Chemistry Reference
In-Depth Information
c
guest
b
guest
a
guest
b
host
a
guest
b
guest
c
host
a
host
c
guest
Fig. 17 The host-guest composite structure of the tI19 phase of Na shown in projection down the
c
-axis. The host atoms are shown in
light grey
and guest atoms in
dark grey
. The monoclinic guest
component unit cell is outlined with
dashed lines
, and a separate perspective view of the guest
structure is shown
confirmation of the e-density pockets in hP4-Na will require very high accuracy
diffraction data to be collected to high diffraction angles. This represents a substan-
tial experimental challenge for crystallographic studies, and remains slightly
beyond current capabilities.
5 Twenty-First Century High-Pressure Crystallography
After the revolutions of the 1990s in both X-ray and neutron diffraction techniques,
and the consolidation and exploitation of the last 10 years, what does the future hold
for high-pressure crystallography? DACs are currently capable of obtaining pres-
sures in excess of 400 GPa (4 Mbar) and there is a current international effort to
extend this limit to 500 GPa (0.5 TPa) and beyond. Diamond anvil cells are now
being equipped with large man-made single-crystal [
253
,
254
] or nanocrystalline
[
121
,
124
] anvils, and recent advances in pulsed laser heating provide access to ever
higher temperatures [
255
-
257
]. X-ray beams are becoming ever smaller, with
nanoscale X-ray probes being employed to reduce stress gradients at multimegabar
pressures, and to perform single-crystal diffraction studies of individual single
grains of powder samples at ultrahigh pressures [
258
]. In further exploitation of
the P-E press, we are developing anvils with transparent insets that will allow many
of the benefits of diamond anvil cells (such optical access for spectroscopy, laser
heating and monitoring in situ crystal growth) to be used with much larger samples.
