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_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
