> Like Rp, Rwp, Chi2, RB are respectively from 3.96, 5.33, 2.68, 5.80 > to 3.35, 4.42, 1.85, 3.07.
This should pass a test of statistical significance.
The preferred orientation (PO) problem is recurrent at the rietveld_l.
This is normal since PO is one of the biggest problem encountered almost systematically when dealing with powder diffraction (but less frequently - or at least less intensely - in neutron experiments with cylindrical sample holders and transmission geometry than in X-ray experiments with flat sample holders and r eflection geometry).
It seems that the information about PO is not sufficiently clearly distributed to powder diffractionists, and especially to neutron powder beamline users (who should never discover PO for the first time at home, back from the neutron experiment).
The best significance test about PO is an experimental test.
It is not easy to perform that test with neutrons, unless the beamline is dedicated to you. But it is easy in the lab with a conventional X-ray powder diffractometer. With Bragg-Brentano geometry and horizontal sample, you just have to record 2 patterns : one with maxi PO by pressing the sample and one with mini PO (try dusting the sample through a sieve on a the frosty surface of a glass sample holder). Then if there are strong differences, your test of statistical significance can be considered as being positive...
In principle, if you see a few PO with neutron transmission through a large-diameter cylinder, then the PO will be huge by X-ray reflection on a packed flat sample. Unless making large efforts, you will not be able to pack the sample in the cylinder as much as on a flat sample holder surface.
Given that a neutron experiment is very expensive, some
recommendations should be inside of the powder neutron
beamline checklist distributed to the users (preferably
coloured IN RED, and bold) before they come to the neutron
'facility' :
- Check at home (X-rays) if your sample is prone to preferred
orientation (make 2 powder patterns, one favouring PO, and
the other avoiding PO, and compare).
- If such PO is susceptible to occur, then do not pack too much
your sample in the cylinder before the neutron experiment.
Moreover, the grain size for a good powder is supposed to be
close to 5 microns (be aware that milling the sample too
much can produce lne-broadening).That X-ray experiment can also be made after the neutron experiment though the reverse is really preferable. Experiment will convince more surely the referee of your manuscript than any test of statistical significance, concerning PO
Armel
