Chemistry Reference
In-Depth Information
state (EoS). This technique has the added advantage of enabling the pressure to be
determined from the diffraction measurement itself, rather than making a separate
optical measurement. However, the technique does have the drawback that the
sample pressure can only be determined
after
the diffraction measurement, rather
than the sample pressure being adjusted to a required value
before
the data collection.
The calibrant included with the sample should have a number of desired qualities.
It should have high symmetry and a small unit cell (to reduce the number of
diffraction peaks that might overlap those from the sample); its volume should be
strongly pressure dependent in order to maximise pressure sensitivity; it should not
react with the sample or the pressure transmitting fluid; and it should be strongly
scattering so that little of the calibrant is needed. Popular materials include NaCl
[
151
], quartz [
152
] and a number of cubic elemental metals such as Pt, Au, Cu and Ta
[
153
,
154
]. The latter materials are most widely used for ultrahigh-pressure studies.
3.3 Diffraction Techniques
There are four principal methods of determining crystal structures at high pressures
employing powders or single-crystals, using X-rays or neutrons. Here I will give a
brief review of the relevant diffraction techniques and analysis methods used with
each technique, focusing on recent developments.
3.3.1 X-Ray Powder Diffraction
While this has long been the most popular method to study crystal structures, it was
not until the introduction of ADXRD on synchrotron sources in the 1990s that it
became routine to determine atomic position parameters using this technique at
high pressure. While such studies have been performed principally at synchrotron
sources, laboratory-based studies have also been performed; see for example
[
155
-
158
].
The use of powder samples greatly simplifies sample loading. In addition,
since many pressure-induced phase transitions are strongly first order (the Si-I
to Si-II phase transition at 11.4 GPa has a volume change
V
/
V
trans
of 20.4%
[
159
]), samples which are initially loaded as single crystals do not typically
remain single crystals as the pressure is increased. As described in Sect.
2
,prior
to 1990 high-pressure powder-diffraction studies were typically conducted using
energy-dispersive diffraction (EDX) techniques at synchrotron sources in which
a polychromatic incident beam was diffracted at a fixed angle (defined by tightly-
collimating slits) into an energy-resolving detector (see for example, [
160
]).
Although the extremely high intensity of the “white” synchrotron beam meant
that this technique could be used to pressures in excess of 300 GPa [
23
], the
relatively poor energy resolution of the detector, and the very tight collimation of
both the incident and diffracted beams, meant that the diffraction patterns have
D
