{{{ SNIP, SNIP, SNIP }}}
Granted, and I think everyone's well aware of the small limitations in the
data base as regards the age of the data vs. its perceived accuracy (or
inaccuracy). What's of more concern are things like the lack of calculated
patterns (which, of course, must be carefully checked to ensure agreement
with experimental data) and the age of some of the information (which is
where the perception of 'inaccuracy' comes into play). =20
For example, I could easily provide an updates on magnetite, for instance,
and I'm sure others could for many compounds - in the past year, I've
examined in detail 3 different samples of natural magnetites, and 7
synthesized samples of differing particle size, which are pure phase, for my
'catalyst morphology' dissertation, and I will be doing more by XRay, and by
neutron analysis. I see the 331 trace every time, get very low angle data
(with the accompanying problems with absorption and fluorescence), and scan
to high angles( I scan 10=B0 to 120=B0 every time, and have a few scans
ranging from 5 to 155 =B0).
In the course of our work, we synthesize and analyze pure carbide phases and
alpha-iron for reference, also. I simulate the data as a matter of course.
I'm sure none of this data (experimental or calculated) will ever show up in
the data base, more's the pity, especially since I'm not a novice
crystallographer, I have to do it anyway, and I must guarantee my
accuracy/precison and the instrument calibration at the time I'm working,
regardless of whether the data just go into my dissertation or not.
My frustrations, as I stated them, stem from all the information I had to
learn the 'hard way' when I first switched from single crystal to powder
work, and fell into all the holes that, apparently, a lot of others have
fallen into. At least I KNEW there were problems, and could work out
solutions. Many don't know there IS a problem, or how to remedy it.
I don't rely on either version of the database much except for confirmation,
whether performing service work or research. Unfortunately, many people do,
and mistakes are made. I spend a quarter of my time correcting other
people's search-match foobahs, and I'm NOT the lab manager or employed by
the lab. For research purposes, mistakes can be costly with regards to
intangibles. But then, the people I help are not experienced, just in the
lab when I am, frustrated, and want help. I have no fault with the ICDD
researchers - if I implied so, I do apologize.
{{{ SNIP, SNIP, SNIP }}}
>This leads me to another question: how well does the Rietveld code behave=
if
>the dispersion correction (which is not so well defined for X-rays in the
>first place) comes into play by being too close to the absorption edge
>wavelength? Has anybody any experience, does it matter at all? It appears
>that with the advent and convenience of the diffracted beam graphite
>monochromator, anything which diffracts, fluorescent or not, is probed with
>CuKa. At least, one has to keep in mind that the use of CuKa radiation,=
with
>or without graphite monochromator, reduces the penetration depth by a=
factor
>~8 in (pure) Fe-samples when compared to MoKa radiation and the diffracted
>intensity is diminished accordingly. Thus, precious sampling volume is lost
>and surface effects due to absorption are substantially enhanced. With all
>the enthusiasm for Rietveld analysis (which I greatly share), the basics of
>the diffraction experiment must not be ignored.=20
>
{{{ SNIP, SNIP, SNIP }}}
Interesting question, and one I think I'd best look into. I don't have an
answer at present, I'm afraid, but sure someone's going to want to know
before I finish. I use a diffracted beam instrument with graphite
monochromator and copper radiation, but have not had difficulties in
analysis with the pure iron samples with respect to absorption. In fact, I
get lovely spectra from step-scanned samples - suitable for framing }:^).
Linda
Linda D. Mansker mansker@unm.edu
213 Farris Engineering Center Chemical and Nuclear Engineering Department
505) 277-5841 Albuquerque, NM 87131-1341
(505) 254-7859 FAX: (505) 277-5433
"We shape clay into a pot, but it is the emptiness inside that holds=20
whatever we want." TAO TE CHING