Chemistry Reference
In-Depth Information
broad peaks, and the powder averaging of the sample (the number of diffracting
crystallites) was poor. Combined with the many wavelength-dependent correc-
tions that need to be applied to the data (for example, sample and pressure
cell absorption, scattering power, detector efficiency), these limitations meant
that accurate intensities were not measurable. “Structure” determination thus
remained at the level of lattice type or structure type, e.g. tetragonal, “
-tin” or
“distortions” thereof. The term “distorted” typically meant that additional reflec-
tions were observed, but were not interpretable.
The advent of angle-dispersive techniques with full conical-aperture
DACs revolutionised powder diffraction [
30
]. The technique uses monochromatic
radiation, and a highly-sensitive image-plate detector to collect the 2D Debye-
Scherrer (D-S) pattern [
28
,
29
]; see Fig.
4a
. This is then integrated azimuthally to
provide a standard 1D diffraction profile (Fig.
4b
) which has high resolution, and,
because of the averaging around the D-S rings, both accurate peak intensities and
an extremely high signal-to-noise [
30
,
161
,
162
]. The image-plate techniques
were first utilised with DACs designed for energy-dispersive studies - which
required only narrow slot-apertures in the pressure cell body in order to provide
access and egress for the X-ray beam [
28
]. But such cells enabled only a small
portion of the D-S pattern to be collected, and integrated azimuthally. Develop-
ments at the SRS synchrotron in the UK made three significant improvements on
the original methods [
29
,
30
]. Firstly, the introduction of DACs with full conical
apertures allowed the full two-dimensional D-S pattern to be collected, leading to
greatly improved signal-to-noise. These DACs, which were originally designed
for single-crystal diffraction studies, used Be seats for the diamond anvils.
Although such seats are inherently weaker than the tungsten carbide used for
slot-aperture seats, the use of specially hardened Be allowed these cells, which
had full conical apertures of 4
b
80
, to be used to pressures above 25 GPa
[
163
], eventually reaching 75 GPa or more.
Second, shielding, collimation and alignment methods were developed to the
extent that all non-sample scattering was removed from the data. This enabled
extremely weak sample reflections to be uncovered, which revealed the unexpected
structural complexity that had previously been masked by the lower-quality energy-
dispersive data. Thirdly, the first GUI interactive software for integrating full two-
dimensional images was developed [
30
,
162
].
The resulting increase in data quality is illustrated in Fig.
5
, which shows
a comparison of ADX and EDX data from the same sample of InSb. The quality
achieved was such that more sophisticated techniques could be applied, such as
the use of anomalous scattering to distinguish similarly-scattering elements, like
In (Z
y ¼
51) [
164
,
165
]. And the combination of the high-resolu-
tion 2D data and the GUI software made it possible to distinguish mixed phases
simply from the difference in appearance of the diffraction rings [
164
,
245
].
The 1D diffraction profiles are also ideally suited to profile refinement, as
illustrated in Figs.
9
and
13
in Sect.
4
, from which atomic coordinates can be
obtained using standard Rietveld methods.
¼
49) and Sb (Z
¼
