Anticipating FAQ

Please submit complement of information and correct errors so as this document may eventually serve as response to FAQ. For instance and by anticipation (although there is no chance that somebody post such questions):

Q1- Why some people continue to use in-laboratory diffractometers so largely surclassed by synchrotrons for structure determination and/or refinement from powder diffraction data ?
R1- Not everybody disposes of a synchrotron in less than a couple of weeks to try to solve his problems. The normal way to access is by a proposal. As a mean, 3 months or much more may take place between the proposal redaction and the experiment, if accepted. For motivated Academic Researchers, awaiting more than a couple of days make difficult to keep interest on the subject : so they usually try with their lab instrument with chances of success that can be estimated (see the question What chance of success with a lab instrument ?). Imagine you wait 6 months and you do not succeed in obtaining a good synchrotron pattern ! Also think about the Commision judgement about your proposal if not any preliminary work has been done on a lab instrument... Now, some researchers may have access to synchrotron by other ways than an official proposal.

Some statistics on data used for determination :

         Sync         31              
         XC1          59         |     Lab.         |   144
         XC12         85         |  instruments     |
         N            17
         Total =     192
Q2- Does the alpha-2 presence lead to the determination of less complex structures than with alpha-1 alone ?
R2- No.

Q3- What chance of success with a lab instrument ?
R3- From experimental cases with a lab. instrument (see above), the current largest structures determined culminate to 38 independent atoms, 118 refined xyz parameters, up to 18 independent atoms simultaneously located as a starting model allowing then Fourier syntheses. Not so bad. From synchrotron (eventually + neutron) data one can find cases approximately twice more complex.

Q4- What is the first structure ever determined from powder diffraction data ?
R4-Two papers are sometimes quoted to be the first :
- The crystal structure of beta plutonium metal, W.H. Zachariasen & F.H. Ellinger, Acta Cryst. 16 (1963) 369-375.
- On the use of Guinier-Hagg film data for structure analysis. The crystal structure of tetraammonium aa'-u-oxobis-{[gied'-u3(S)- malato-O(1),O(2),O(4),O(4')]-di-u-oxabis[dioxomolybdate(VI)]} monohydrate, J.-E. Berg & P.-E. Werner, Z. Kristallogr. 145 (1977) 310-320.