PRESSURE-DEPENDENCE OF THE ELASTIC-MODULI IN ALUMINUM-RICH AL-LI COMPOUNDS

I have carried out numerical first-principles calculations of the pressure dependence of the elastic moduli for several ordered structures in the aluminum-lithium system, specifically fcc Al, fcc and bcc Li, L1(2) Al3Li, and an ordered fcc Al7Li supercell. The calculations were performed using the full-potential linear augmented plane-wave method (LAPW) to calculate the total energy as a function of strain, after which the data were fit to a polynomial function of the strain to determine the modulus. A procedure for estimating the errors in this process is also given. The predicted equilibrium lattice parameters are slightly smaller than found experimentally, consistent with other local-density-approximation (LDA) calculations. The computed elastic moduli are within approximately 10% of the experimentally measured moduli, provided the calculations are carried out at the experimental lattice constant. The LDA equilibrium shear modulus C-11-C-12 increases from 59.3 GPa in Al, to 76.0 GPa in Al7Li, to 106.2 GPa in Al3Li. The modulus C44 increases from 38.4 GPa in Al to 46.1 GPa in Al7Li, then falls to 40.7 GPa in Al3Li. All of the calculated elastic moduli increase with pressure with the exception of bcc Li, which becomes elastically unstable at about 2 GPa, where C-11-C-12 vanishes.