The lanthanide and actinide metals have complex electronic and magnetic
behaviour at high pressures and low temperatures, respectively, owing to the
f electrons buried within the atoms participating in the bonding of those metals.
As one traverses the lanthanide series with increasing Z at ambient conditions,
each element adds one electron to the 4f shell, starting with cerium (Ce) and
ending with a full 4f electron shell in lutetium (Lu). This thesis focuses on
the trivalent lanthanides, which are considered to be all the lanthanides except
Ce, europium (Eu) and ytterbium (Yb), and their structural systematics at
high pressures. The trivalent lanthanides exhibit a common, pressure-induced,
structural phase transition sequence: hexagonal close-packed (hcp) ! Sm-type
or -Sm ! double-hcp (dhcp) ! face-centred cubic (fcc) ! distorted-fcc (dfcc)
! \volume-collapsed". This sequence is thought to arise from the 5s ! 5d
transfer of electrons, and, at ultra-high pressures, from the 4f electrons becoming
delocalised and then participating in the bonding of these elements, leading to
volume-collapsed phases with low-symmetry structures. However, there is still
debate over the exact structures of the low-symmetry collapsed phases.
This thesis describes x-ray di raction studies of the high-pressure structures of the
collapsed phases of six trivalent lanthanide elements (neodymium (Nd), samarium
(Sm), praseodymium (Pr), gadolinium (Gd), terbium (Tb) and dysprosium
(Dy)), and also yttrium (Y) { a rare-earth element. Using diamond anvil cell
(DAC) techniques and x-ray powder di raction at the Diamond Light Source
and PETRA-III synchrotrons, the structures of the collapsed phases of six of the
seven elements are found to be face-centred orthorhombic, spacegroup Fddd, with
either 8 atoms (Nd and Sm) or 16 atoms (Gd, Tb, Dy and Y) in the unit cell.
The 8-atom structure (oF8) is the same as that seen previously in the actinides
plutonium (Pu), americium (Am), curium (Cm) and californium (Cf), greatly
strengthening the structural relationships between the 4f lanthanides and the 5f
actinides. The 16-atom structure (oF16) found in Gd, Tb, Dy and Y is previously
unknown in the elements, but, along with the oF8 structure, and hP3 structure
seen in Nd and Sm at lower pressures, it is a member of a new structural family
comprising di erent stackings of quasi-hcp layers.
This thesis also describes higher-pressure studies with the aim of determining the
structural systematics of the post-oF8/oF16 phases. Pr and Nd have been shown
to exhibit the -uranium ( -U) structure, oC4, in their collapsed phases. In Pr,
a primitive orthorhombic structure was previously reported above 147 GPa, the
post- -U phase, a structure not seen in any other element. This thesis shows that
the true structure is body-centred tetragonal, the same as the highest-pressure
phase seen in the neighbouring lanthanide Ce, and also in the actinides thorium
(Th) and uranium (U), suggesting that it is the \next" structure in the transition
sequence of the trivalent lanthanides.
This work contained in this thesis completely rewrites the structural behaviour
of the trivalent lanthanide elements at high-pressure and reveals that the highpressure
behaviour of the lanthanide and actinide metals is much more similar
than previously believed.
i
