Chemistry Reference
In-Depth Information
in approximately the plane of the (typically X-ray transparent) gasket. Using
such cells, the volume of reciprocal space accessible in single-crystal studies is
increased considerably. A DAC that allows data to be collected in both trans-
verse and transmission geometries has also been designed [
118
]. However, while
they offer increased access to reciprocal space, the complex X-ray attenuation
corrections that need to be used on data collected in DACs with the transverse
geometry means that they are not as well suited to crystallography as those using
the transmission geometry.
DAC technology continues to develop. Recently, the development of Boehler-
Almax anvil seats [
119
] means that DACs no longer need to use X-ray transparent
beryllium or BN seats in order to have the widest possible angular apertures. Single-
crystal data can thus be collected to high angles without any contaminating diffrac-
tion lines from the anvil seats themselves [
68
]. The Boehler-Almax seats also
enable cells with full conical apertures to be used at pressures to 150 GPa [
120
].
A still more recent development has been to use transparent anvils made from
sintered nanocrystalline diamond [
121
-
123
]. Such anvils do away with the effects
of the intense Bragg reflections from the anvils (although these are replaced by
powder-diffraction lines). These sintered anvils are also tougher than single-crystal
diamond anvils, and their lower thermal conductivity is advantageous for laser-
heating studies [
124
].
3.1.2 Cells for Neutron Diffraction
The large sample sizes required for neutron-diffraction studies means that the
diamond anvil cells so widely used for X-ray studies cannot be used routinely for
neutron diffraction studies - although attempts have been made to use DACs with
extremely large anvils [
125
]. However, the low neutron-absorption of many mate-
rials means that pressure cells constructed from Al alloys and other strong materials
can be used. Pressure cells for neutron diffraction typically fall into three different
categories: (1) cells utilising high-pressure gas to compress the sample, (2) those
using a piston-cylinder arrangement and (3) cells with opposed anvils. The gas
cell design of Paureau and Vettier [
126
] is capable of compressing several cubic
centimetres of sample in a perfectly hydrostatic environment to 1 GPa by using He
gas as a pressure transmitting medium. Such cells have been used extensively at the
Institut Laue-Langevin (ILL) reactor source in Grenoble and at the IPNS neutron
source at Argonne National Laboratory, and have produced extremely high quality
crystallographic data on a wide range of different materials. In piston-cylinder
designs [
127
,
128
], the cells use a liquid pressure medium, and pressures of 2 GPa
are achievable with both powder and single-crystal samples. The cells are typically
pressurised off-line using a press, and then clamped to maintain the sample pres-
sure. These cells have also been designed to be inserted into cryostats, extending
studies to temperatures of 4 K and below.
The widest range of pressures has been achieved with opposed-anvil designs.
Cells for single-crystal diffraction studies have been based on the DAC and have
