At 07:08 PM 5/10/99 +0200, Armel wrote:
>PS- take the Rietveld Round Robin PbSO4 X-ray pattern and omit
>the O atoms, you will have RB~15%. Make a Fourier difference
>and you will see if the "light" atoms are so light, when using good data.
>When I remember my crystallography courses 25 years ago, maybe,
>I see the decomposition of the structure factor by atom pairs. O atoms
>are involved not only in O-O pairs, but also in Pb-O and S-O pairs,
>fortunately. In many cases, like Li2TbF6, or LiSbWO6 and so on, I had
>no difficulty to see the Li atoms in the Fourier syntheses, by X-ray.
>Hence not a large need of neutrons, but in a few cases.
Armel could "see" the Li atoms from X-ray data, and I found the H in
AlSiO3(OH) (Schmidt et al., American Mineralogist 83, 881, 1998) with
powder data in a 3-phase mixture. It was clearly visible in the difference
Fourier and could be refined with reasonable distances and thermal factors.
I think all would agree that this was an unusual case, and that one would
not normally expect to find H atoms from X-ray powder data.
Clearly it is important to have a variety of tools in your arsenal and use
whatever of them you need. In addition, make certain that your scientific
administrators and legislators understand the need for neutron facilities!
Larry
--
Larry W. Finger [EMAIL PROTECTED]
Geophysical Laboratory Phone: +1 (202) 686-2410 X 2464
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