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Scherrer XRD Shape factors

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Salish99
#1
Jul28-09, 06:35 PM
P: 29
The Scherrer equation is used to determine crystallite sizes from XRD spectra.
they contain K, a shape factor, that varies from 0.89 for spherical to 0.94 for cubic particles. Usually, this is set to 0.9 for particles of unknown size.
Does anyone know where we could find a table with the exact, published values for different crystallites (e.g. monoclinic, cubic, etc.)?

Scherrer's original papers are published here
http://gdz.sub.uni-goettingen.de/dms...uery]=scherrer
but a) in German, and b) do not contain such a table.

thanks.

The discussions in
http://www.physicsforums.com/showthr...=122682&page=2
and
http://www.physicsforums.com/showthread.php?t=322033
simply assume it's 0.9. No reference is given there. I need the exact table, and a reference.
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nasu
#2
Jul31-09, 07:58 AM
P: 1,969
I think you are mixing the crystallite shape with the unit cell of the crystal.
As you say yourself, K depends on the shape of the crystallite and not that of the unit cell.
There may be a realtion between unit cell and crystallite shape (NaCl has cubic unit cell and may crystalize in cubic crystallites) but is not unique and not well determined.

If you take cubic crystallites and grind them in a mill you may end up with spherical particles or irregularly shaped particles, depending on the milling conditions.
There are also crystals with cubic unit cell (ferrites for example) that may crystallize in elongated (needle-like) crystals.
Salish99
#3
Jul31-09, 11:19 AM
P: 29
Attached a table from the International Union of Crystallography, Vol 3 Section 5.2, 1986 pp.318-323. It shows some shape factors to be used for the cubic system, depending on the reflection plane, and the shape (octahedron, cube, tetrahedron) of the actual crystal.
Interesting to see is that only one value is below 1, whereas is all this forum, as well as in many published papers, 0.9 is usually used "when the shape is unknown".
I would like to have someone tell me where that information comes from, with reference please.
thanks!

(and, no, outdated student pages like http://www.d.umn.edu/~bhar0022/dpcalculator/index.php and http://www.eng.uc.edu/~gbeaucag/Clas...herrerEqn.html do not have any references either - maybe people don't think about that factor usually)
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nshtkmar1977
#4
Aug6-09, 05:33 AM
P: 9
Scherrer XRD Shape factors

I have value of 2 theta 35.546 deg, d spacing 2.5235 A, fwhm (2 th) 0.148 deg, can somone use it to get size of crystallite. after getting data analysed by a program expertplus i have got this, thankyou


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