The trivalent lanthanides have long been known to exhibit a series of phase transformations with increasing pressure: hcp → Sm-type → dhcp → fcc → dfcc (distorted-fcc) → “volume-collapsed”. These have long been ascribed to changes in electronic structure [1]. Beyond the dfcc phase, considerably more complicated, low-symmetry “collapsed” structures are seen, most commonly the mC4 monoclinic structure with space group C2/m.

However, despite previous extensive study in many of the lanthanide elements, there is still a strong disagreement as to the true structures of the post-dfcc phases in a number of lanthanides (Nd, Sm and the heavy trivalent lanthanides). Previous structural assignment via powder X-ray diffraction for the heavy trivalent lanthanides, in particular Gd, Tb and Dy, has depended upon the “analysis by analogy” approach, where structural determination at high pressures in these elements depends on the solutions obtained with higher-quality data in the lower-Z elements  at lower pressures [2,3]. Currently, analysis on the post-dfcc phases of Gd, Tb and Dy made by our group shows that the high pressure systematics of the trivalent lanthanides is incorrect (see Figure 1).

In addition, new data we have obtained from Sm above 100 GPa reveal that while it is more like Nd than previously reported [4], it too has peaks unexplained by the C2/m structure.

References

[1] Young, “Phase Diagrams of the Elements”, (University of California Press, 1991).

[2] McMahon & Nelmes PRL 78, 3884 (1997).

[3] Evans et al. PRB 80, 134105 (2009).

[4] Chesnut (PhD thesis, Univ Alabama, 2001).