ELEMENTS OF RARE EARTH MAGNETISM - Rare Earth Magnetism: Structures and Excitations - page 28

constant, was therefore consistent with the standard model. However the
positron-annihilation experiments of Gustafson and Mackintosh (1964)
showed that the change in f occupancy, when the transition was induced
by a change in temperature, was much less than one, and indeed that
the results in both phases were consistent with about one f electron per
ion. Similar results were obtained by Gustafson et al. (1969) when the
transition was driven by pressure, and they concluded that it involves
not primarily a change in the f occupancy but rather a change in the f
state , from being localized in the γ -phase to being an itinerant band elec-
tron in the α -phase. This idea was taken up by Johansson (1974) who,
from a consideration of spectroscopic, cohesive and thermodynamic evi-
dence, proposed that the γ - α transition should be considered as a Mott
localized-delocalized transition among the f electrons. Glotzel (1978)
used density-functional theory to calculate the ground state properties
and showed that the equation of state in the α -phase can be accounted
for rather satisfactorily by including the f electrons in the band struc-
ture, and furthermore that a transition to a spin-polarized state should
occur at a lattice constant close to that of γ -Ce, though at a (nega-
tive) pressure considerably lower than that deduced from experiment.
Eriksson et al. (1990) have recently shown that this discrepancy may be
substantially reduced by including the l - l coupling, which is responsible
for the second of Hund's rules, in the calculation of the 4 f bands. This
leads to a ground state in γ -Ce in which the 4 f electrons are almost
fully polarized, thus occupying the Hund's-rule ground state on each
site. Despite the fact that they are described in the band picture, they
may thus be considered as localized , making very little contribution to
the cohesive properties. The calculated atomic volumes in both phases
are in good agreement with experiment. Podloucky and Glotzel (1983)
found a cohesive energy for α -Ce in accord with the measured value,
while that of a 'promotional' state with no f electrons is far too small.
They were also able to account for the Compton-scattering experiments
of Kornstadt et al. (1980), who had verified that the change in f occu-
pancy at the transition is small. Skriver (1985) calculated the crystal
structure and equation of state of α -Ce up to high pressures, finding
very good agreement with experiment (Staun Olsen et al. 1985), pro-
vided that the f bands are included, but very poor agreement if the
f electrons are promoted to the d bands, or are assumed to be local-
ized, and therefore to make a negligible contribution to the electronic
pressure. The relative stability at high pressures of low-symmetry con-
figurations such as the α -U structure, which is observed experimentally,
is a strong indicator that there are f electrons in the conduction bands,
as in the light actinides, where they play a decisive role in determining
the structure (Skriver 1985).