Ideally you should rotate your crystal around the long axis during data
collection. This is however easier said than done since often such
crystals grow a plates with the long axis perpendicular to the plane of
the plate.
As long as the long axis is more or less oriented perpendicular to the
x-ray beam (+/-45 deg but depending on the "severity of the case") you
will get away with it and you might even consider large oscillation
ranges. For orientations with the long axis in a direction close to the
X-ray beam, you will have to reduce the oscillation range, perhaps even
to 0.1deg to avoid excessive spot overlap. Here things depend heavily on
mosaicity, which should be low (preferably < 0.2 deg). Otherwise spots
may always overlap, even on still images.
Very often (in my experience at least) crystals with one particularly
long axis have high symmetry, and collecting a large wedge covering the
"good" orientations can be sufficient. Of course I don't know if you are
that lucky. If you are not sure which part to collect, collect at least
180 deg to be sure you covered the whole unique part of reciprocal space.
BTW, you didn't say what is "long". My post extreme case until now was
P3121 with a=b= 50, c = 360 and this resulted in very good 2.4 A data
from a rystal with mosaicity 0.1-0.15. Crystals with mosiacity around
1.0 deg were useless because of too much overlap.
Remy
Op 5/04/2011 7:05, dengzq1987 schreef:
hello all,
does anyone have the experience of
Collecting Data from Long Unit Cell Axes ? I have a crystal that
diffracts to about 4 A. in some /direction/ the spots overlap. we
can't use the data to index .we think it is because that there is a
long unit cell axes. so is there any method to solve this problem?
best wishes.
2011-04-05
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dengzq1987
