Dynamics of elemental lithium at megabar pressures

A recent report on the Raman observation of a high-pressure broken-symmetry phase of Li at 120 megabar pressures is examined with first-principles electron structure and phonon calculations. The results show that the observed very high-frequency $\mathrm{Li}\text{−}\mathrm{Li}$ vibration, approximately $1800{\mathrm{cm}}^{-1}$, cannot be reproduced by the “dimer” structure proposed earlier or a recently found energetically competitive orthorhombic structure. Model calculations show that such a high Li vibrational frequency could, in principle, be achieved at a $\mathrm{Li}\text{−}\mathrm{Li}$ contact less than $1.2\mathrm{\AA }$ in a linear chain. However, no stable structure was found in the megabar pressure range that satisfies this condition.

Figure 1

The phonon band structure of (a) the broken symmetry $\alpha \text{−}\mathrm{Ga}$ structure at $137\mathrm{GPa}$ and (b) Cmca-24 structure at $189\mathrm{GPa}$ of dense Li.