Workshop Announcement Fundamental Parameter Approach / EPDIC6

Arnt Kern Arnt.Kern at khe.siemens.de
Wed Jun 24 01:08:23 EST 1998


---------- First Announcement ----------

The Bruker-AXS GmbH cordially invites you to the following workshop, which
will be held as a satellite meeting accompanying the 6th EPDIC conference
in Budapest:


"A New Fundamental Parameters Approach in Profile Analysis of Powder Data"

Line profile shapes are a convolution of (i) the emission profile, (ii) an
instrument component and (iii) specimen aberrations. This approach to line
profile analysis is well known latest since the pioneering work of Klug &
Alexander in 1974. Nevertheless, until today current profile fit routines
still apply empirical models to the peaks which do not distinguish between
these contributions and are often inadequate in modeling observed peak
shapes in X-ray data. 

A common problem is the description of peak asymmetry which is mainly a
result of axial divergence effects. In addition, the extraction of sample
effects, such as crystal size/strain broadening relies on the (time
consuming) measurement of a large crystal size, strain free 'standard'
sample under the same instrumental conditions as the 'unknown' sample. 

The problem with an empirical/standard approach is that a large number of
refinable parameters are required to accurately describe line profiles over
the whole 2Theta range. This parametrization as well as the insufficient
description of instrumental contributions (such as asymmetry) performed by
recent profile fit routines result either in inadequate fits or in
erroneous interpretation of the specimen contributions.

The new fundamental parameters approach presented in this workshop uses a
convolution based method to synthesize line profiles. Instrumental and
specimen aberrations are convoluted with the emission profile to form the
final line profile. Peak position, shape and asymmetry are described by the
instrument and sample contributions resulting in accurate estimates of
Bragg angle and profile shape. Sample related effects, such as specimen
absorption, crystallite size and strain broadening, are entered as
refinable values. The physical parameters of the diffractometer, such as
the receiving slit length, horizontal divergence, and the primary and
secondary Soller slit angles, are measurable quantities and are not usually
refined but can be if required. This theoretical modeling of line profiles
provides information on diffractometer misalignment and/or geometric
irregularities. Significantly the fundamental parameters approach reduces
the number of refined parameters and thus decreases parameter correlation
in general. 

Computationally demanding numerical convolutions, indicative of the
fundamental parameters approach, have in the past made it an unlikely
approach for routine line profile or Rietveld analysis (e.g. Howard &
Snyder, 1989), but this has been overcome with the inclusion of a number of
fast algorithms and by using a direct convolution-interpolation procedure.
Efficiencies in the refinement procedure together with extremely fast
algorithms has resulted in an approach that is at least three to ten times
faster than conventional profile fit programs - even for non-convolution
based fit routines. 

The procedures presented at the workshop are expected to overcome the
recent, empirical profile fitting methods.

---------- 

The topics to be discussed are:
1. Possibilities and limits of recent profile analysis techniques
2. Fundamental parameter approach vs. conventional fitting techniques
    a) Principles & core mathematics
    b) Generation of line profiles
    c) Emission profile, instrument characterisation, sample contributions 
3. STANDARDLESS real structure analysis (crystallite size, microstrain,
absorption, ...)
4. Instrument characterisation (axial divergence, horizontal divergence or
flat specimen, slit dimensions, ...)
5. Use of any parameter couplings and of any constraints (linear and
non-linear) 
6. "How many peaks are there?"
7. Instrument alignment aspects
8. Software

---------- 

Preliminary date:
Monday, 24. August, 16:30 – 18:30.
The final schedule will be provided in the 2nd announcement and at the
EPDIC registration desc.

Workshop language is ENGLISH.
Participation is free.

---------- 

For further information please contact:

Dr. Arnt Kern
Bruker-AXS GmbH
Oestliche Rheinbrueckenstr. 50
D-76181 Karlsruhe

Fax:      ++49-721-595-6693
EMail:   Arnt.Kern at bruker-axs.de






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