Dear Arie, what you mention is a rather common occurrence, which may depend on (at least) to factors:
a) the presence of a short axis or a dominant zone (making the whole powder pattern indexable by a 2D reciprocal lattice)
b) sample morphology (say, needles) leading to a partial sampling of the reciprocal lattice.
The obvious way out from this second problem can be a different preparation of the sample, changing "texture coefficients", just aiming to detect the 'missing informative peaks'.
If, instead, the lattice metrics are such that one (short) axis can barely be identified (for short d's), (as it happens in many fully aromatic planar organics), you may want to 'constrain' this value by geometric, database, energetic, etc. considerations.
Even if unobservable at all, the short axis can indeed confidently estimated by density/volume calculations, etc.
Otherwise, check consistency of the indexing by a Le Bail fit and step-by-step increase of the short axis (the other two being fixed at their nominal 'best' values).
Best Regards
Norberto
At 13.07 04/06/04 +0200, you wrote:
Dear all, It is generally assumed that a high value of the M20 index gives a high probability that the solution is correct. I have a particular problem for which I have a couple of different solutions with M20 between 40 and 100. The problem resides in the fact that the a and c axes of the monoclinic cell are much larger (both around 20 A) than the b-axis (3-6A?) The solution depends therefore on the angular range which is taken for the indexation process. Taking the first 20 reflections with significant intensity gives a 6A b-axis with the 010 reflection as the last one in the list. Taking 5 reflections more, the length of the b-axis decreases and the 010 remains the last in the list. And so on. The compound is organic and there is not much scattering left beyond 2Theta=30degr, giving a total of 31 reflections to be used for the indexation. How to pick up the right solution between these high M20 solutions?
Thanks in advance, Arie ******************************************* A. van der Lee Institut Européen des Membranes (UMR 5635) Université de Montpellier II - cc 047 Place E. Bataillon 34095 Montpellier Cedex 5 FRANCE
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