Dynamics of elemental lithium at megabar pressures

Abstract

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 $\mathrm{Li}\text{\ensuremath{-}}\mathrm{Li}$ vibration, approximately $1800\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, 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 $\mathrm{Li}\text{\ensuremath{-}}\mathrm{Li}$ contact less than $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ in a linear chain. However, no stable structure was found in the megabar pressure range that satisfies this condition.