High-Pressure Crystallography - Advanced X-Ray Crystallography - page 74

Pressure Calibrant

Gasket

Gasket

Sample

Hydrostatic Fluid

Diamond Anvil

Anvil Seat

Fig. 1 Schematic diagram of a diamond anvil cell (DAC). The sample, pressure calibrant and
hydrostatic fluid are loaded into the gasket hole, which is then compressed between the culets of
the two diamond anvils

quasi-hydrostatic to 50 GPa or so [ 113 - 116 ]. Along with the sample, it is also
necessary to include a pressure calibrant with the sample. This might be a small
(5 m diameter) piece of ruby or other fluorescing material, the fluorescence

wavelength of which is known to change with pressure, or a calibrant (Au, Cu,
Ta) whose equation of state has been calibrated previously (see Sect. 3.2 ).
For crystallographic studies, the typical diffraction geometry is that in which the
incident X-ray beam enters the sample through one anvil, and the diffracted X-rays
exit through the other. This is the so-called transmission geometry. In order to
ensure that the maximum amount of powder-diffraction data are accessible, the
opening angle in the anvil seat on the exit side of the pressure cell should be as large
as possible. Anvil seats with conical apertures with a full opening angle of 70 are
now used routinely (see Sect. 3.3 ).
For single-crystal studies, it is important that the opening angle in the anvil
seat on the incident-beam side of the DAC is also as large as possible. Even so,
during a single-crystal study, only a limited fraction of all single-crystal reflec-
tions are typically accessible, because the particular orientation of the sample
crystal requires the incident or diffracted beam, or both, to be outside the DAC
apertures. In particular, all reflections with scattering vectors along or close to
the axis of the pressure cell are not accessible [ 63 , 111 ]. This restriction on
accessible reflections can have serious consequences for the refinement of
certain crystallographic parameters, and can be overcome by using a DAC
with a transverse diffraction geometry [ 117 ], in which diffraction takes place

m