A 1ST PRINCIPLES STUDY ON THE STRUCTURE AND THE ELASTIC PROPERTIES OFLI-MG ALLOYS

Elastic properties of the Li15Mg alloy have been studied by using the first principles structural optimization method. The results show that at zero pressure the relative distance between the Mg atom and the ne arest Li atoms is larger than that in the ideal crystal and when compr essing the lattice this distance will increase further. A simple model is introduced to interpret this relaxation. It is also shown that str uctural optimization has to be taken into account when investigating t he bulk properties of alloys. The bulk modulus of the optimized Li15Mg is smaller than that of the ideal structure although the volume of th e optimized structure is smaller than the volume of the ideal structur e. Hence bulk modulus is determined not only by change in the crystal volume but also by local changes in the crystal structure near to the Mg atom. An interpolation formula to calculate compressibility at any composition is presented.