RE: [sdpd] Optimal instrument configuration

["Yokochi, Alexandre" <alex.yokochi...@orst.edu>]

Hi,

	As I have a couple instruments of both kinds, I'll add some general
comments, not only aimed at SDPD.

	As Armel said, the primary beam monochromator+PSD does lead to data
stripped of Ka2 from the start, and you WILL get bad fluorescence background
problems if you use 1st row transition metals (but if you use this data
un-massaged, you can get beautifully low R's, which are absolutely
meaningless).  And yes, data collection is faster, at least from my
experience.



	If you do not have diffracted beam Soller slits (you will probably
have to invent something of your own as I am currently doing) your low angle
peaks (and by that I mean below 15degrees or so) will look very strange and
anisotropic.  However, this does not limit you to above 5degrees two-theta,
it just gives you strange looking data.  I have gone down to 0.5 degrees for
normal scattered data, and below zero for thickness and roughness
measurements by reflectometry.

	Low angle data from a conventional setup is usually pretty good, as
expected.



	One advantage you have is that for regular powders, any crystal can
be used as a zero background sample holder - Just make sure that you do not
have an allowed reflection anywhere close to the scattering condition.  I
usually scavenge Si(111) wafers from old film depositions for sample
holders.  For high temperature work, old MgO or Al2O3 substrates are great
too.

	For a conventional BB instrument, you have to buy specially cut
single crystals for your zero background sample holders.  They're really not
that expensive anyway.



	Also, for thin film work, I find that having single crystal
substrates is not a problem - you may have a few weak and broad humps from
the substrate, which if you know they are there, you can easily remove.  In
fact, from my setup you can do a wonderful job of subtracting experimental
backgrounds to obtain "pure sample" diffractograms (e.g., you can remove
that annoying glass hump from your thin films data).

	The same goes for the conventional instrument.



	Finally, for thin film work, the instrument based on the PSD allows
you to collect data whose scattering vectors do not lie perpendicular to the
substrate.  This was the primary consideration for us when we designed/built
our thin films diffractometer.  The diffraction data is a lot richer and
looks more like what you would expect from a regular powder.  Also, you can
try to do a texture study from only a few diffraction patterns, rather than
having to collect a thousand intensities for several reflections.

	On a conventional instrument with BB geometry data from a highly
oriented film usually ends up having one or two reflections, which also
gives you useful data.
	


	Hope someone finds it useful, if only to decide what a misguided
fool I am.  

	AlexY


Dr. Alexandre F. T. Yokochi
Assistant Professor (Senior Research)
Director, X-ray Crystallographic Facilities
Department of Chemistry
Oregon State University
Corvallis, OR 97331-4003

Ph#  (541) 737-6724 Email: alex.yokochi...@orst.edu
Fax# (541) 737-2062 Web Page: www.chem.orst.edu/yokochi


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