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.
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Wang et al. (1986) studied this question.
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