Dear all, Isnt it much more natural to publish only atomic coordinates in a direct space without inversion ?
Therefore I propose, that publishing data in other units should be
avoided. Sounds very tough. Best regards, -- Olga Smirnova Laboratoire de Cristallographie 24, quai Ernest-Ansermet CH-1211 Geneva 4, Switzerland Phone : [+[41] 22] 37 962 14, FAX : [+[41] 22] 37 961 08 On 2/21/07, Klaus-Dieter Liss <[EMAIL PROTECTED]> wrote:
Dear Powder-Diffraction User, with the advancement of modern research infrastructure such as instruments, computing, complementary techniques, I like to raise again the necessity to present powder diffraction data in Q-space rather than in instrumental units. Other communities are already well ahead (single-xtal, SANS, SAXS, reflectometry etc) and to my view, only the powder diffractionist stick to their out-dated units (2-theta, TOF, d-spacing...). there is a poll I started a while ago under http://elpopo.ing.unitn.it:8064/maudFor/viewtopic.php?t=205 which, so far, is not very representative and I would encourage you to give your opinions. I suppose, all of us have learned the basics of crystallography somewhere during the career and the laws of Bragg diffraction. So, all of us are familiar with reciprocal space, where, for example, a reciprocal lattice can be constructed in order to represent the crystal symmetry in the natural space of diffraction. The Ewald construction and the Laue equation are examples which make most use of this. Further, reciprocal space is LINEAR, i.e. A second order reflection has double the distance from the origin than the fundamental reflection and a 110 reflection sqrt(2) times the distance than a 100 of a cubic system, etc. This alone would be a very good reason to plot all diffraction patterns as a function of reciprocal space coordinates Q. For Powder Diffraction, this means, patterns should NOT be plotted as a function of 2-theta, d, tof etc but Q which is the only natural unit! The relations are: Q = 4 * Pi * sin(theta) / lambda; or Q = 2 * Pi / d; The benefits of plotting and publishing data in this representation are obvious: * reciprocal space is the NATURAL space diffraction takes place; * reciprocal space is LINEAR and symmetries can be identified by eye; * the representation directly reflects crystal SYMMETRY; * the representation is INDEPENDENT of the instrument, type of radiation (electrons, neutrons, X-rays, light, atoms...) * the representation is INDEPENDENT of the wavelength used; * presentations and publications are directly COMPARABLE; * reciprocal space is WIDELY USED outside the powder diffraction community, such as single crystal diffraction SA(NX)S or reflectometry; Therefore I propose, that publishing data in other units should be avoided. _________________ Klaus-Dieter Liss -- Dr. Klaus-Dieter Liss Research Scientist, Bragg Institute Australian Nuclear Science and Technology Organisation PMB 1, Menai NSW 2234, Australia T: +61-2-9717+9479 F: +61-2-9717+3606 M: 0419 166 978 E: [EMAIL PROTECTED] http://www.ansto.gov.au/ansto/bragg/staff/s_liss.html see also: http://liss.freeshell.org
