I think Gerard has answered the original question. To introduce additional complications (do people want complications?), there is apparently an issue with the ease of getting reflections in a powder pattern.
A new theory for X-ray diffraction Fewster, P. F. (2014). Acta Cryst. A70, 257-282. http://journals.iucr.org/a/issues/2014/03/00/sc5066/index.html A couple of quotes (I don't think they are out of context) For powder diffraction, this theory " explains why diffraction peaks are obtained from samples with very few crystallites, which cannot be explained with the conventional theory." "If the whole diffraction process is considered as an interference problem then the contributions are not confined to the Bragg condition." 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 > > -- =============================================================== * * * Gerard Bricogne g...@globalphasing.com * * * * Global Phasing Ltd. * * Sheraton House, Castle Park Tel: +44-(0)1223-353033 * * Cambridge CB3 0AX, UK Fax: +44-(0)1223-366889 * * * =============================================================== -- This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. If you are not the intended addressee or an authorised recipient of the addressee please notify us of receipt by returning the e-mail and do not use, copy, retain, distribute or disclose the information in or attached to the e-mail. Any opinions expressed within this e-mail are those of the individual and not necessarily of Diamond Light Source Ltd. Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message. Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom
