties at zero temperature, as a function of Li concentration. The predicted heats of formation for all the different ground state superstructures result in a representative stability profile, which shows that the L1(2), B32 and DO3 structures are the most stable amongst various phases having Al3Li, AlLi and AlLi3 compositions, respectively. Moreover, we have parameterized the cohesive energies using the Connolly-Williams cluster expansion method and estimated the effective many-body interactions for the fee lattice in an octahedron-tetrahedron cluster approximation, and for the bcc lattice in an irregular tetrahedron cluster approximation. These volume dependent but configuration (as well as concentration) independent interactions coming out of the TB-LMTO-CWM approach are not only important for first principles calculation of phase diagram but are also useful for predicting the evolutionary path of ordering processes. (C) 1996 Elsevier Science Limited.