Chemistry Reference
In-Depth Information
that are possible at present in Sect.
4
, the next section reviews high-pressure
crystallographic methods.
3 Experimental Methods
3.1 Pressure Cells
3.1.1 Diamond-Anvil Cell
While a great deal of high-pressure science was conducted prior to the invention of
the DAC in 1958, very few diffraction studies were performed prior to this date
[
6
,
7
]. Those that were conducted mostly involved the use of either Be cylinders
[
103
], or a diamond in a piston-cylinder arrangement where the diamond, with a
small hole drilled through it, acted as the cylinder [
11
]. The invention of the DAC,
in which the sample is compressed between two opposing single-crystal diamond
anvils, opened the door to X-ray diffraction science. In initial studies, a powdered
sample was compressed directly between the two anvils. However, the subsequent
introduction of the gasketed DAC in 1962 [
104
] in which a thin sheet of metal is placed
between the diamonds, with a small hole to contain the sample, greatly increased
the scope of the DAC. It enabled higher pressures to be reached by providing
support to the tip (culet) of the diamonds, and also enabled liquids and gases to be
loaded and compressed, and therefore the use of a hydrostatic fluid to achieve more
hydrostatic pressures and hence better quality diffraction patterns. It also allowed
single-crystals to be loaded and studied. The use and history of the DAC has been
extensively reviewed by others, and the reader is referred to these excellent reviews
for detailed further information [
19
,
20
,
61
,
63
,
105
-
111
].
The basics of the DAC are shown in Fig.
1
. A detailed description of how to align
a DAC, and the details of its loading, have been described by Miletich et al. [
63
],
and the reader is directed to this excellent description if they wish more details. The
diamond anvils are flawless gem-quality stones of typically 0.25 carats, with a culet
size of 50-800
m
m: the smaller the culet, the higher the achievable pressure (but the
smaller the sample). For pressures above 100 GPa, where the tip of the diamond
undergoes significant distortion, so-called bevelled diamonds are used [
112
]. The
anvils are mounted on anvil seats, made of a hard material such as Be or BN, both of
which are X-ray transparent, or tungsten carbide. The metallic gasket between the
anvils is initially 200-250
m
before the sample chamber is drilled by electrical spark erosion or laser drilling.
The diameter of the sample chamber hole is typically one-third of the diameter of
the culets.
The sample is loaded into the gasket hole, and surrounded by a pressure
transmitting fluid - which might be an alcohol:water mixture (which provides
a quasi-hydrostatic medium to ~10-16 GPa) or a crystalline gas such as Ar,
Ne or He (loaded either at high pressures or cryogenically as a liquid) which is
m
m thick, but is preindented to a thickness of 5-50
m
