Structure and magnetism of collapsed lanthanide elements

Abstract

Using synchrotron X-ray diffraction, we show that the long-accepted monoclinic structure of the "collapsed" high-pressure phases reported in seven lanthanide elements (Nd, Tb, Gd, Dy, Ho, Er and (probably) Tm) is incorrect. In Tb, Gd, Dy, Ho, Er and Tm we show that the collapsed phases have a 16-atom orthorhombic structure (oF 16) not previously seen in the elements, while in Nd we show that it has an 8-atom orthorhombic structure (oF 8) previously reported in several actinide elements. oF 16 and oF 8 are members of a new family of layered elemental structures, the discovery of which reveals that the high-pressure structural systematics of the lanthanides, actinides and group 3 elements (Sc and Y) are much more related that previously imagined. Electronic structure calculations of Tb, combined with quantum many body corrections, confirm the experimental observation, and calculate that the collapsed orthorhombic phase is a ferromagnet, nearly degenerate with an anti-ferromagnetic state between 60 and 80 GPa. We find that the magnetic properties of Tb survive to the highest pressures obtained in our experiments (110 GPa). Further calculations of the collapsed phases of Gd and Dy, again using the correct crystal structure, show the former to be a type-A antiferromagnet, while the latter is ferromagnetic.

High-pressure structural systematics in samarium up to 222 GPa

“…The structure of Sm metal above 93 GPa is found to be face-centred orthorhombic (oF 8), iso-structural with that observed in Nd, Am, Cm and Cf at high pressures, and iso-symmetric with the oF 16 structure observed in Tb, Gd, Dy, Ho, Er and (probably) Tm 18 . High precision measurements of the compressibility of Sm reveal that it becomes less compressible after the transition to the hP 3 phase at 43 GPa, and that above 65 GPa its compressibility is that of a "regular" metal such as Au or Pt.…”

“…The compressibility of Sm to 222 GPa, including the data of Husband et al below 50 GPa 27 , is shown in Figure 7. The similarities of the hP3 and oF8 structures, which differ only in the stacking sequence of their hcplike layers 18 , results in a sizable pressure range (for example, 93-157 GPa in one of our samples -see Figure 3) over which mixed hP3-oF8 profiles are observed, and in extensive peak-overlap in this mixed-phase region. The P6 2 22 and Fddd space groups are not group-subgroup related and so the transition between the hP3 and oF8 structures need not be second order.…”

“…However, the nature of the stacking in the two structures is the same, with the atoms in each layer centred above the midpoint of two atoms in the previous layer, resulting in both having 10-fold (6+2+2) coordination. We have also reported that the hP3 structure of Sm actually has spacegroup P6 2 22 rather than P3 1 21, and that it has the same stacking as the oF8 and oF16 structures, although with a 3-layer ABC stacking sequence 18 . Finally, we have noted that the reported diffraction profiles from the mC4 phase of Nd 22 are remarkably similar to those from the oF8 phase of Am, Cm and Cf, and that the published d-spacings of Nd at 89 GPa 22 can be fitted perfectly with this orthorhombic structure 18 .…”

“…We have also reported that the hP3 structure of Sm actually has spacegroup P6 2 22 rather than P3 1 21, and that it has the same stacking as the oF8 and oF16 structures, although with a 3-layer ABC stacking sequence 18 . Finally, we have noted that the reported diffraction profiles from the mC4 phase of Nd 22 are remarkably similar to those from the oF8 phase of Am, Cm and Cf, and that the published d-spacings of Nd at 89 GPa 22 can be fitted perfectly with this orthorhombic structure 18 . The collapsed oF8, oF16 and hP3 phases are thus all members of the same family of quasi-close-packed layer structures, differing only in the stacking sequence of the layers.…”

“…We have recently shown that the long-reported mC4 structure is incorrect in Tb, and also in Gd, Dy, Ho, Er and Tm, and that the true structure of these collapsed phases is orthorhombic, space group Fddd, with 16 atoms per unit cell (hereafter oF16) 18 . While isosymmetric with the 8-atom Fddd structure found in Am, Cm and Cf at high pressures [19][20][21] , the oF16 structure comprises an 8-layer ABCADCBD stacking of quasi-hcp layers, as opposed to the 4-layer ABCD repeat seen in oF8see Figure 1.…”