High-Pressure Crystallography - Advanced X-Ray Crystallography - page 70

advent in the late 1950s of pressure cells utilising single-crystal diamonds to
compress samples that heralded the dawn of high-pressure crystallography. Initial
designs of the pressure cell used the diamonds in a piston-cylinder arrangement,
with a small cylindrical sample chamber being drilled through the diamond
[ 10 , 11 ]. But the invention of the diamond anvil cell (DAC) in 1958 by Jamieson
et al. and Weir et al. [ 12 , 13 ] was the key turning point in high-pressure crystallo-
graphy. The first diffraction studies were performed almost immediately afterwards
by Jamieson, Weir and colleagues, who determined the structure-types of the high-
pressure phases of elements by matching observed X-ray diffraction patterns with
those obtained from other materials at ambient pressure [ 14 - 17 ]. The small Merrill-
Bassett DAC, developed in 1974, allowed DACs to be used on commercial dif-
fractometers [ 18 ], and was used extremely successfully by researchers at the
Geophysical Laboratory at the Carnegie Institution of Washington to explore
the structures of complex minerals at high pressure, and to pioneer high-pressure
single-crystal diffraction techniques. Much of the work conducted at this time
is described in the seminal topic by R.M. Hazen and L.W. Finger [ 19 ]. The DAC
celebrated its 50th anniversary in 2009 and, to mark this, W.B. Bassett, an early
pioneer of high-pressure mineralogy, has recently reviewed its history [ 20 ].
The greatly increased X-ray flux available from synchrotron light sources in the
1970s enabled high-pressure diffraction studies to be pushed to ever higher pres-
sures: 1 Mbar (100 GPa) in 1976 [ 21 ], 200 GPa in 1984 [ 22 ], 300 GPa in 1989 [ 23 ]
and 400 GPa in 1990 [ 24 ] or 2010 [ 25 ]. However, such studies typically used
energy-dispersive X-ray powder-diffraction methods in order to utilise the extreme
intensity of the polychromatic synchrotron X-ray beam, and it was not possible
to determine accurate Bragg peak intensities and therefore structure factors. As
a result, while equations of state of known materials could be followed to very high
pressures [ 26 ], diffraction information on many high-pressure phases remained at
the level of structure type (cubic, tetragonal, “ -tin”) or “distortions” thereof. (See,

for example, many of the phase diagrams reproduced in [ 27 ].)
A major transformation in the power of high-pressure powder diffraction came
about through, first, the pioneering application of image-plate detectors for angle-
dispersive X-ray diffraction (ADXRD) in Japan in the late 1980s [ 28 ], and then, in
the early 1990s, the development of advanced ADXRD techniques using DACs and
an image-plate detector at the SRS synchrotron in the UK [ 29 , 30 ]. In a short period
of two or three years, it became possible to perform crystallography to extreme
pressures comparable in resolution and quality to ambient-pressure studies. The use
of a sensitive two-dimensional (2D) image-plate detector, and short-wavelength
monochromatic X-rays, allowed Rietveld refinement of high-pressure powder-
diffraction data for the first time [ 30 , 31 ]. The replacement of the restricted-aperture
pressure cells used for energy-dispersive studies by cells with wide conical aper-
tures, as developed for single-crystal applications, allowed full 2D Debye-Scherrer
(DS) diffraction patterns to be collected, giving data with the high resolution of
ADXRD techniques coupled with high signal-to-background and unmatched sensi-
tivity to weak diffraction features [ 30 , 32 ]. Two things became immediately appa-
rent from the use of these new techniques: (1) many of the structural descriptions

b