High-Pressure Crystallography - Advanced X-Ray Crystallography - page 82

Energy Dispersive
In and Sb
Fluorescence Peaks
Angle Dispersive
Fig. 5 Diffraction profiles collected from the same powdered sample of InSb at ~2 GPa, using
( top ) energy- and ( bottom ) angle-dispersive diffraction. The angle-dispersive data clearly have
higher angular resolution, and are not contaminated by X-ray fluorescence peaks. The tick marks
below the angle-dispersive data mark the positions of some of the weak superlattice reflections that
were essential to determining the structure of the InSb-IV phase [ 165 ]
crystal by recrystallisation is different from sample to sample. For example, in
Ba a single crystal of the high-pressure phase Ba-IV can be grown by increasing
the pressure very slowly through the Ba-II to Ba-IV phase transition at 12 GPa
[ 166 ]. And in Bi, a single crystal of Bi-III can be grown by first increasing the
pressure on a quasi-single-crystal of Bi-I into the higher-pressure Bi-V at ~10 GPa,
before slowly reducing the pressure through the Bi-V to Bi-III phase transition
at ~8 GPa [ 167 ].
3.3.2 X-Ray Single-Crystal Diffraction
As described previously, the invention of the small, gasketed Merrill-Bassett DAC
in 1974 meant that single-crystal studies could be performed on standard single-
crystal diffractometers [ 18 ]. Such studies require as wide an access as possible to
reciprocal space, and in the Merrill-Bassett DAC the diamond anvils thus sit on
beryllium anvil seats, which are highly transparent to X-rays [ 18 ]. Optical access to
the sample is via a small (~1 mm) axial hole in the Be seat. While such an
arrangement provides greatly increased angular access on both incident and dif-
fracted sides of the cell, the tensile strength of Be is quite low, limiting the pres-
sures available with such cells to ~10 GPa. It was such Be-equipped cells that were