Structure of the distorted fcc high-pressure phase of the trivalent rare-earth metals

The crystal structure of the high-pressure distorted fcc phase, which is so far the last in the sequence of crystal structures for the trivalent rare-earth metals, is assigned the trigonal space group $P{3}_{1}21\left(D_3^4\right)$ or $P{3}_{2}21\left(D_3^6\right)$ with $Z=6\mathrm{}$. In this space group the distortion of the fcc lattice occurs continuously and is consistent with the second-order nature of this phase transition. The observed intensity ratios of reflections for the distorted fcc phase of Y, La, and Pr are satisfactorily explained. This structure can be described by the softening of a zone-boundary longitudinal phonon mode accompanied by the shearing of the hexagonal layers in the basal plane.