Two very good points by Armel:
>all the very good PDF studies ...are made by using synchrotron data or
neutron data from spallation
>sources
This is because they are the only means to get to high Q (i.e., high
resolution in real space) and sufficiently high resolution (in reciprocal
space) simultaneously. The RMC method is somewhat similar and does not
require such high Q, but it has the drawback of requiring a starting model
(arguably, there is also a uniqueness issue with RMC). One nice feature of
the latest generation of TOF instruments is that one does not have to choose
in advance between PDF and crystallography, as long as one has appropriate
references (empty can, empty instrument etc.), which are collected as a
matter of course anyway. PDF analysis requires better statistics, but, in
the context of a large phase diagram study, it is always possible to collect
a few data point to PDF accuracy.
>So, this PDF advantage does not impress me a lot
True, in most cases PDF=Rietveld + Common Sense. However, there are some
exceptions. For some nice cases see the work of Simon Hibble et al. (e.g.,
Hibble SJ, Hannon AC, Cheyne SM Structure of AuCN determined from total
neutron diffraction INORG CHEM 42 (15): 4724-4730 JUL 28 2003 and references
cited therein) and that by Simon Billinge (e.g. Petkov V, Billinge SJL,
Larson P, et al.
Structure of nanocrystalline materials using atomic pair distribution
function analysis: Study of LiMoS2 PHYS REV B 65 (9): art. no. 092105 MAR 1
2002 ). Particularly, Simon Billinge makes the point that the future of PDF
is in the study of materials with short and intermediate-range order but no
long-range order ("nano-crystallography"). It is an interesting point of
view, although, at the moment, there are not very many examples of this in
the literature.
Paolo Radaelli
