Evidence for a Soft Phonon Mode and a New Structure in Rare-Earth Metals under Pressure

Abstract

High-pressure x-ray diffraction experiments up to 30-40 GPa on lanthanum, praseodymium, and yttrium metals demonstrate that there is one more crystal structure in the regular rare-earth crystal-structure sequence. This structure results from a second-order phase transition in fcc phase and could be described by a zone-boundary soft phonon mode with $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=(\frac{2\ensuremath{\pi}}{{a}_{0}})(\frac{1}{2}, \frac{1}{2}, \frac{1}{2})$.