Dear Jacob, In terms of the effect of crystal (lattice) defects on diffraction spot profiles, there are two great papers by Colin Nave that discuss this: http://journals.iucr.org/d/issues/1998/05/00/issconts.html and http://journals.iucr.org/d/issues/1998/05/00/issconts.html . There is also this paper on the nature of mosaic micro-domains: http://journals.iucr.org/d/issues/2000/08/00/en0024/en0024.pdf.
I am sure there must be other references for the 'nature' of lattice disorder, and it anyone can point to them, I would be grateful. Cheers, Oliver On Fri, Apr 25, 2014 at 6:20 AM, Keller, Jacob <[email protected]>wrote: > >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. > > If you wanted to do it with a single crystal, you would have to rotate the > crystal through all possible rotations in 3d, not just around the axis of > the beam, because you would then miss all the reflections which were not in > the diffraction condition at that phi angle. I agree that it could be done > this way (not sure why this is important though.) > > >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. > > You are talking about "powderness," which I would guess is a measure of > the completeness of the sampling of all possible orientations of the > constituent crystals, and I agree with what you say would happen. I said, > however, mosaicity, which is a measure of the breadth of the distribution > of the orientation angles of the microdomains/microcrystals. By decreasing > "powderness," one would do nothing to mosaicity. If one could arrange the > microdomains into some range of orientation angles, one would reduce the > mosaicity, and get arcs. I wish I had a picture of an arc-containing > diffraction pattern--I've seen them from time to time, and they're always > of course bad news. Anyone on the list have such a diffraction pattern > handy? > > JPK > > > > > 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:[email protected]] On Behalf Of > > Ian Tickle Sent: Thursday, April 24, 2014 7:01 PM To: > > [email protected] Subject: Re: [ccp4bb] AW: [ccp4bb] Twinning VS. > > Disorder > > > > > > Dear Herman On 24 April 2014 22:32, > > <[email protected]<mailto:[email protected]>> > > 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_Vartan > > iants.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 > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.12 (GNU/Linux) > Comment: Using GnuPG with Icedove - http://www.enigmail.net/ > > iD8DBQFTWiJpUxlJ7aRr7hoRAvhpAKCWt3PwAQsPnUgMlHjYoGS/7lVlGACglWpz > K+rZPikLZBwe+CrK29WhBnc= > =4a9F > -----END PGP SIGNATURE----- > -- Dr. Oliver B. Zeldin Brunger Group Stanford University
