As sent to the neutron mailing list

[Armel Le Bail]

It is not usual to reply to a mail on another mailing list than
the original.

Fortunately I am a subscriber of both the Rietveld and neutron
mailing lists.

Anyway, no, it is not a joke.

On about 400 structure determinations from powder diffraction
data, less than 5% were done from neutron data, and each time
the complexity of the structure was low.

Just see http://sdpd.univ-lemans.fr/iniref/topc2.html

I think that this is really due to a gap between what is called
"high resolution" for neutron data and for X-ray data. Another
reason is that almost all the atoms should be find in the
initial structure model, since there is no heavy atom in neutron
diffraction, of course.

But what is the main reason : lowest resolution or difficulty
to locate all atoms before being able to refine ? At least
fifty-fifty, I guess, explains the high level of SDPD failure with
neutron data.

Let us examine the lowest FWHM measured on powder patterns
at constant wavelength :
      neutron :  minimal FWHM ~0.10° (2-theta), pushing the machine
      conventional X-ray :  minimal FWHM ~0.04°
      synchrotron radiation : minimal FWHM < 0.01°

There is an order of magnitude between the neutron high
resolution and the synchrotron high resolution.

This is just a fact, not a joke.

Best,

Armel Le Bail


At 14:05 23/09/99 +0100, K. Shankland wrote:
>This expanded version of a message sent to the
>Rietveld list has just been brought to my 
>attention.  Surely the reference to resolution
>and neutrons must be intended as a joke ?
>
>>To: neutron at anl.gov
>>From: Armel Le Bail <armel at fluo.univ-lemans.fr>
>>Reply-To: Armel Le Bail <armel at fluo.univ-lemans.fr>
>>Subject: SDPD Internet Course
>>
>>Though neutron data are underprivileged by lowest resolution
>>than X-ray data (conventional as well as synchrotron
>>radiation sources) for SDPD, this may be of some interest.
>>
>>A new Internet course is proposed at the University
>>of Maine (France). Learn how to succeed in a Structure
>>Determination by Powder Diffractometry (SDPD)
>>with the help of experts.
>> 
>
>This is scarely believable after HRPD (yes, I'm biased
>towards the RAL diffractometer since I work
>here) has been running and producing high
>quality, high resolution data for 14 years.
>And before 'someone' asks, yes there are plenty
>of papers out there to back this up.
>
>Regards,
>
>Kenneth Shankland
>
>  
>