Not sure if this has been brought up before or it may be common
knowledge in some quarters?
I have been giving some commercial Powder XRD training seminars -
including search-match on PDF-1 database using commercial software
the laboratories have purchased (in this case Philips PC-Identify). =20
I am curious about the following and am interested in other know=20
of similar phases and/or want to trade some phase ID pitfall stories.
----
Of the many and various issues on the pitfalls of search-match:
1) One of the points I like to mention about Phase ID is that you
can't trust the person who gives you the sample. (That "Calcium
containing phase" may really be an Aluminium phase?)
2) The other is not placing 100% trust in reference data. Which you
may not realize/appreciate unless you encounter this=20
the hard way. =20
I am curious if there are other examples of the following??
-----
One of the pitfalls I see people fall into (having beaten
the addiction myself after much pain) is failing=20
to identify a trace peak - that turns out to be due to
a major phase - as the ICDD data does not include the trace=20
peak in the card. In many cases, this only comes to light when=20
doing a quantitative Rietveld analysis on the sample. A spurious,
problem trace peak that stood out like (to use the common=20
Australian expression) "dog's balls" disappears into the Rietveld=20
fit, which has been "correctly" modeled as part of one of the=20
major phases.
----
An example of this is to do a search match on a lead-acid battery
sample supposedly containing (PbO2) ~50% plattnerite, ~50 Scrutinyite. =20
Search on the PDF-1 database and you identify this=20
as 45-1416 - Scrutinyite and 35-1222 - Plattnerite - Syn.
But this leaves a peak at 2.436=C5 (36.87=B0 2-theta with CuKa). If you=20
are not used to this, trying to identify this peak can put you=20
in a world of pain - or perhaps more hair loss - or perhaps=20
matching the peak with an incorrect phase (perhaps resulting
in an expensive goose chase trying to eliminate a phantom phase
from a product - or rejecting a million dollar shipment as impure?)
As the saying goes - send lawyers, guns and money - though in the=20
above it is likely that only lawyers would be involved?
However, do a quant Rietveld refinement on the sample and=20
this fits the trace peak as:
h k l 2-theta
1 1 1 36.988 =20
This also matches a very minor hump at 2.218=C5 (40.68=B0).
1 2 0 40.691 =20
(In this case, and it hindsight, it is interesting that is you=20
do a PDF-1 search on deleted patterns, you can match the=20
deleted 25-447 Plattnerite which does match all the most
obvious trace peaks - including the minor hump at 2.218=C5 (40.691=B0))
----
Another looks like magnetite?
A sample with magnetite can be matched with ICDD/JCPDS card=20
19-629 Magnetite. This leaves a trace peak at 1.929=C5 (47.19=B0)
Do a Rietveld quant analysis and this is found to be the:
h k l 2-theta
1 3 3 47.156
----
I'm not sure if people are used to "experience/gut feeling"=20
when determining the sensitivity of Powder XRDs to trace peaks.
When helping out with a Rietveld Quant Analysis course I had
a query from two people who were refreshing their knowledge.
They were very curious about a trace impurity peak in the fluorite
standard we were using for this course at 1.57=C5 (58.6=B02theta CuKa). =20
They did XRD since the "good old days" and never saw a fluorite=20
standard with an impurity peak in that position. Clicking on the=20
"run Rietveld" button showed it to be the 2 2 2 reflection.
This is routinely visible as a trace peak when run on 1050 based=20
Philips XRD with Graphite monochromator - And I assume with any=20
other good diffractometer(?) - Rietveld style data collection.
----
Modern Powder XRDs are easily this sensitive to detect trace
peaks on routine, quick phase identification scans - definitely=20
with Scrutinyite - maybe with Magnetite (at last with standard 1050
base Philips gear I have used). These are not what I would call=20
rare or obscure minerals(?).
----
While discussed before, I think a case can be made for having
multiple data types (single crystal calculated, rietveld,=20
transmission, reflection) for phases in the ICDD/JCPDS database.
Could a future for Rietveld Analysis be as part of a future,
reliable, black box, expert, phase identification system? =20
Press a button, it not only does the phase ID but then the=20
quant analysis?
----
Hope this is legible at this time of the morning! :-)
Cheers,
Lachlan.
PS: On another point. Would anyone feel it safe to be performing=20
search match with only the PDF-1 data? As an example, and from memory,=20
it is easy to match various Lead Oxides to some samples. However, it is
the information in the PDF-2 that tells you that some of the matches
are high pressure high-temp phases - and would not be expected in
a room temperature sample without some good explaining to do.
If someone has purchased a PDF-1 database with their search-match
software, does anyone know of a cheap upgrade path to a PDF-2 database?
Having to pay US$6,100 for the CD-ROM from scratch sounds rather cruel
when you have already forked out a major (possibly excessive) amount
of money for a PDF-1 database?
I can't see evidence of an PDF-1 to PDF-2 upgrade path in=20
the ICDD Web page at http://www.icdd.com???
(I have already given one PDF-1 database user a near heart attack
saying they need the PDF-2 CD-ROM. Hopefully this is not considered=20
improper/dodgy advice for doing phase ID on complete unknowns?)
Another query, what are the recommended counting statistics or
data collection times for running a Grant-in-Aid pattern?
Using Rietveld to calculate a pattern could help you determine
the appropriate data collection conditions?
Lachlan Cranswick - Melbourne, Australia _--_|\ =20
Phone/Fax : (613) 9455-1345 / \
E-mail : lachlan@melbpc.org.au \_.--._/
Mobile Phone/Voice Mail : 0412-1141-31 v
Crystallographic WWW : http://www.unige.ch/crystal/stxnews/stx/volnteer.htm