Pressure Dependence of the Elastic Moduli in Aluminum Rich Al-Li Compounds

TL;DR

This study uses first principles calculations to analyze how pressure affects the elastic moduli of various Aluminum-Lithium compounds, revealing increases in elastic moduli with pressure and instability in BCC Lithium at high pressure.

Contribution

It provides detailed first-principles predictions of pressure-dependent elastic moduli for Al-Li compounds, including error estimation and comparison with experimental data.

Findings

Elastic moduli increase with pressure for most compounds.

BCC Lithium becomes elastically unstable at around 2 GPa.

Computed moduli are within 10% of experimental values when using experimental lattice constants.

Abstract

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 Al_3Li, and an ordered FCC Al_7Li 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 was 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 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 C11-C12 increases from 59.3 GPa in Al, to 76.0 GPa in Al_7Li, to 106.2 GPa in Al_3Li. The modulus C_44 increases from 38.4 GPa in Al to 46.1 GPa in Al_7Li, then falls to 40.7 GPa in Al_3Li. 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.