New structural systematics in the lanthanide elements at high pressure

The pressure-induced structural phase transitions in the lanthanide elements provide insight into changes in their electronic structures at high densities. After a series of transitions via close-packed structures, the regular trivalent lanthanides (La to Lu, excluding Ce, Eu and Yb) undergo first-order transitions to so-called collapsed phases, the structures of which have long been reported as monoclinic. However, the diffraction data from these phases are not well fitted by this monoclinic structure, and the patterns from Nd and Sm are distinctly different to those from the higher-Z lanthanides (Gd→). Here we present results from recent diffraction studies on Tb, Gd, Sm, Nd and Y to above 300 GPa, which reveal that there are two different collapsed structures, neither of which is monoclinic. High-precision equation of state studies of the same elements reveal distinct changes in compressibility once the collapsed phases are obtained. We also show that these new structures are strikingly similar to those observed in the higher-Z actinides at high pressure, greatly strengthening the structural systematics of the 4f and 5f elements.