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Dear Jakob,
your Gedankenexperiment on powder diffraction is not correct. You
would record a powder diffraction pattern if you rotated a single
crystal around the beam axis and record the result on a single image.
This rotation does not affect the mosaicity and the mosaicity of a
powder sample related only to the mosaicity of the micro crystals
present in the powder. You also do not get arcs when reducing the
powderness but you start seeing single spots. This can often be
observed in the presence of ice rings.
Best,
Tim
On 04/25/2014 09:32 AM, Keller, Jacob wrote:
> Is the following being neglected?
>
> In a crystal with these putative mosaic microdomains, there will be
> interference between microdomains at their edges/borders (at
> least), but since most microdomains are probably way smaller than
> the coherence length of 3-10 microns, presumably all unit cells in
> domain A interfere with all unit cells in domains B, C, etc, which
> are in the same coherence volume. In fact, as I said too unclearly
> in a previous post, as the putative microdomains become smaller and
> smaller to the limit of one unit cell, they become
> indistinguishable from unit cell parameter variation. So I am
> becoming increasingly suspicious about the existence of
> microdomains, and wonder what hard evidence there is for their
> existence?
>
> As a thought experiment, one can consider the microdomain theory
> taken to its limit: a powder diffraction image. In powder
> diffraction, there are so many crystals (read: microdomains) that
> each spot is manifested at its Bragg angle at every possible radial
> position on the detector. Mosaicity would be, what, 360 degrees?
> So, now imagine decreasing the mosaicity to lower values, and one
> gets progressively shorter arcs which at lower values become spots.
> Doesn’t this mean that the contribution from microdomain mosaicity
> should be to make the spots more like arcs, as we sometimes see in
> terrible diffraction patterns, and not just general broadening of
> spots? Put another way: mosaicity should broaden spots in the
> radial direction (arcs), and unit cell parameter variation should
> produce straight broadening in the direction of the unit cell
> variation of magnitude proportional to the degree of variation in
> that direction.
>
> JPK
>
>
> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf
> Of Ian Tickle Sent: Thursday, April 24, 2014 7:01 PM To:
> [log in to unmask] Subject: Re: [ccp4bb] AW: [ccp4bb] Twinning
> VS. Disorder
>
>
> Dear Herman On 24 April 2014 22:32,
> <[log in to unmask]<mailto:[log in to unmask]>>
> wrote:
>
> The X-ray coherent length is depending on the crystal, not the
> synchrotron and my gut feeling is that it is at least several
> hundred unit cells, but here other experts may correct me.
>
>
> I assume you meant that the coherence length is a property of the
> beam (e.g. for a Cu target source it's related to the lifetime of
> the excited Cu K-alpha state), not the crystal, e,g, see
> http://www.aps.anl.gov/Users/Meeting/2010/Presentations/WK2talk_Vartaniants.pdf
> (slides 8-11). The relevant property of the crystal is the size of
> the microdomains. You don't get interference because coherence
> length << domain size, i.e. the beam is not coherent over more than
> 1 domain. This is true for in-house sources & synchrotrons, I
> guess for FELs it's different, i.e. much greater coherence length?
> This relates to a question I asked on the BB some time ago: if the
> coherence length is long enough would you start to see the effects
> of interference in twinned crystals, i.e. would the summation of
> intensities break down? I defer to the experts on synchrotrons &
> FELs! Cheers -- Ian
>
- --
- --
Dr Tim Gruene
Institut fuer anorganische Chemie
Tammannstr. 4
D-37077 Goettingen
GPG Key ID = A46BEE1A
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