New Structural Systematics in the Lanthanide Elements at High Pressure

The pressure-induced phase transitions in the lanthanide elements provide insight into their electronic structures at high densities. After a series of transitions via close-packed structures, the regular trivalent lanthanides undergo first-order transitions to so-called "collapsed" phases, the structures of which are reported as monoclinic. However, the diffraction data from these phases are not well fitted by the monoclinic structure, and the patterns from Nd and Sm are different to those from the higher-Z lanthanides. Here we present diffraction studies on Sm, Nd, Tb, and Y, which reveal that there are two collapsed structures, neither of which is monoclinic. 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.