Thank you all for the helpful discussions. sincerely, Kianoush
On Jul 9, 2014, at 2:49 AM, Tim Gruene wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > Hi Colin, > > On 07/09/2014 09:52 AM, Colin Nave wrote: >> [...] "If the whole diffraction process is considered as an >> interference problem then the contributions are not confined to >> the Bragg condition." > Isn't this how textbooks on crystallography usually start? Drenth, > e.g. starts with the scattering from a single electron, then builds up > a molecule and the lattice and you find the everything except the > Bragg peaks vanish in the noise. How does one NOT see the diffraction > process as an interference 'problem'? > > Curiously, > Tim > >> >> Colin >> >> -----Original Message----- From: Gerard Bricogne >> [mailto:g...@globalphasing.com] Sent: 09 July 2014 00:38 To: ccp4bb >> Subject: Re: [ccp4bb] question about powder diffraction >> >> Dear all, >> >> The downstream end of this thread seems to have drifted into >> learned considerations of spelling, so I am getting back to this >> early reply. >> >> I am surprised that nobody has mentioned the role of the >> wavelength in all this: there is no way that one can directly link >> the first four planes in a Nickel crystal to a fixed set of 2theta >> values. The values you quote, Kianoush, must have been observed for >> a certain wavelength, but they would be different for another >> wavelength. So if you want one of the powder rings to come out at >> a 2theta of 45 degrees, adjust the wavelength accordingly so that >> Bragg's law be satisfied for the spacing between the corresponding >> planes. >> >> There also seems to be a confusion in the last question (unless I >> have completely misunderstood it) about the orientation of a >> crystal and the Bragg angle at which it will contribute to the >> ring pattern of the powder it belongs to. If there is a crystal >> oriented with some if its planes at 45 degrees from the X-ray beam, >> that will simply determine where on each ring its diffraction spots >> will contribute: it will have no effect on the Bragg angles of >> those spots, that depend purely on the internal spacings between >> atoms within the crystal, not on the orientation of the crystal. At >> the same wavelength at which you quote the 2theta values for those >> four rings, the crystal at 45 degrees from the beam will still have >> its diffraction spots contribute to the rings at 44, 52, 76 and 93 >> degrees. >> >> Again, forgive me if I have completely misunderstood the initial >> question. >> >> >> With best wishes, >> >> Gerard. >> >> -- On Tue, Jul 08, 2014 at 04:13:59PM -0400, Edward A. Berry >> wrote: >>> The plane will scatter, and all atoms in the plane will scatter >>> in phase if angle of incidence equals angle of reflection. this >>> is how a mirror reflects. Furthermore all the parallel planes >>> will also reflect at this angle. Trouble is the beams scattered >>> from the different parallel planes are systematically out of >>> phase with each other unless Bragg's law is met for that set of >>> planes, so interference is destructive and adds up to nothing. >>> At least that's how I understand it, eab >>> >>> >>> >>> >>> On 07/08/2014 03:53 PM, Kianoush Sadre-Bazzaz wrote: >>>> Hi >>>> >>>> If a sample of powder crystal (say Nickel) is shot with >>>> monochromatic >> x-rays, one will observe reflections from planes that satisfy >> Bragg's Law. For Ni the first four planes are (111, 200, 202, 311) >> with 2theta (44, 52, 76, 93 degrees) respectively. >>>> >>>> Why doesn't one observe a reflection at, say, 45 degrees? >>>> There will be >> a grain oriented in the powder such that x-rays reflect at 45 >> degrees and so forth. I would expect a continuum of reflections... >> >>>> >>>> thanks for the insight. >>>> >>>> Kianoush >>>> >> > > - -- > - -- > 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/ > > iD8DBQFTvRAeUxlJ7aRr7hoRAoJAAKDA0VZMXMz7+sbOxVjGB/lPLzI+tgCePKu9 > uDKSAlSudLR0OGprtltg9oE= > =eNZt > -----END PGP SIGNATURE-----
