-----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-----
