Lattice dynamics of lithium at low temperature

Below a temperature of 80 K, lithium transforms from its room-temperature bcc structure to a close-packed rhombohedral phase (9R) having three atoms per primitive unit cell. The phonon spectrum of the 9R phase is calculated from a pseudopotential model that fits the experimental bcc phonon spectrum. The predicted zone-center optical modes are a doubly degenerate LO mode at 7.9 THz and a fourfold-degenerate TO mode at 3.4 THz.