(Rietveld Calc Data) vs (ICDD Ref Data lacking trace peaks)

Lab. Rayos X ( (no email) )
Mon, 30 Jun 1997 17:18:39 -0400 (EDT)

>Hi All,
>
>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).
>I am curious about the following and am interested in other know
>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?)
>

Hi Lachlan,

I remember once I was advised, by the person who gave me the sample, about
what phase was on it. Then , to save time, I reduced the measurement
interval and started at 25 degrees (the 'expected' phase had no peaks below
this angle). The pattern was not as expected and it was impossible to
identify
the phase, after two hours work I decided to rerun the sample starting from
5 degrees. Then the identification was easy: the phase had an important peak
below 25 degrees.
Trying to save 15 minutes I spent more than two hours!!

>2) The other is not placing 100% trust in reference data. Which you
> may not realize/appreciate unless you encounter this
> the hard way.
>
>I am curious if there are other examples of the following??
>

A friend once was working on a sample for which there is a deleted and an
updated card. When he tried to identify the phase the deleted card
matched better than the updated one. Some peaks from the deleted card
were not in the updated one because of a difference in the measurement
interval.

Using Rietveld for phase ID is a nice thing but to do this you need the
structural data and for people without access to structural databases, as
myself, it does not seem to be the way.

Regards

Ariel

Ariel Gomez Gonzalez
X-Ray Diffraction Lab,
Division of Chemistry,
National Center for Scientific Research,
P.O. Box 6990
Playa, Ciudad de La Habana,
Cuba.

Tel. (53-7) 211235 and 218066 Fax: (53-7) 330497
e-mail: xray@infomed.sld.cu
ariel@quimica.cneuro.cu
Wed page: http://www.unige.ch/crystal/stxnews/nexus/cuba/cnic/index.htm