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[sdpd] Re: preferred axes
> I have a sample with severe problems of preferred orientation and, in
> order to get the right fitting, i'd like to know what criteria do you
> use to know on which axes is my sample mainly orientated.
It is difficult to give a generally applicable advice: I can just share
my experience.
When the structure model is absent or incomplete one can guess the most
probable preferred orientation direction from the shape of (000)
Patterson peak. If the preferred orientation is appreciable enough the
shape of the (000) Patterson peak tends to "follow" the crystallite
shape.
If the structure model is more or less complete, I use the following
preferred orientation correction (included in DDM program):
1/[1+(P1*h^2+P2*k^2+P3*l^2+P4*2hk+P5*2hl+P6*2kl)*0.001d^2]^1.5
where hkl are the Miller indexes, d is the d-spacing and P1...P6 are
refinable parameters. Using this correction you do not need to know the
preferred direction, you just refine P1...P6 parameter applying proper
symmetry restrictions when needed.
Best regards,
Leonid
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