Re: Fundamental Parameter Approach versus Empirical

Matteo Leoni ( leonim@ing.unitn.it )
Thu, 25 Jun 1998 16:39:46 +0200 (MET DST)

Armel Le Bail wrote:

> You probably know the most widely used method for extracting
> microstructure sample effects from a powder pattern : the
> Warren-Averbach method. In the more exact version of this method,
> the profiles should never be modelled, but experimentally measured
> (using two patterns, one from a defect-free sample and the other
> from the defective sample) and then deconvoluted. No modelling
> at all is the best approach...

I totally agree with you....not always feasible since both
defective and defect-free samples are (generally) not available ;o)

> In this sense, pretending that the fundamental approach is better than
> the empirical is nonsense because none is as good as measuring profile
> shapes. This third approach was also applied to the Rietveld method
> in the so-called learnt-profile methods used in XRS-82 or ARIT1 (...).
> Of course, the problem of these learnt-profiles is that they apply perfectly
> only at the angular position from where they are parametrized. Models
> come back for their extrapolation to the whole powder pattern.
>
> Fundamental parameters and empirical are both approximated
> approaches. The BGMN program uses another terminology which
> is "Exact Peak Shape Model". Sorry, I don't like it anymore,
> because a model is by definition not exact.
>
> Anyway, I am ready to adopt the best model, including a model
> based on "fundamental parameters", if this model provides the
> best fit. As soon as empirical approaches will produce a better
> fit, I will come back to them :-|.

...personally I still use a mixed model: modelling of the instrumental
profile by using a "defect-free" powder (KCl or LaB6...) and modelling of
the size/strain behaviour using a Fourier approach... 50% empirism - 50%
modelling ;o)... with the advantage that cases that cannot be treated with
fundamental parameters methods (I can hardly believe that any FPA method
can model the instrumental function of Station 2.3 at the Daresbury
Lab...) can be easily taken into account, and a direct comparison with
traditional profile analysis techniques (Fourier ones...) is possible...
I think is a good compromise between exact Warren-Averbach (measurement of
the instrumental function....) and meeting the requirement of the
Rietveld method (need for generating the broadened profile).... and with
reduced efforts it is possible to take into account anisotropic
broadening...

Matteo

...By the way... it's worth trying fundamental parameters approach on KCl,
LaB6, BaF2... ;o) ...

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