Chemistry Reference
In-Depth Information
~2 GPa [
185
,
186
]. As the design of this cell has limited access for the incident and
diffracted beams, data collection is limited to perhaps only one layer of reciprocal
space (although a few reflections from other layers might be visible if the crystal is
slightly tilted relative to the window in the cell body). The crystal must thus be
mounted in the cell so as to have the reflections of interest accessible, and also
aligned so that the reflections remain visible as the pressure is increased. Other
(perhaps perpendicular) layers of reflections can be obtained using crystals with
different orientations. Data collection using this cell does not require a four-circle
diffractometer, and a lifting-counter diffractometer has been used successfully at
the ILL [
185
,
186
]. Although the pre-compressed Al
2
O
3
cylinder within these cells
produces an intense background, the effected regions of the diffraction pattern are
omitted from the fit.
Prompted by the success of the DAC, opposed-anvil cells equipped with large,
normally sapphire, anvils have been used in a number of high-resolution diffraction
studies that have used classical four-circle diffractometers [
187
-
189
] to perform
high quality studies to above 2 GPa. The quality of the data is excellent, particularly
if collected using small area detectors which became available in the late 1980s
[
190
], and the use of which is now widespread.
Using the VX variant of the P-E cell, specifically designed for single-crystal
diffraction [
137
], Bull et al. have recently pushed high-resolution single-crystal
structural studies above 10 GPa using time-of-flight techniques at the ISIS neutron
source and crystals with volumes of several mm
3
[
78
]. Careful design of the anvil
geometry was necessary in order to be able to correct for their considerable
absorption [
191
]. One disadvantage of the standard P-E anvils is that they are either
sintered WC or sintered diamond, both of which are optically opaque. It is therefore
not possible to inspect visually the single crystal on compression, nor to be able to
monitor the sample chamber in order to grow single crystals in situ from, say, the
melt. Again, using the VX variant of the P-E press, Bull et al. have overcome this
by using anvils which are large synthetic diamonds with a culet size of 3 mm [
192
].
This enabled them to monitor the growth of a single-crystal of ice-VI as it was
grown from the melt at 1.3 GPa. The structure was subsequently studied at 1.3 GPa
and 10 K. Very recently, the same authors have performed single-crystal studies on
the D9 diffractometer at the ILL in full 4-circle geometry to 3 GPa, with the small
pressure cell mounted on the chi-circle, and to 10 GPa with a larger pressure cell
mounted on the omega circle and a lifting arm detector in normal-beam geometry.
They have also made preliminary studies at ISIS to 17 GPa using Ar as a pressure
transmitting medium.
Although the VX variant of the P-E press was designed specifically for single-
crystal studies, and has wider angular apertures than the original design, large segments
of reciprocal space are still obscured by the frame press. This has been overcome by
the development of a rotation mechanism that allows the anvils to be rotated within
the frame of the cell, while maintaining the load on the sample, to allow much greater
access to reciprocal space for single-crystal studies [
193
].
The advantages of using polychromatic radiation from a reactor source, coupled
with Laue diffraction methods, have been investigated by McIntyre et al. for high-
pressure studies [
194
]. Using a 0.5-mm
3
crystal in a moissanite anvil pressure cell
