Wojciech pointed out an important issue regarding ADS measurements:
Wojciech Paszkowicz schrieb:
Dear All,
Additional comment concerning the usefulness of variable slits for structure
refinement: The angular distribution of the radiation from an X-ray tube is
not necessarily uniform, so using variable slits for collection of
Rietveld-refined (Bragg-Brentano geometry) data may imply the need for an
additional intensity correction for this effect.
Because the intensity distribution versus take-off angle of a tube is a
priori unknown, one must find an working compromise, see again our
comments at
http://www.bgmn.de/vardiv.html
Our comment: "Maximum divergence angle should be much less than take-off
angle of X-rays from tube anode. The latter angle usually is 6°. "
An additional restriction of the maximum slit opening may be given by
the limited length of a secondary beam monochromator. The crystal may
act like a fixed antiscatter slit and may cut the intensity if a certain
ADS divergence is reached. But this can be modeled by the Monte-Carlo
approach.
Moreover, the opening of
the variable slits requires an extremely high precision mechanics so there
may be a doubt concerning the smoothness of the flux variation with varying
two theta, at the given instrument.
This is true for conventional stepper-motor driven slits at low angles,
say below 15 °2theta. In that range the smallest steps of the slit may
be too large to fix the irradiated sample length over two theta. But the
intensity data at very low angles may be biased by a number of additonal
factors like sample roughness and misalignment of the sample surface.
Thus, high accuracy/reproducibility of the intensity at low angles
cannot be expected at all, and excluding this range from the refinement
is common practice.
For higher angles, my personal experiences with ADS are positive, even
for older instruments (built from 1986 to 1995, from 3 different
manufacturers, 3 different types of stepper-motor driven ADS systems).
Other people have been less happy with their ADS system, see
Peplinski, B. & Wenzel, J. (2000) Mat.Sc.Forum, Vols. 321-324, pp. 144-149.
It should be pointed out that it is necessary to check the a alignment
and programming of the ADS in the way that Ian Madsen recommended in a
previous mail (refining the temperature factors of a known standard).
See also the literatur cited above. For a first coarse check of the
primary beam ADS, a simple look on a fluorescent screen with a scale
during a measurement (2° step, 2 sec per step) helps to see coarse
angular fluctuations of the irradiated length, for example by a wrong
programming. To check the zero point alignment of the slit, a
"divergence slit scan" from negative to positive opening at 0 °2theta
without sample (of course at reduced tube power and with absorber) can
be done. If the instrument or the control software is not flexible
enough to do this, one should ask the manufacturer service for help
respective for a demonstration that the ADS is correctly aligned.
It is clear that such additional efforts that are neccessary for the
correct use of the ADS system (compared to fixed slit systems) must be
counterbalanced to the advantages of an ADS. Here, everyone must do his
own judging.
Regards
Reinhard Kleeberg
