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 Li-Li vibration, approximately 1800 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 Li-Li contact less than 1.2 angstrom in a linear chain. However, no stable structure was found in the megabar pressure range that satisfies this condition.