Let start to cut the crap and go to Armel's real reason not to use a
combined refinement:
> Of course this is only kidding and provocative opinion, as
> usual. I love both radiations, indeed. However, I tend to think
> that a simultaneous refinement could eventually degrade
> the heavy atom position accuracy AND degrade the light
> atoms accuracy. The former being better from the X-ray only
> and the latter from the neutron only, not to speak of the various
> problems that hurt me like, for instance :
Do not agree!
If you work on any transition metal oxide, what I am doing all the time,
you will find that the oxide position are basically not determined. OK I
work a lot with heavy elements like lanthanide's and iridium and the like.
But in general the x-ray are very insensitive to where the oxides are. Not
the forget the oxide content! So they do not effect the positions as
required by the neutrons. Things can be even more true the other way
around. e.g when you have a random dirtribution of either Ti or Mn and
another metal you can have scattering lengths of zero.
> - the need to refine two sets of cell parameters, inevitably
> slightly different if both data are of high resolution.
Basically what you mean is that the neutron diffractometer is not properly
calibrated. Well a combined refinement in which you refine the neutron
wavelength is than the answer! Recently I ran two different samples on
D2b, and refined lambda:
sample 1 1.59362(5)
sample 2 1.59354(5)
well that is very similar isn't it. I have read people stating on this
e-mail list that they don't trust cell parameters after the third decimal.
> - not exactly the same temperature
True, but you can control that if you want to, and in general does bot
seem to be the biggest pain.
> - bulk with neutrons, surface with X-ray
>
What? Do you propose to throw all the single crystall structure
determinations in the bin, out of the window or where else because it is a
surface technique? Of course the penetration depth of x-ray is a lot
smaller than that of neutrons, but it remains a bulk technique. Thank god
other wise we would still be cleaving those large single crystals. I dare
to say that you can study the bulk structure even with electrons, which
have an even smaller penetration depth.
Of course you have to be carefull as Brian Toby pointed out. But in those
cases you either a non-homogeneous sample, or a phase transformation which
does happen on the slow cooling central parts of the particles but not on
the fast cooling outer regions of the particle. In those cased it is back
to the preparation lab.
> Giving the results of 2 independent refinements seems better
> to me.
But which journal is going to accept that!
Finally Armel, in my view an attempt the split the Rietveld community in
two, i.e. in house X-ray and central facility neutrons, is artificial. By
teh way did you note that someone called Mark Weller was on your neutron
list as well. As far as I know, he has been working in Southampton for
some time now. So there is hope for ordinairy university based scientist
to use neutrons!
Best
Jaap
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