Alan Hewat wrote:
>Before some-one else says they can see hydrogen with x-rays, Mike
>Glazer, Bill David and I saw vanadium with neutrons. So let's just
>say "insensitive" but not "completely". Otherwise I agree :-)
>Combined refinements are sometimes necessary and a good thing.
>
>1. David, W. I. F., Glazer, A. M. and Hewat, A. W. (1979) Phase
Transitions 1, 155-69.
>The structure and ferroelastic phase transition of bismuth vanadate (BiVO4).
>
I'd agree with Alan that neutrons are a little "insensitive" to vanadium.
I've been looking at some vanadium substituted cuprates - in which the
vanadium is tetrahedral as "VO43-". We got this information from 51V
MAS-NMR - the 51V nucleus has spin I=7/2, a natural abundance of 99.76% and
high relative sensitivity. Using this information we could set up the
tetrahedral vanadium coordination environment in the model for the powder
neutron data - allowing us to model the vanadium position with more
confidence (if not precision). I'm no expert on 51V-NMR but there is a lot
of stuff in the literature (e.g. O.B. Lapina et al., Progress in NMR
spectroscopy, 24, 457-525, 1992) indicating that the degree of distortion
around the 51V nucleus can be extracted from a detailed analysis of such
NMR data. Lets assume we have near perfect "VO4" tetrahedra - it seems to
me that a smart way to deal with the "insensitivity" of neutrons toward
vanadium would then be to set up the the coordination around vanadium as a
rigid body in GSAS with the bond angles fixed (initially) at 109o 28', and
then allow the bond lengths to relax. That said, I can't actually figure
out how to do this with GSAS though!! (But I'm persevering...)
Best regards,
Neil Hyatt.
**********
Neil Hyatt tel: +44-(0)121 414 4370
School of Chemistry fax: +44-(0)121 414 4442
University of Birmingham email: [EMAIL PROTECTED]
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