Chemistry Reference
In-Depth Information
If one wants to attain temperatures higher than those attainable with resistive
heating, then this can be achieved with laser heating, in which either ~1-
m
mor
~10-
m infrared radiation from Nd:YAG or CO
2
lasers, respectively, is focused
onto the sample. The focus spots can be as small as ~10
m
m across, with tempera-
ture gradients of many 100 s of K per micron. To ensure as uniformly heated spot
as possible, the sample should be heated from both sides simultaneously, and
the lasers defocused slightly, or passed through beam-shaping optics, to increase
the size of the heated region, and to give more uniform heating. It is essential that
the X-ray beam be centred precisely within the heated spot, thereby ensuring that the
diffraction data are collected only from the heated region. Recent advances in laser
technology have resulted in a move to fibre lasers, the small size of which adds
a flexibility that is well suited to installation on synchrotron beamlines. Using
two such lasers, Tateno et al. have recently obtained high quality diffraction data
from iron at 377 GPa and 5,700 K, which corresponds to Earth inner-core condi-
tions [
209
]. There is an extremely large number of papers describing laser-heating
systems, and the reader is pointed to the review of Eremets [
109
] and the recent
papers by Prakapenka et al. [
210
], Dubrovinsky et al. [
211
] and Goncharov et al.
[
212
] for more details of the technique.
Resistive heating techniques have also been widely used in pressure cells suitable
for neutron diffraction studies. However, additional problems are raised by the neces-
sity of having to keep the larger-volume samples necessary for neutron-diffraction
studies heated for the typically longer data collection times. Using an internal
cylindrical furnace held within the gasket, the P-E press has been used for neutron
diffraction studies to 10 GPa and 1,500 K [
135
,
213
-
216
]. Neutron radiography was
used to determine the temperature in the furnace with a precision of
m
20 K. A P-E
press, combined with a T-cup multi-anvil stage, has been developed for angle-
dispersive X-ray power diffraction studies to 25 GPa and 2,000 K [
217
].
Low-temperature studies have typically been more popular in high-pressure
neutron diffraction studies than in X-ray studies, perhaps because of the combined
use of neutron scattering techniques and low temperatures to study magnetism.
All of the various pressure cells used for neutron diffraction studies can be cooled
with liquid-He cryostats or closed-cycle refrigerators. The P-E press has also been
cooled more simply by direct liquid-N
2
cooling, with good temperature control then
possible between 77 and 200 K. High-pressure low-temperature crystallographic
studies using X-rays have proved somewhat rarer, although more studies are
appearing as cryostats have begun to appear on some high-pressure synchrotron
beamlines as standard sample environment equipment [
218
,
219
].
4 Experimental Examples
The following four examples have been chosen to illustrate the variety of different
crystal structures now being found in simple materials at high pressure, and
the quality of diffraction information that was necessary to solve the crystal
